// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';
import 'dart:collection';
import 'dart:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter/gestures.dart' show DragStartBehavior;

import 'app_bar.dart';
import 'bottom_sheet.dart';
import 'button_bar.dart';
import 'colors.dart';
import 'divider.dart';
import 'drawer.dart';
import 'flexible_space_bar.dart';
import 'floating_action_button.dart';
import 'floating_action_button_location.dart';
import 'material.dart';
import 'snack_bar.dart';
import 'snack_bar_theme.dart';
import 'theme.dart';
import 'theme_data.dart';

// Examples can assume:
// TabController tabController;
// void setState(VoidCallback fn) { }
// String appBarTitle;
// int tabCount;
// TickerProvider tickerProvider;

const FloatingActionButtonLocation _kDefaultFloatingActionButtonLocation = FloatingActionButtonLocation.endFloat;
const FloatingActionButtonAnimator _kDefaultFloatingActionButtonAnimator = FloatingActionButtonAnimator.scaling;

// When the top of the BottomSheet crosses this threshold, it will start to
// shrink the FAB and show a scrim.
const double _kBottomSheetDominatesPercentage = 0.3;
const double _kMinBottomSheetScrimOpacity = 0.1;
const double _kMaxBottomSheetScrimOpacity = 0.6;

enum _ScaffoldSlot {
  body,
  appBar,
  bodyScrim,
  bottomSheet,
  snackBar,
  persistentFooter,
  bottomNavigationBar,
  floatingActionButton,
  drawer,
  endDrawer,
  statusBar,
}

/// The geometry of the [Scaffold] after all its contents have been laid out
/// except the [FloatingActionButton].
///
/// The [Scaffold] passes this pre-layout geometry to its
/// [FloatingActionButtonLocation], which produces an [Offset] that the
/// [Scaffold] uses to position the [FloatingActionButton].
///
/// For a description of the [Scaffold]'s geometry after it has
/// finished laying out, see the [ScaffoldGeometry].
@immutable
class ScaffoldPrelayoutGeometry {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const ScaffoldPrelayoutGeometry({
    @required this.bottomSheetSize,
    @required this.contentBottom,
    @required this.contentTop,
    @required this.floatingActionButtonSize,
    @required this.minInsets,
    @required this.scaffoldSize,
    @required this.snackBarSize,
    @required this.textDirection,
  });

  /// The [Size] of [Scaffold.floatingActionButton].
  ///
  /// If [Scaffold.floatingActionButton] is null, this will be [Size.zero].
  final Size floatingActionButtonSize;

  /// The [Size] of the [Scaffold]'s [BottomSheet].
  ///
  /// If the [Scaffold] is not currently showing a [BottomSheet],
  /// this will be [Size.zero].
  final Size bottomSheetSize;

  /// The vertical distance from the Scaffold's origin to the bottom of
  /// [Scaffold.body].
  ///
  /// This is useful in a [FloatingActionButtonLocation] designed to
  /// place the [FloatingActionButton] at the bottom of the screen, while
  /// keeping it above the [BottomSheet], the [Scaffold.bottomNavigationBar],
  /// or the keyboard.
  ///
  /// The [Scaffold.body] is laid out with respect to [minInsets] already. This
  /// means that a [FloatingActionButtonLocation] does not need to factor in
  /// [minInsets.bottom] when aligning a [FloatingActionButton] to
  /// [contentBottom].
  final double contentBottom;

  /// The vertical distance from the [Scaffold]'s origin to the top of
  /// [Scaffold.body].
  ///
  /// This is useful in a [FloatingActionButtonLocation] designed to
  /// place the [FloatingActionButton] at the top of the screen, while
  /// keeping it below the [Scaffold.appBar].
  ///
  /// The [Scaffold.body] is laid out with respect to [minInsets] already. This
  /// means that a [FloatingActionButtonLocation] does not need to factor in
  /// [minInsets.top] when aligning a [FloatingActionButton] to [contentTop].
  final double contentTop;

  /// The minimum padding to inset the [FloatingActionButton] by for it
  /// to remain visible.
  ///
  /// This value is the result of calling [MediaQuery.padding] in the
  /// [Scaffold]'s [BuildContext],
  /// and is useful for insetting the [FloatingActionButton] to avoid features like
  /// the system status bar or the keyboard.
  ///
  /// If [Scaffold.resizeToAvoidBottomInset] is set to false, [minInsets.bottom]
  /// will be 0.0.
  final EdgeInsets minInsets;

  /// The [Size] of the whole [Scaffold].
  ///
  /// If the [Size] of the [Scaffold]'s contents is modified by values such as
  /// [Scaffold.resizeToAvoidBottomInset] or the keyboard opening, then the
  /// [scaffoldSize] will not reflect those changes.
  ///
  /// This means that [FloatingActionButtonLocation]s designed to reposition
  /// the [FloatingActionButton] based on events such as the keyboard popping
  /// up should use [minInsets] to make sure that the [FloatingActionButton] is
  /// inset by enough to remain visible.
  ///
  /// See [minInsets] and [MediaQuery.padding] for more information on the appropriate
  /// insets to apply.
  final Size scaffoldSize;

  /// The [Size] of the [Scaffold]'s [SnackBar].
  ///
  /// If the [Scaffold] is not showing a [SnackBar], this will be [Size.zero].
  final Size snackBarSize;

  /// The [TextDirection] of the [Scaffold]'s [BuildContext].
  final TextDirection textDirection;
}

/// A snapshot of a transition between two [FloatingActionButtonLocation]s.
///
/// [ScaffoldState] uses this to seamlessly change transition animations
/// when a running [FloatingActionButtonLocation] transition is interrupted by a new transition.
@immutable
class _TransitionSnapshotFabLocation extends FloatingActionButtonLocation {

  const _TransitionSnapshotFabLocation(this.begin, this.end, this.animator, this.progress);

  final FloatingActionButtonLocation begin;
  final FloatingActionButtonLocation end;
  final FloatingActionButtonAnimator animator;
  final double progress;

  @override
  Offset getOffset(ScaffoldPrelayoutGeometry scaffoldGeometry) {
    return animator.getOffset(
      begin: begin.getOffset(scaffoldGeometry),
      end: end.getOffset(scaffoldGeometry),
      progress: progress,
    );
  }

  @override
  String toString() {
    return '$runtimeType(begin: $begin, end: $end, progress: $progress)';
  }
}

/// Geometry information for [Scaffold] components after layout is finished.
///
/// To get a [ValueNotifier] for the scaffold geometry of a given
/// [BuildContext], use [Scaffold.geometryOf].
///
/// The ScaffoldGeometry is only available during the paint phase, because
/// its value is computed during the animation and layout phases prior to painting.
///
/// For an example of using the [ScaffoldGeometry], see the [BottomAppBar],
/// which uses the [ScaffoldGeometry] to paint a notch around the
/// [FloatingActionButton].
///
/// For information about the [Scaffold]'s geometry that is used while laying
/// out the [FloatingActionButton], see [ScaffoldPrelayoutGeometry].
@immutable
class ScaffoldGeometry {
  /// Create an object that describes the geometry of a [Scaffold].
  const ScaffoldGeometry({
    this.bottomNavigationBarTop,
    this.floatingActionButtonArea,
  });

  /// The distance from the [Scaffold]'s top edge to the top edge of the
  /// rectangle in which the [Scaffold.bottomNavigationBar] bar is laid out.
  ///
  /// Null if [Scaffold.bottomNavigationBar] is null.
  final double bottomNavigationBarTop;

  /// The [Scaffold.floatingActionButton]'s bounding rectangle.
  ///
  /// This is null when there is no floating action button showing.
  final Rect floatingActionButtonArea;

  ScaffoldGeometry _scaleFloatingActionButton(double scaleFactor) {
    if (scaleFactor == 1.0)
      return this;

    if (scaleFactor == 0.0) {
      return ScaffoldGeometry(
        bottomNavigationBarTop: bottomNavigationBarTop,
      );
    }

    final Rect scaledButton = Rect.lerp(
      floatingActionButtonArea.center & Size.zero,
      floatingActionButtonArea,
      scaleFactor,
    );
    return copyWith(floatingActionButtonArea: scaledButton);
  }

  /// Creates a copy of this [ScaffoldGeometry] but with the given fields replaced with
  /// the new values.
  ScaffoldGeometry copyWith({
    double bottomNavigationBarTop,
    Rect floatingActionButtonArea,
  }) {
    return ScaffoldGeometry(
      bottomNavigationBarTop: bottomNavigationBarTop ?? this.bottomNavigationBarTop,
      floatingActionButtonArea: floatingActionButtonArea ?? this.floatingActionButtonArea,
    );
  }
}

class _ScaffoldGeometryNotifier extends ChangeNotifier implements ValueListenable<ScaffoldGeometry> {
  _ScaffoldGeometryNotifier(this.geometry, this.context)
    : assert (context != null);

  final BuildContext context;
  double floatingActionButtonScale;
  ScaffoldGeometry geometry;

  @override
  ScaffoldGeometry get value {
    assert(() {
      final RenderObject renderObject = context.findRenderObject();
      if (renderObject == null || !renderObject.owner.debugDoingPaint)
        throw FlutterError(
            'Scaffold.geometryOf() must only be accessed during the paint phase.\n'
            'The ScaffoldGeometry is only available during the paint phase, because\n'
            'its value is computed during the animation and layout phases prior to painting.'
        );
      return true;
    }());
    return geometry._scaleFloatingActionButton(floatingActionButtonScale);
  }

  void _updateWith({
    double bottomNavigationBarTop,
    Rect floatingActionButtonArea,
    double floatingActionButtonScale,
  }) {
    this.floatingActionButtonScale = floatingActionButtonScale ?? this.floatingActionButtonScale;
    geometry = geometry.copyWith(
      bottomNavigationBarTop: bottomNavigationBarTop,
      floatingActionButtonArea: floatingActionButtonArea,
    );
    notifyListeners();
  }
}

// Used to communicate the height of the Scaffold's bottomNavigationBar and
// persistentFooterButtons to the LayoutBuilder which builds the Scaffold's body.
//
// Scaffold expects a _BodyBoxConstraints to be passed to the _BodyBuilder
// widget's LayoutBuilder, see _ScaffoldLayout.performLayout(). The BoxConstraints
// methods that construct new BoxConstraints objects, like copyWith() have not
// been overridden here because we expect the _BodyBoxConstraintsObject to be
// passed along unmodified to the LayoutBuilder. If that changes in the future
// then _BodyBuilder will assert.
class _BodyBoxConstraints extends BoxConstraints {
  const _BodyBoxConstraints({
    double minWidth = 0.0,
    double maxWidth = double.infinity,
    double minHeight = 0.0,
    double maxHeight = double.infinity,
    @required this.bottomWidgetsHeight,
    @required this.appBarHeight,
  }) : assert(bottomWidgetsHeight != null),
       assert(bottomWidgetsHeight >= 0),
       assert(appBarHeight != null),
       assert(appBarHeight >= 0),
       super(minWidth: minWidth, maxWidth: maxWidth, minHeight: minHeight, maxHeight: maxHeight);

  final double bottomWidgetsHeight;
  final double appBarHeight;

  // RenderObject.layout() will only short-circuit its call to its performLayout
  // method if the new layout constraints are not == to the current constraints.
  // If the height of the bottom widgets has changed, even though the constraints'
  // min and max values have not, we still want performLayout to happen.
  @override
  bool operator ==(Object other) {
    if (super != other)
      return false;
    return other is _BodyBoxConstraints
        && other.bottomWidgetsHeight == bottomWidgetsHeight
        && other.appBarHeight == appBarHeight;
  }

  @override
  int get hashCode {
    return hashValues(super.hashCode, bottomWidgetsHeight, appBarHeight);
  }
}

// Used when Scaffold.extendBody is true to wrap the scaffold's body in a MediaQuery
// whose padding accounts for the height of the bottomNavigationBar and/or the
// persistentFooterButtons.
//
// The bottom widgets' height is passed along via the _BodyBoxConstraints parameter.
// The constraints parameter is constructed in_ScaffoldLayout.performLayout().
class _BodyBuilder extends StatelessWidget {
  const _BodyBuilder({
    Key key,
    @required this.extendBody,
    @required this.extendBodyBehindAppBar,
    @required this.body,
  }) : assert(extendBody != null),
       assert(extendBodyBehindAppBar != null),
       assert(body != null),
       super(key: key);

  final Widget body;
  final bool extendBody;
  final bool extendBodyBehindAppBar;

  @override
  Widget build(BuildContext context) {
    if (!extendBody && !extendBodyBehindAppBar)
      return body;

    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        final _BodyBoxConstraints bodyConstraints = constraints as _BodyBoxConstraints;
        final MediaQueryData metrics = MediaQuery.of(context);

        final double bottom = extendBody
          ? math.max(metrics.padding.bottom, bodyConstraints.bottomWidgetsHeight)
          : metrics.padding.bottom;

        final double top = extendBodyBehindAppBar
          ? math.max(metrics.padding.top, bodyConstraints.appBarHeight)
          : metrics.padding.top;

        return MediaQuery(
          data: metrics.copyWith(
            padding: metrics.padding.copyWith(
              top: top,
              bottom: bottom,
            ),
          ),
          child: body,
        );
      },
    );
  }
}

class _ScaffoldLayout extends MultiChildLayoutDelegate {
  _ScaffoldLayout({
    @required this.minInsets,
    @required this.textDirection,
    @required this.geometryNotifier,
    // for floating action button
    @required this.previousFloatingActionButtonLocation,
    @required this.currentFloatingActionButtonLocation,
    @required this.floatingActionButtonMoveAnimationProgress,
    @required this.floatingActionButtonMotionAnimator,
    @required this.isSnackBarFloating,
    @required this.extendBody,
    @required this.extendBodyBehindAppBar,
  }) : assert(minInsets != null),
       assert(textDirection != null),
       assert(geometryNotifier != null),
       assert(previousFloatingActionButtonLocation != null),
       assert(currentFloatingActionButtonLocation != null),
       assert(extendBody != null),
       assert(extendBodyBehindAppBar != null);

  final bool extendBody;
  final bool extendBodyBehindAppBar;
  final EdgeInsets minInsets;
  final TextDirection textDirection;
  final _ScaffoldGeometryNotifier geometryNotifier;

  final FloatingActionButtonLocation previousFloatingActionButtonLocation;
  final FloatingActionButtonLocation currentFloatingActionButtonLocation;
  final double floatingActionButtonMoveAnimationProgress;
  final FloatingActionButtonAnimator floatingActionButtonMotionAnimator;

  final bool isSnackBarFloating;

  @override
  void performLayout(Size size) {
    final BoxConstraints looseConstraints = BoxConstraints.loose(size);

    // This part of the layout has the same effect as putting the app bar and
    // body in a column and making the body flexible. What's different is that
    // in this case the app bar appears _after_ the body in the stacking order,
    // so the app bar's shadow is drawn on top of the body.

    final BoxConstraints fullWidthConstraints = looseConstraints.tighten(width: size.width);
    final double bottom = size.height;
    double contentTop = 0.0;
    double bottomWidgetsHeight = 0.0;
    double appBarHeight = 0.0;

    if (hasChild(_ScaffoldSlot.appBar)) {
      appBarHeight = layoutChild(_ScaffoldSlot.appBar, fullWidthConstraints).height;
      contentTop = extendBodyBehindAppBar ? 0.0 : appBarHeight;
      positionChild(_ScaffoldSlot.appBar, Offset.zero);
    }

    double bottomNavigationBarTop;
    if (hasChild(_ScaffoldSlot.bottomNavigationBar)) {
      final double bottomNavigationBarHeight = layoutChild(_ScaffoldSlot.bottomNavigationBar, fullWidthConstraints).height;
      bottomWidgetsHeight += bottomNavigationBarHeight;
      bottomNavigationBarTop = math.max(0.0, bottom - bottomWidgetsHeight);
      positionChild(_ScaffoldSlot.bottomNavigationBar, Offset(0.0, bottomNavigationBarTop));
    }

    if (hasChild(_ScaffoldSlot.persistentFooter)) {
      final BoxConstraints footerConstraints = BoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: math.max(0.0, bottom - bottomWidgetsHeight - contentTop),
      );
      final double persistentFooterHeight = layoutChild(_ScaffoldSlot.persistentFooter, footerConstraints).height;
      bottomWidgetsHeight += persistentFooterHeight;
      positionChild(_ScaffoldSlot.persistentFooter, Offset(0.0, math.max(0.0, bottom - bottomWidgetsHeight)));
    }

    // Set the content bottom to account for the greater of the height of any
    // bottom-anchored material widgets or of the keyboard or other
    // bottom-anchored system UI.
    final double contentBottom = math.max(0.0, bottom - math.max(minInsets.bottom, bottomWidgetsHeight));

    if (hasChild(_ScaffoldSlot.body)) {
      double bodyMaxHeight = math.max(0.0, contentBottom - contentTop);

      if (extendBody) {
        bodyMaxHeight += bottomWidgetsHeight;
        bodyMaxHeight = bodyMaxHeight.clamp(0.0, looseConstraints.maxHeight - contentTop).toDouble();
        assert(bodyMaxHeight <= math.max(0.0, looseConstraints.maxHeight - contentTop));
      }

      final BoxConstraints bodyConstraints = _BodyBoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: bodyMaxHeight,
        bottomWidgetsHeight: extendBody ? bottomWidgetsHeight : 0.0,
        appBarHeight: appBarHeight,
      );
      layoutChild(_ScaffoldSlot.body, bodyConstraints);
      positionChild(_ScaffoldSlot.body, Offset(0.0, contentTop));
    }

    // The BottomSheet and the SnackBar are anchored to the bottom of the parent,
    // they're as wide as the parent and are given their intrinsic height. The
    // only difference is that SnackBar appears on the top side of the
    // BottomNavigationBar while the BottomSheet is stacked on top of it.
    //
    // If all three elements are present then either the center of the FAB straddles
    // the top edge of the BottomSheet or the bottom of the FAB is
    // kFloatingActionButtonMargin above the SnackBar, whichever puts the FAB
    // the farthest above the bottom of the parent. If only the FAB is has a
    // non-zero height then it's inset from the parent's right and bottom edges
    // by kFloatingActionButtonMargin.

    Size bottomSheetSize = Size.zero;
    Size snackBarSize = Size.zero;
    if (hasChild(_ScaffoldSlot.bodyScrim)) {
      final BoxConstraints bottomSheetScrimConstraints = BoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: contentBottom,
      );
      layoutChild(_ScaffoldSlot.bodyScrim, bottomSheetScrimConstraints);
      positionChild(_ScaffoldSlot.bodyScrim, Offset.zero);
    }

    // Set the size of the SnackBar early if the behavior is fixed so
    // the FAB can be positioned correctly.
    if (hasChild(_ScaffoldSlot.snackBar) && !isSnackBarFloating) {
      snackBarSize = layoutChild(_ScaffoldSlot.snackBar, fullWidthConstraints);
    }

    if (hasChild(_ScaffoldSlot.bottomSheet)) {
      final BoxConstraints bottomSheetConstraints = BoxConstraints(
        maxWidth: fullWidthConstraints.maxWidth,
        maxHeight: math.max(0.0, contentBottom - contentTop),
      );
      bottomSheetSize = layoutChild(_ScaffoldSlot.bottomSheet, bottomSheetConstraints);
      positionChild(_ScaffoldSlot.bottomSheet, Offset((size.width - bottomSheetSize.width) / 2.0, contentBottom - bottomSheetSize.height));
    }

    Rect floatingActionButtonRect;
    if (hasChild(_ScaffoldSlot.floatingActionButton)) {
      final Size fabSize = layoutChild(_ScaffoldSlot.floatingActionButton, looseConstraints);

      // To account for the FAB position being changed, we'll animate between
      // the old and new positions.
      final ScaffoldPrelayoutGeometry currentGeometry = ScaffoldPrelayoutGeometry(
        bottomSheetSize: bottomSheetSize,
        contentBottom: contentBottom,
        contentTop: contentTop,
        floatingActionButtonSize: fabSize,
        minInsets: minInsets,
        scaffoldSize: size,
        snackBarSize: snackBarSize,
        textDirection: textDirection,
      );
      final Offset currentFabOffset = currentFloatingActionButtonLocation.getOffset(currentGeometry);
      final Offset previousFabOffset = previousFloatingActionButtonLocation.getOffset(currentGeometry);
      final Offset fabOffset = floatingActionButtonMotionAnimator.getOffset(
        begin: previousFabOffset,
        end: currentFabOffset,
        progress: floatingActionButtonMoveAnimationProgress,
      );
      positionChild(_ScaffoldSlot.floatingActionButton, fabOffset);
      floatingActionButtonRect = fabOffset & fabSize;
    }

    if (hasChild(_ScaffoldSlot.snackBar)) {
      if (snackBarSize == Size.zero) {
        snackBarSize = layoutChild(_ScaffoldSlot.snackBar, fullWidthConstraints);
      }
      final double snackBarYOffsetBase = floatingActionButtonRect != null && isSnackBarFloating
        ? floatingActionButtonRect.top
        : contentBottom;
      positionChild(_ScaffoldSlot.snackBar, Offset(0.0, snackBarYOffsetBase - snackBarSize.height));
    }

    if (hasChild(_ScaffoldSlot.statusBar)) {
      layoutChild(_ScaffoldSlot.statusBar, fullWidthConstraints.tighten(height: minInsets.top));
      positionChild(_ScaffoldSlot.statusBar, Offset.zero);
    }

    if (hasChild(_ScaffoldSlot.drawer)) {
      layoutChild(_ScaffoldSlot.drawer, BoxConstraints.tight(size));
      positionChild(_ScaffoldSlot.drawer, Offset.zero);
    }

    if (hasChild(_ScaffoldSlot.endDrawer)) {
      layoutChild(_ScaffoldSlot.endDrawer, BoxConstraints.tight(size));
      positionChild(_ScaffoldSlot.endDrawer, Offset.zero);
    }

    geometryNotifier._updateWith(
      bottomNavigationBarTop: bottomNavigationBarTop,
      floatingActionButtonArea: floatingActionButtonRect,
    );
  }

  @override
  bool shouldRelayout(_ScaffoldLayout oldDelegate) {
    return oldDelegate.minInsets != minInsets
        || oldDelegate.textDirection != textDirection
        || oldDelegate.floatingActionButtonMoveAnimationProgress != floatingActionButtonMoveAnimationProgress
        || oldDelegate.previousFloatingActionButtonLocation != previousFloatingActionButtonLocation
        || oldDelegate.currentFloatingActionButtonLocation != currentFloatingActionButtonLocation
        || oldDelegate.extendBody != extendBody
        || oldDelegate.extendBodyBehindAppBar != extendBodyBehindAppBar;
  }
}

/// Handler for scale and rotation animations in the [FloatingActionButton].
///
/// Currently, there are two types of [FloatingActionButton] animations:
///
/// * Entrance/Exit animations, which this widget triggers
///   when the [FloatingActionButton] is added, updated, or removed.
/// * Motion animations, which are triggered by the [Scaffold]
///   when its [FloatingActionButtonLocation] is updated.
class _FloatingActionButtonTransition extends StatefulWidget {
  const _FloatingActionButtonTransition({
    Key key,
    @required this.child,
    @required this.fabMoveAnimation,
    @required this.fabMotionAnimator,
    @required this.geometryNotifier,
    @required this.currentController,
  }) : assert(fabMoveAnimation != null),
       assert(fabMotionAnimator != null),
       assert(currentController != null),
       super(key: key);

  final Widget child;
  final Animation<double> fabMoveAnimation;
  final FloatingActionButtonAnimator fabMotionAnimator;
  final _ScaffoldGeometryNotifier geometryNotifier;

  /// Controls the current child widget.child as it exits.
  final AnimationController currentController;

  @override
  _FloatingActionButtonTransitionState createState() => _FloatingActionButtonTransitionState();
}

class _FloatingActionButtonTransitionState extends State<_FloatingActionButtonTransition> with TickerProviderStateMixin {
  // The animations applied to the Floating Action Button when it is entering or exiting.
  // Controls the previous widget.child as it exits.
  AnimationController _previousController;
  Animation<double> _previousScaleAnimation;
  Animation<double> _previousRotationAnimation;
  // The animations to run, considering the widget's fabMoveAnimation and the current/previous entrance/exit animations.
  Animation<double> _currentScaleAnimation;
  Animation<double> _extendedCurrentScaleAnimation;
  Animation<double> _currentRotationAnimation;
  Widget _previousChild;

  @override
  void initState() {
    super.initState();

    _previousController = AnimationController(
      duration: kFloatingActionButtonSegue,
      vsync: this,
    )..addStatusListener(_handlePreviousAnimationStatusChanged);
    _updateAnimations();

    if (widget.child != null) {
      // If we start out with a child, have the child appear fully visible instead
      // of animating in.
      widget.currentController.value = 1.0;
    } else {
      // If we start without a child we update the geometry object with a
      // floating action button scale of 0, as it is not showing on the screen.
      _updateGeometryScale(0.0);
    }
  }

  @override
  void dispose() {
    _previousController.dispose();
    super.dispose();
  }

  @override
  void didUpdateWidget(_FloatingActionButtonTransition oldWidget) {
    super.didUpdateWidget(oldWidget);
    final bool oldChildIsNull = oldWidget.child == null;
    final bool newChildIsNull = widget.child == null;
    if (oldChildIsNull == newChildIsNull && oldWidget.child?.key == widget.child?.key)
      return;
    if (oldWidget.fabMotionAnimator != widget.fabMotionAnimator || oldWidget.fabMoveAnimation != widget.fabMoveAnimation) {
      // Get the right scale and rotation animations to use for this widget.
      _updateAnimations();
    }
    if (_previousController.status == AnimationStatus.dismissed) {
      final double currentValue = widget.currentController.value;
      if (currentValue == 0.0 || oldWidget.child == null) {
        // The current child hasn't started its entrance animation yet. We can
        // just skip directly to the new child's entrance.
        _previousChild = null;
        if (widget.child != null)
          widget.currentController.forward();
      } else {
        // Otherwise, we need to copy the state from the current controller to
        // the previous controller and run an exit animation for the previous
        // widget before running the entrance animation for the new child.
        _previousChild = oldWidget.child;
        _previousController
          ..value = currentValue
          ..reverse();
        widget.currentController.value = 0.0;
      }
    }
  }

  static final Animatable<double> _entranceTurnTween = Tween<double>(
    begin: 1.0 - kFloatingActionButtonTurnInterval,
    end: 1.0,
  ).chain(CurveTween(curve: Curves.easeIn));

  void _updateAnimations() {
    // Get the animations for exit and entrance.
    final CurvedAnimation previousExitScaleAnimation = CurvedAnimation(
      parent: _previousController,
      curve: Curves.easeIn,
    );
    final Animation<double> previousExitRotationAnimation = Tween<double>(begin: 1.0, end: 1.0).animate(
      CurvedAnimation(
        parent: _previousController,
        curve: Curves.easeIn,
      ),
    );

    final CurvedAnimation currentEntranceScaleAnimation = CurvedAnimation(
      parent: widget.currentController,
      curve: Curves.easeIn,
    );
    final Animation<double> currentEntranceRotationAnimation = widget.currentController.drive(_entranceTurnTween);

    // Get the animations for when the FAB is moving.
    final Animation<double> moveScaleAnimation = widget.fabMotionAnimator.getScaleAnimation(parent: widget.fabMoveAnimation);
    final Animation<double> moveRotationAnimation = widget.fabMotionAnimator.getRotationAnimation(parent: widget.fabMoveAnimation);

    // Aggregate the animations.
    _previousScaleAnimation = AnimationMin<double>(moveScaleAnimation, previousExitScaleAnimation);
    _currentScaleAnimation = AnimationMin<double>(moveScaleAnimation, currentEntranceScaleAnimation);
    _extendedCurrentScaleAnimation = _currentScaleAnimation.drive(CurveTween(curve: const Interval(0.0, 0.1)));

    _previousRotationAnimation = TrainHoppingAnimation(previousExitRotationAnimation, moveRotationAnimation);
    _currentRotationAnimation = TrainHoppingAnimation(currentEntranceRotationAnimation, moveRotationAnimation);

    _currentScaleAnimation.addListener(_onProgressChanged);
    _previousScaleAnimation.addListener(_onProgressChanged);
  }

  void _handlePreviousAnimationStatusChanged(AnimationStatus status) {
    setState(() {
      if (status == AnimationStatus.dismissed) {
        assert(widget.currentController.status == AnimationStatus.dismissed);
        if (widget.child != null)
          widget.currentController.forward();
      }
    });
  }

  bool _isExtendedFloatingActionButton(Widget widget) {
    return widget is FloatingActionButton
        && widget.isExtended;
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      alignment: Alignment.centerRight,
      children: <Widget>[
        if (_previousController.status != AnimationStatus.dismissed)
          if (_isExtendedFloatingActionButton(_previousChild))
            FadeTransition(
              opacity: _previousScaleAnimation,
              child: _previousChild,
            )
          else
            ScaleTransition(
              scale: _previousScaleAnimation,
              child: RotationTransition(
                turns: _previousRotationAnimation,
                child: _previousChild,
              ),
            ),
        if (_isExtendedFloatingActionButton(widget.child))
          ScaleTransition(
            scale: _extendedCurrentScaleAnimation,
            child: FadeTransition(
              opacity: _currentScaleAnimation,
              child: widget.child,
            ),
          )
        else
          ScaleTransition(
            scale: _currentScaleAnimation,
            child: RotationTransition(
              turns: _currentRotationAnimation,
              child: widget.child,
            ),
          ),
      ],
    );
  }

  void _onProgressChanged() {
    _updateGeometryScale(math.max(_previousScaleAnimation.value, _currentScaleAnimation.value));
  }

  void _updateGeometryScale(double scale) {
    widget.geometryNotifier._updateWith(
      floatingActionButtonScale: scale,
    );
  }
}

/// Implements the basic material design visual layout structure.
///
/// This class provides APIs for showing drawers, snack bars, and bottom sheets.
///
/// To display a snackbar or a persistent bottom sheet, obtain the
/// [ScaffoldState] for the current [BuildContext] via [Scaffold.of] and use the
/// [ScaffoldState.showSnackBar] and [ScaffoldState.showBottomSheet] functions.
///
/// {@tool sample --template=stateful_widget_material}
/// This example shows a [Scaffold] with a [body] and [FloatingActionButton].
/// The [body] is a [Text] placed in a [Center] in order to center the text
/// within the [Scaffold]. The [FloatingActionButton] is connected to a
/// callback that increments a counter.
///
/// ![The Scaffold has a white background with a blue AppBar at the top. A blue FloatingActionButton is positioned at the bottom right corner of the Scaffold.](https://flutter.github.io/assets-for-api-docs/assets/material/scaffold.png)
///
/// ```dart
/// int _count = 0;
///
/// Widget build(BuildContext context) {
///   return Scaffold(
///     appBar: AppBar(
///       title: const Text('Sample Code'),
///     ),
///     body: Center(
///       child: Text('You have pressed the button $_count times.')
///     ),
///     floatingActionButton: FloatingActionButton(
///       onPressed: () => setState(() => _count++),
///       tooltip: 'Increment Counter',
///       child: const Icon(Icons.add),
///     ),
///   );
/// }
/// ```
/// {@end-tool}
///
/// {@tool sample --template=stateful_widget_material}
/// This example shows a [Scaffold] with a blueGrey [backgroundColor], [body]
/// and [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in
/// order to center the text within the [Scaffold]. The [FloatingActionButton]
/// is connected to a callback that increments a counter.
///
/// ![](https://flutter.github.io/assets-for-api-docs/assets/material/scaffold_background_color.png)
///
/// ```dart
/// int _count = 0;
///
/// Widget build(BuildContext context) {
///   return Scaffold(
///     appBar: AppBar(
///       title: const Text('Sample Code'),
///     ),
///     body: Center(
///       child: Text('You have pressed the button $_count times.')
///     ),
///     backgroundColor: Colors.blueGrey.shade200,
///     floatingActionButton: FloatingActionButton(
///       onPressed: () => setState(() => _count++),
///       tooltip: 'Increment Counter',
///       child: const Icon(Icons.add),
///     ),
///   );
/// }
/// ```
/// {@end-tool}
///
/// {@tool sample --template=stateful_widget_material}
/// This example shows a [Scaffold] with an [AppBar], a [BottomAppBar] and a
/// [FloatingActionButton]. The [body] is a [Text] placed in a [Center] in order
/// to center the text within the [Scaffold]. The [FloatingActionButton] is
/// centered and docked within the [BottomAppBar] using
/// [FloatingActionButtonLocation.centerDocked]. The [FloatingActionButton] is
/// connected to a callback that increments a counter.
///
/// ![](https://flutter.github.io/assets-for-api-docs/assets/material/scaffold_bottom_app_bar.png)
///
/// ```dart
/// int _count = 0;
///
/// Widget build(BuildContext context) {
///   return Scaffold(
///     appBar: AppBar(
///       title: Text('Sample Code'),
///     ),
///     body: Center(
///       child: Text('You have pressed the button $_count times.'),
///     ),
///     bottomNavigationBar: BottomAppBar(
///       shape: const CircularNotchedRectangle(),
///       child: Container(height: 50.0,),
///     ),
///     floatingActionButton: FloatingActionButton(
///       onPressed: () => setState(() {
///         _count++;
///       }),
///       tooltip: 'Increment Counter',
///       child: Icon(Icons.add),
///     ),
///     floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
///   );
/// }
/// ```
/// {@end-tool}
///
/// ## Scaffold layout, the keyboard, and display "notches"
///
/// The scaffold will expand to fill the available space. That usually
/// means that it will occupy its entire window or device screen. When
/// the device's keyboard appears the Scaffold's ancestor [MediaQuery]
/// widget's [MediaQueryData.viewInsets] changes and the Scaffold will
/// be rebuilt. By default the scaffold's [body] is resized to make
/// room for the keyboard. To prevent the resize set
/// [resizeToAvoidBottomInset] to false. In either case the focused
/// widget will be scrolled into view if it's within a scrollable
/// container.
///
/// The [MediaQueryData.padding] value defines areas that might
/// not be completely visible, like the display "notch" on the iPhone
/// X. The scaffold's [body] is not inset by this padding value
/// although an [appBar] or [bottomNavigationBar] will typically
/// cause the body to avoid the padding. The [SafeArea]
/// widget can be used within the scaffold's body to avoid areas
/// like display notches.
///
/// ## Troubleshooting
///
/// ### Nested Scaffolds
///
/// The Scaffold was designed to be the single top level container for
/// a [MaterialApp] and it's typically not necessary to nest
/// scaffolds. For example in a tabbed UI, where the
/// [bottomNavigationBar] is a [TabBar] and the body is a
/// [TabBarView], you might be tempted to make each tab bar view a
/// scaffold with a differently titled AppBar. It would be better to add a
/// listener to the [TabController] that updates the AppBar.
///
/// {@tool snippet}
/// Add a listener to the app's tab controller so that the [AppBar] title of the
/// app's one and only scaffold is reset each time a new tab is selected.
///
/// ```dart
/// TabController(vsync: tickerProvider, length: tabCount)..addListener(() {
///   if (!tabController.indexIsChanging) {
///     setState(() {
///       // Rebuild the enclosing scaffold with a new AppBar title
///       appBarTitle = 'Tab ${tabController.index}';
///     });
///   }
/// })
/// ```
/// {@end-tool}
///
/// Although there are some use cases, like a presentation app that
/// shows embedded flutter content, where nested scaffolds are
/// appropriate, it's best to avoid nesting scaffolds.
///
/// See also:
///
///  * [AppBar], which is a horizontal bar typically shown at the top of an app
///    using the [appBar] property.
///  * [BottomAppBar], which is a horizontal bar typically shown at the bottom
///    of an app using the [bottomNavigationBar] property.
///  * [FloatingActionButton], which is a circular button typically shown in the
///    bottom right corner of the app using the [floatingActionButton] property.
///  * [Drawer], which is a vertical panel that is typically displayed to the
///    left of the body (and often hidden on phones) using the [drawer]
///    property.
///  * [BottomNavigationBar], which is a horizontal array of buttons typically
///    shown along the bottom of the app using the [bottomNavigationBar]
///    property.
///  * [SnackBar], which is a temporary notification typically shown near the
///    bottom of the app using the [ScaffoldState.showSnackBar] method.
///  * [BottomSheet], which is an overlay typically shown near the bottom of the
///    app. A bottom sheet can either be persistent, in which case it is shown
///    using the [ScaffoldState.showBottomSheet] method, or modal, in which case
///    it is shown using the [showModalBottomSheet] function.
///  * [ScaffoldState], which is the state associated with this widget.
///  * <https://material.io/design/layout/responsive-layout-grid.html>
class Scaffold extends StatefulWidget {
  /// Creates a visual scaffold for material design widgets.
  const Scaffold({
    Key key,
    this.appBar,
    this.body,
    this.floatingActionButton,
    this.floatingActionButtonLocation,
    this.floatingActionButtonAnimator,
    this.persistentFooterButtons,
    this.drawer,
    this.endDrawer,
    this.bottomNavigationBar,
    this.bottomSheet,
    this.backgroundColor,
    this.resizeToAvoidBottomPadding,
    this.resizeToAvoidBottomInset,
    this.primary = true,
    this.drawerDragStartBehavior = DragStartBehavior.start,
    this.extendBody = false,
    this.extendBodyBehindAppBar = false,
    this.drawerScrimColor,
    this.drawerEdgeDragWidth,
  }) : assert(primary != null),
       assert(extendBody != null),
       assert(extendBodyBehindAppBar != null),
       assert(drawerDragStartBehavior != null),
       super(key: key);

  /// If true, and [bottomNavigationBar] or [persistentFooterButtons]
  /// is specified, then the [body] extends to the bottom of the Scaffold,
  /// instead of only extending to the top of the [bottomNavigationBar]
  /// or the [persistentFooterButtons].
  ///
  /// If true, a [MediaQuery] widget whose bottom padding matches the
  /// the height of the [bottomNavigationBar] will be added above the
  /// scaffold's [body].
  ///
  /// This property is often useful when the [bottomNavigationBar] has
  /// a non-rectangular shape, like [CircularNotchedRectangle], which
  /// adds a [FloatingActionButton] sized notch to the top edge of the bar.
  /// In this case specifying `extendBody: true` ensures that that scaffold's
  /// body will be visible through the bottom navigation bar's notch.
  ///
  /// See also:
  ///
  ///  * [extendBodyBehindAppBar], which extends the height of the body
  ///    to the top of the scaffold.
  final bool extendBody;

  /// If true, and an [appBar] is specified, then the height of the [body] is
  /// extended to include the height of the app bar and the top of the body
  /// is aligned with the top of the app bar.
  ///
  /// This is useful if the app bar's [AppBar.backgroundColor] is not
  /// completely opaque.
  ///
  /// This property is false by default. It must not be null.
  ///
  /// See also:
  ///
  ///  * [extendBody], which extends the height of the body to the bottom
  ///    of the scaffold.
  final bool extendBodyBehindAppBar;

  /// An app bar to display at the top of the scaffold.
  final PreferredSizeWidget appBar;

  /// The primary content of the scaffold.
  ///
  /// Displayed below the [appBar], above the bottom of the ambient
  /// [MediaQuery]'s [MediaQueryData.viewInsets], and behind the
  /// [floatingActionButton] and [drawer]. If [resizeToAvoidBottomInset] is
  /// false then the body is not resized when the onscreen keyboard appears,
  /// i.e. it is not inset by `viewInsets.bottom`.
  ///
  /// The widget in the body of the scaffold is positioned at the top-left of
  /// the available space between the app bar and the bottom of the scaffold. To
  /// center this widget instead, consider putting it in a [Center] widget and
  /// having that be the body. To expand this widget instead, consider
  /// putting it in a [SizedBox.expand].
  ///
  /// If you have a column of widgets that should normally fit on the screen,
  /// but may overflow and would in such cases need to scroll, consider using a
  /// [ListView] as the body of the scaffold. This is also a good choice for
  /// the case where your body is a scrollable list.
  final Widget body;

  /// A button displayed floating above [body], in the bottom right corner.
  ///
  /// Typically a [FloatingActionButton].
  final Widget floatingActionButton;

  /// Responsible for determining where the [floatingActionButton] should go.
  ///
  /// If null, the [ScaffoldState] will use the default location, [FloatingActionButtonLocation.endFloat].
  final FloatingActionButtonLocation floatingActionButtonLocation;

  /// Animator to move the [floatingActionButton] to a new [floatingActionButtonLocation].
  ///
  /// If null, the [ScaffoldState] will use the default animator, [FloatingActionButtonAnimator.scaling].
  final FloatingActionButtonAnimator floatingActionButtonAnimator;

  /// A set of buttons that are displayed at the bottom of the scaffold.
  ///
  /// Typically this is a list of [FlatButton] widgets. These buttons are
  /// persistently visible, even if the [body] of the scaffold scrolls.
  ///
  /// These widgets will be wrapped in a [ButtonBar].
  ///
  /// The [persistentFooterButtons] are rendered above the
  /// [bottomNavigationBar] but below the [body].
  final List<Widget> persistentFooterButtons;

  /// A panel displayed to the side of the [body], often hidden on mobile
  /// devices. Swipes in from either left-to-right ([TextDirection.ltr]) or
  /// right-to-left ([TextDirection.rtl])
  ///
  /// Typically a [Drawer].
  ///
  /// To open the drawer programmatically, use the [ScaffoldState.openDrawer]
  /// function.
  ///
  /// {@tool sample --template=stateful_widget_material}
  /// To disable the drawer edge swipe, set the [Scaffold.drawerEdgeWidth]
  /// to 0. Then, use [ScaffoldState.openDrawer] to open the drawer.
  ///
  /// ```dart
  /// final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
  ///
  /// void _openDrawer() {
  ///   _scaffoldKey.currentState.openDrawer();
  /// }
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return Scaffold(
  ///     key: _scaffoldKey,
  ///     appBar: AppBar(title: Text('Drawer Demo')),
  ///     body: Center(
  ///       child: RaisedButton(
  ///         onPressed: _openDrawer,
  ///         child: Text('Open Drawer'),
  ///       ),
  ///     ),
  ///     drawer: Drawer(
  ///       child: Center(child: Text('This is the Drawer')),
  ///     ),
  ///     drawerEdgeDragWidth: 0.0, // Disable opening the drawer with a swipe gesture.
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  final Widget drawer;

  /// A panel displayed to the side of the [body], often hidden on mobile
  /// devices. Swipes in from right-to-left ([TextDirection.ltr]) or
  /// left-to-right ([TextDirection.rtl])
  ///
  /// Typically a [Drawer].
  ///
  /// To open the drawer programmatically, use the [ScaffoldState.openEndDrawer]
  /// function.
  ///
  /// {@tool sample --template=stateful_widget_material}
  /// To disable the drawer edge swipe, set the [Scaffold.drawerEdgeWidth]
  /// to 0. Then, use [ScaffoldState.openEndDrawer] to open the drawer.
  ///
  /// ```dart
  /// final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
  ///
  /// void _openEndDrawer() {
  ///   _scaffoldKey.currentState.openEndDrawer();
  /// }
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return Scaffold(
  ///     key: _scaffoldKey,
  ///     appBar: AppBar(title: Text('Drawer Demo')),
  ///     body: Center(
  ///       child: RaisedButton(
  ///         onPressed: _openEndDrawer,
  ///         child: Text('Open End Drawer'),
  ///       ),
  ///     ),
  ///     endDrawer: Drawer(
  ///       child: Center(child: Text('This is the Drawer')),
  ///     ),
  ///     drawerEdgeDragWidth: 0.0, // Disable opening the drawer with a swipe gesture.
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  final Widget endDrawer;

  /// The color to use for the scrim that obscures primary content while a drawer is open.
  ///
  /// By default, the color is [Colors.black54]
  final Color drawerScrimColor;

  /// The color of the [Material] widget that underlies the entire Scaffold.
  ///
  /// The theme's [ThemeData.scaffoldBackgroundColor] by default.
  final Color backgroundColor;

  /// A bottom navigation bar to display at the bottom of the scaffold.
  ///
  /// Snack bars slide from underneath the bottom navigation bar while bottom
  /// sheets are stacked on top.
  ///
  /// The [bottomNavigationBar] is rendered below the [persistentFooterButtons]
  /// and the [body].
  final Widget bottomNavigationBar;

  /// The persistent bottom sheet to display.
  ///
  /// A persistent bottom sheet shows information that supplements the primary
  /// content of the app. A persistent bottom sheet remains visible even when
  /// the user interacts with other parts of the app.
  ///
  /// A closely related widget is a modal bottom sheet, which is an alternative
  /// to a menu or a dialog and prevents the user from interacting with the rest
  /// of the app. Modal bottom sheets can be created and displayed with the
  /// [showModalBottomSheet] function.
  ///
  /// Unlike the persistent bottom sheet displayed by [showBottomSheet]
  /// this bottom sheet is not a [LocalHistoryEntry] and cannot be dismissed
  /// with the scaffold appbar's back button.
  ///
  /// If a persistent bottom sheet created with [showBottomSheet] is already
  /// visible, it must be closed before building the Scaffold with a new
  /// [bottomSheet].
  ///
  /// The value of [bottomSheet] can be any widget at all. It's unlikely to
  /// actually be a [BottomSheet], which is used by the implementations of
  /// [showBottomSheet] and [showModalBottomSheet]. Typically it's a widget
  /// that includes [Material].
  ///
  /// See also:
  ///
  ///  * [showBottomSheet], which displays a bottom sheet as a route that can
  ///    be dismissed with the scaffold's back button.
  ///  * [showModalBottomSheet], which displays a modal bottom sheet.
  final Widget bottomSheet;

  /// This flag is deprecated, please use [resizeToAvoidBottomInset]
  /// instead.
  ///
  /// Originally the name referred [MediaQueryData.padding]. Now it refers
  /// [MediaQueryData.viewInsets], so using [resizeToAvoidBottomInset]
  /// should be clearer to readers.
  @Deprecated(
    'Use resizeToAvoidBottomInset to specify if the body should resize when the keyboard appears. '
    'This feature was deprecated after v1.1.9.'
  )
  final bool resizeToAvoidBottomPadding;

  /// If true the [body] and the scaffold's floating widgets should size
  /// themselves to avoid the onscreen keyboard whose height is defined by the
  /// ambient [MediaQuery]'s [MediaQueryData.viewInsets] `bottom` property.
  ///
  /// For example, if there is an onscreen keyboard displayed above the
  /// scaffold, the body can be resized to avoid overlapping the keyboard, which
  /// prevents widgets inside the body from being obscured by the keyboard.
  ///
  /// Defaults to true.
  final bool resizeToAvoidBottomInset;

  /// Whether this scaffold is being displayed at the top of the screen.
  ///
  /// If true then the height of the [appBar] will be extended by the height
  /// of the screen's status bar, i.e. the top padding for [MediaQuery].
  ///
  /// The default value of this property, like the default value of
  /// [AppBar.primary], is true.
  final bool primary;

  /// {@macro flutter.material.drawer.dragStartBehavior}
  final DragStartBehavior drawerDragStartBehavior;

  /// The width of the area within which a horizontal swipe will open the
  /// drawer.
  ///
  /// By default, the value used is 20.0 added to the padding edge of
  /// `MediaQuery.of(context).padding` that corresponds to [alignment].
  /// This ensures that the drag area for notched devices is not obscured. For
  /// example, if `TextDirection.of(context)` is set to [TextDirection.ltr],
  /// 20.0 will be added to `MediaQuery.of(context).padding.left`.
  final double drawerEdgeDragWidth;

  /// The state from the closest instance of this class that encloses the given context.
  ///
  /// {@tool sample --template=freeform}
  /// Typical usage of the [Scaffold.of] function is to call it from within the
  /// `build` method of a child of a [Scaffold].
  ///
  /// ```dart imports
  /// import 'package:flutter/material.dart';
  /// ```
  ///
  /// ```dart main
  /// void main() => runApp(MyApp());
  /// ```
  ///
  /// ```dart preamble
  /// class MyApp extends StatelessWidget {
  ///   // This widget is the root of your application.
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     return MaterialApp(
  ///       title: 'Flutter Code Sample for Scaffold.of.',
  ///       theme: ThemeData(
  ///         primarySwatch: Colors.blue,
  ///       ),
  ///       home: Scaffold(
  ///         body: MyScaffoldBody(),
  ///         appBar: AppBar(title: Text('Scaffold.of Example')),
  ///       ),
  ///       color: Colors.white,
  ///     );
  ///   }
  /// }
  /// ```
  ///
  /// ```dart
  /// class MyScaffoldBody extends StatelessWidget {
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     return Center(
  ///       child: RaisedButton(
  ///         child: Text('SHOW A SNACKBAR'),
  ///         onPressed: () {
  ///           Scaffold.of(context).showSnackBar(
  ///             SnackBar(
  ///               content: Text('Have a snack!'),
  ///             ),
  ///           );
  ///         },
  ///       ),
  ///     );
  ///   }
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// {@tool sample --template=stateless_widget_material}
  /// When the [Scaffold] is actually created in the same `build` function, the
  /// `context` argument to the `build` function can't be used to find the
  /// [Scaffold] (since it's "above" the widget being returned in the widget
  /// tree). In such cases, the following technique with a [Builder] can be used
  /// to provide a new scope with a [BuildContext] that is "under" the
  /// [Scaffold]:
  ///
  /// ```dart
  /// Widget build(BuildContext context) {
  ///   return Scaffold(
  ///     appBar: AppBar(
  ///       title: Text('Demo')
  ///     ),
  ///     body: Builder(
  ///       // Create an inner BuildContext so that the onPressed methods
  ///       // can refer to the Scaffold with Scaffold.of().
  ///       builder: (BuildContext context) {
  ///         return Center(
  ///           child: RaisedButton(
  ///             child: Text('SHOW A SNACKBAR'),
  ///             onPressed: () {
  ///               Scaffold.of(context).showSnackBar(SnackBar(
  ///                 content: Text('Have a snack!'),
  ///               ));
  ///             },
  ///           ),
  ///         );
  ///       },
  ///     ),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// A more efficient solution is to split your build function into several
  /// widgets. This introduces a new context from which you can obtain the
  /// [Scaffold]. In this solution, you would have an outer widget that creates
  /// the [Scaffold] populated by instances of your new inner widgets, and then
  /// in these inner widgets you would use [Scaffold.of].
  ///
  /// A less elegant but more expedient solution is assign a [GlobalKey] to the
  /// [Scaffold], then use the `key.currentState` property to obtain the
  /// [ScaffoldState] rather than using the [Scaffold.of] function.
  ///
  /// If there is no [Scaffold] in scope, then this will throw an exception.
  /// To return null if there is no [Scaffold], then pass `nullOk: true`.
  static ScaffoldState of(BuildContext context, { bool nullOk = false }) {
    assert(nullOk != null);
    assert(context != null);
    final ScaffoldState result = context.findAncestorStateOfType<ScaffoldState>();
    if (nullOk || result != null)
      return result;
    throw FlutterError.fromParts(<DiagnosticsNode>[
      ErrorSummary(
        'Scaffold.of() called with a context that does not contain a Scaffold.'
      ),
      ErrorDescription(
        'No Scaffold ancestor could be found starting from the context that was passed to Scaffold.of(). '
        'This usually happens when the context provided is from the same StatefulWidget as that '
        'whose build function actually creates the Scaffold widget being sought.'
      ),
      ErrorHint(
        'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
        'context that is "under" the Scaffold. For an example of this, please see the '
        'documentation for Scaffold.of():\n'
        '  https://api.flutter.dev/flutter/material/Scaffold/of.html'
      ),
      ErrorHint(
        'A more efficient solution is to split your build function into several widgets. This '
        'introduces a new context from which you can obtain the Scaffold. In this solution, '
        'you would have an outer widget that creates the Scaffold populated by instances of '
        'your new inner widgets, and then in these inner widgets you would use Scaffold.of().\n'
        'A less elegant but more expedient solution is assign a GlobalKey to the Scaffold, '
        'then use the key.currentState property to obtain the ScaffoldState rather than '
        'using the Scaffold.of() function.'
      ),
      context.describeElement('The context used was')
    ]);
  }

  /// Returns a [ValueListenable] for the [ScaffoldGeometry] for the closest
  /// [Scaffold] ancestor of the given context.
  ///
  /// The [ValueListenable.value] is only available at paint time.
  ///
  /// Notifications are guaranteed to be sent before the first paint pass
  /// with the new geometry, but there is no guarantee whether a build or
  /// layout passes are going to happen between the notification and the next
  /// paint pass.
  ///
  /// The closest [Scaffold] ancestor for the context might change, e.g when
  /// an element is moved from one scaffold to another. For [StatefulWidget]s
  /// using this listenable, a change of the [Scaffold] ancestor will
  /// trigger a [State.didChangeDependencies].
  ///
  /// A typical pattern for listening to the scaffold geometry would be to
  /// call [Scaffold.geometryOf] in [State.didChangeDependencies], compare the
  /// return value with the previous listenable, if it has changed, unregister
  /// the listener, and register a listener to the new [ScaffoldGeometry]
  /// listenable.
  static ValueListenable<ScaffoldGeometry> geometryOf(BuildContext context) {
    final _ScaffoldScope scaffoldScope = context.dependOnInheritedWidgetOfExactType<_ScaffoldScope>();
    if (scaffoldScope == null)
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary(
          'Scaffold.geometryOf() called with a context that does not contain a Scaffold.'
        ),
        ErrorDescription(
          'This usually happens when the context provided is from the same StatefulWidget as that '
          'whose build function actually creates the Scaffold widget being sought.'
        ),
        ErrorHint(
          'There are several ways to avoid this problem. The simplest is to use a Builder to get a '
          'context that is "under" the Scaffold. For an example of this, please see the '
          'documentation for Scaffold.of():\n'
          '  https://api.flutter.dev/flutter/material/Scaffold/of.html'
        ),
        ErrorHint(
          'A more efficient solution is to split your build function into several widgets. This '
          'introduces a new context from which you can obtain the Scaffold. In this solution, '
          'you would have an outer widget that creates the Scaffold populated by instances of '
          'your new inner widgets, and then in these inner widgets you would use Scaffold.geometryOf().',
        ),
        context.describeElement('The context used was')
      ]);
    return scaffoldScope.geometryNotifier;
  }

  /// Whether the Scaffold that most tightly encloses the given context has a
  /// drawer.
  ///
  /// If this is being used during a build (for example to decide whether to
  /// show an "open drawer" button), set the `registerForUpdates` argument to
  /// true. This will then set up an [InheritedWidget] relationship with the
  /// [Scaffold] so that the client widget gets rebuilt whenever the [hasDrawer]
  /// value changes.
  ///
  /// See also:
  ///
  ///  * [Scaffold.of], which provides access to the [ScaffoldState] object as a
  ///    whole, from which you can show snackbars, bottom sheets, and so forth.
  static bool hasDrawer(BuildContext context, { bool registerForUpdates = true }) {
    assert(registerForUpdates != null);
    assert(context != null);
    if (registerForUpdates) {
      final _ScaffoldScope scaffold = context.dependOnInheritedWidgetOfExactType<_ScaffoldScope>();
      return scaffold?.hasDrawer ?? false;
    } else {
      final ScaffoldState scaffold = context.findAncestorStateOfType<ScaffoldState>();
      return scaffold?.hasDrawer ?? false;
    }
  }

  @override
  ScaffoldState createState() => ScaffoldState();
}

/// State for a [Scaffold].
///
/// Can display [SnackBar]s and [BottomSheet]s. Retrieve a [ScaffoldState] from
/// the current [BuildContext] using [Scaffold.of].
class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {

  // DRAWER API

  final GlobalKey<DrawerControllerState> _drawerKey = GlobalKey<DrawerControllerState>();
  final GlobalKey<DrawerControllerState> _endDrawerKey = GlobalKey<DrawerControllerState>();

  /// Whether this scaffold has a non-null [Scaffold.appBar].
  bool get hasAppBar => widget.appBar != null;
  /// Whether this scaffold has a non-null [Scaffold.drawer].
  bool get hasDrawer => widget.drawer != null;
  /// Whether this scaffold has a non-null [Scaffold.endDrawer].
  bool get hasEndDrawer => widget.endDrawer != null;
  /// Whether this scaffold has a non-null [Scaffold.floatingActionButton].
  bool get hasFloatingActionButton => widget.floatingActionButton != null;

  double _appBarMaxHeight;
  /// The max height the [Scaffold.appBar] uses.
  ///
  /// This is based on the appBar preferred height plus the top padding.
  double get appBarMaxHeight => _appBarMaxHeight;
  bool _drawerOpened = false;
  bool _endDrawerOpened = false;

  /// Whether the [Scaffold.drawer] is opened.
  ///
  /// See also:
  ///
  ///  * [ScaffoldState.openDrawer], which opens the [Scaffold.drawer] of a
  ///    [Scaffold].
  bool get isDrawerOpen => _drawerOpened;

  /// Whether the [Scaffold.endDrawer] is opened.
  ///
  /// See also:
  ///
  ///  * [ScaffoldState.openEndDrawer], which opens the [Scaffold.endDrawer] of
  ///    a [Scaffold].
  bool get isEndDrawerOpen => _endDrawerOpened;

  void _drawerOpenedCallback(bool isOpened) {
    setState(() {
      _drawerOpened = isOpened;
    });
  }

  void _endDrawerOpenedCallback(bool isOpened) {
    setState(() {
      _endDrawerOpened = isOpened;
    });
  }

  /// Opens the [Drawer] (if any).
  ///
  /// If the scaffold has a non-null [Scaffold.drawer], this function will cause
  /// the drawer to begin its entrance animation.
  ///
  /// Normally this is not needed since the [Scaffold] automatically shows an
  /// appropriate [IconButton], and handles the edge-swipe gesture, to show the
  /// drawer.
  ///
  /// To close the drawer once it is open, use [Navigator.pop].
  ///
  /// See [Scaffold.of] for information about how to obtain the [ScaffoldState].
  void openDrawer() {
    if (_endDrawerKey.currentState != null && _endDrawerOpened)
      _endDrawerKey.currentState.close();
    _drawerKey.currentState?.open();
  }

  /// Opens the end side [Drawer] (if any).
  ///
  /// If the scaffold has a non-null [Scaffold.endDrawer], this function will cause
  /// the end side drawer to begin its entrance animation.
  ///
  /// Normally this is not needed since the [Scaffold] automatically shows an
  /// appropriate [IconButton], and handles the edge-swipe gesture, to show the
  /// drawer.
  ///
  /// To close the end side drawer once it is open, use [Navigator.pop].
  ///
  /// See [Scaffold.of] for information about how to obtain the [ScaffoldState].
  void openEndDrawer() {
    if (_drawerKey.currentState != null && _drawerOpened)
      _drawerKey.currentState.close();
    _endDrawerKey.currentState?.open();
  }

  // SNACKBAR API

  final Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>> _snackBars = Queue<ScaffoldFeatureController<SnackBar, SnackBarClosedReason>>();
  AnimationController _snackBarController;
  Timer _snackBarTimer;
  bool _accessibleNavigation;

  /// Shows a [SnackBar] at the bottom of the scaffold.
  ///
  /// A scaffold can show at most one snack bar at a time. If this function is
  /// called while another snack bar is already visible, the given snack bar
  /// will be added to a queue and displayed after the earlier snack bars have
  /// closed.
  ///
  /// To control how long a [SnackBar] remains visible, use [SnackBar.duration].
  ///
  /// To remove the [SnackBar] with an exit animation, use [hideCurrentSnackBar]
  /// or call [ScaffoldFeatureController.close] on the returned
  /// [ScaffoldFeatureController]. To remove a [SnackBar] suddenly (without an
  /// animation), use [removeCurrentSnackBar].
  ///
  /// See [Scaffold.of] for information about how to obtain the [ScaffoldState].
  ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackbar) {
    _snackBarController ??= SnackBar.createAnimationController(vsync: this)
      ..addStatusListener(_handleSnackBarStatusChange);
    if (_snackBars.isEmpty) {
      assert(_snackBarController.isDismissed);
      _snackBarController.forward();
    }
    ScaffoldFeatureController<SnackBar, SnackBarClosedReason> controller;
    controller = ScaffoldFeatureController<SnackBar, SnackBarClosedReason>._(
      // We provide a fallback key so that if back-to-back snackbars happen to
      // match in structure, material ink splashes and highlights don't survive
      // from one to the next.
      snackbar.withAnimation(_snackBarController, fallbackKey: UniqueKey()),
      Completer<SnackBarClosedReason>(),
      () {
        assert(_snackBars.first == controller);
        hideCurrentSnackBar(reason: SnackBarClosedReason.hide);
      },
      null, // SnackBar doesn't use a builder function so setState() wouldn't rebuild it
    );
    setState(() {
      _snackBars.addLast(controller);
    });
    return controller;
  }

  void _handleSnackBarStatusChange(AnimationStatus status) {
    switch (status) {
      case AnimationStatus.dismissed:
        assert(_snackBars.isNotEmpty);
        setState(() {
          _snackBars.removeFirst();
        });
        if (_snackBars.isNotEmpty)
          _snackBarController.forward();
        break;
      case AnimationStatus.completed:
        setState(() {
          assert(_snackBarTimer == null);
          // build will create a new timer if necessary to dismiss the snack bar
        });
        break;
      case AnimationStatus.forward:
      case AnimationStatus.reverse:
        break;
    }
  }

  /// Removes the current [SnackBar] (if any) immediately.
  ///
  /// The removed snack bar does not run its normal exit animation. If there are
  /// any queued snack bars, they begin their entrance animation immediately.
  void removeCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.remove }) {
    assert(reason != null);
    if (_snackBars.isEmpty)
      return;
    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
    if (!completer.isCompleted)
      completer.complete(reason);
    _snackBarTimer?.cancel();
    _snackBarTimer = null;
    _snackBarController.value = 0.0;
  }

  /// Removes the current [SnackBar] by running its normal exit animation.
  ///
  /// The closed completer is called after the animation is complete.
  void hideCurrentSnackBar({ SnackBarClosedReason reason = SnackBarClosedReason.hide }) {
    assert(reason != null);
    if (_snackBars.isEmpty || _snackBarController.status == AnimationStatus.dismissed)
      return;
    final MediaQueryData mediaQuery = MediaQuery.of(context);
    final Completer<SnackBarClosedReason> completer = _snackBars.first._completer;
    if (mediaQuery.accessibleNavigation) {
      _snackBarController.value = 0.0;
      completer.complete(reason);
    } else {
      _snackBarController.reverse().then<void>((void value) {
        assert(mounted);
        if (!completer.isCompleted)
          completer.complete(reason);
      });
    }
    _snackBarTimer?.cancel();
    _snackBarTimer = null;
  }


  // PERSISTENT BOTTOM SHEET API

  // Contains bottom sheets that may still be animating out of view.
  // Important if the app/user takes an action that could repeatedly show a
  // bottom sheet.
  final List<_StandardBottomSheet> _dismissedBottomSheets = <_StandardBottomSheet>[];
  PersistentBottomSheetController<dynamic> _currentBottomSheet;

  void _maybeBuildPersistentBottomSheet() {
    if (widget.bottomSheet != null && _currentBottomSheet == null) {
      // The new _currentBottomSheet is not a local history entry so a "back" button
      // will not be added to the Scaffold's appbar and the bottom sheet will not
      // support drag or swipe to dismiss.
      final AnimationController animationController = BottomSheet.createAnimationController(this)..value = 1.0;
      LocalHistoryEntry _persistentSheetHistoryEntry;
      bool _persistentBottomSheetExtentChanged(DraggableScrollableNotification notification) {
        if (notification.extent > notification.initialExtent) {
          if (_persistentSheetHistoryEntry == null) {
            _persistentSheetHistoryEntry = LocalHistoryEntry(onRemove: () {
              if (notification.extent > notification.initialExtent) {
                DraggableScrollableActuator.reset(notification.context);
              }
              showBodyScrim(false, 0.0);
              _floatingActionButtonVisibilityValue = 1.0;
              _persistentSheetHistoryEntry = null;
            });
            ModalRoute.of(context).addLocalHistoryEntry(_persistentSheetHistoryEntry);
          }
        } else if (_persistentSheetHistoryEntry != null) {
          ModalRoute.of(context).removeLocalHistoryEntry(_persistentSheetHistoryEntry);
        }
        return false;
      }

      _currentBottomSheet = _buildBottomSheet<void>(
        (BuildContext context) {
          return NotificationListener<DraggableScrollableNotification>(
            onNotification: _persistentBottomSheetExtentChanged,
            child: DraggableScrollableActuator(
              child: widget.bottomSheet,
            ),
          );
        },
        true,
        animationController: animationController,
      );
    }
  }

  void _closeCurrentBottomSheet() {
    if (_currentBottomSheet != null) {
      if (!_currentBottomSheet._isLocalHistoryEntry) {
        _currentBottomSheet.close();
      }
      assert(() {
        _currentBottomSheet?._completer?.future?.whenComplete(() {
          assert(_currentBottomSheet == null);
        });
        return true;
      }());
    }
  }

  PersistentBottomSheetController<T> _buildBottomSheet<T>(
    WidgetBuilder builder,
    bool isPersistent, {
    AnimationController animationController,
    Color backgroundColor,
    double elevation,
    ShapeBorder shape,
    Clip clipBehavior,
  }) {
    assert(() {
      if (widget.bottomSheet != null && isPersistent && _currentBottomSheet != null) {
        throw FlutterError(
          'Scaffold.bottomSheet cannot be specified while a bottom sheet'
          'displayed with showBottomSheet() is still visible.\n'
          'Rebuild the Scaffold with a null bottomSheet before calling showBottomSheet().'
        );
      }
      return true;
    }());

    final Completer<T> completer = Completer<T>();
    final GlobalKey<_StandardBottomSheetState> bottomSheetKey = GlobalKey<_StandardBottomSheetState>();
    _StandardBottomSheet bottomSheet;

    bool removedEntry = false;
    void _removeCurrentBottomSheet() {
      removedEntry = true;
      if (_currentBottomSheet == null) {
        return;
      }
      assert(_currentBottomSheet._widget == bottomSheet);
      assert(bottomSheetKey.currentState != null);
      _showFloatingActionButton();

      void _closed(void value) {
        setState(() {
          _currentBottomSheet = null;
        });

        if (animationController.status != AnimationStatus.dismissed) {
          _dismissedBottomSheets.add(bottomSheet);
        }
        completer.complete();
      }

      final Future<void> closing = bottomSheetKey.currentState.close();
      if (closing != null) {
        closing.then(_closed);
      } else {
        _closed(null);
      }
    }

    final LocalHistoryEntry entry = isPersistent
      ? null
      : LocalHistoryEntry(onRemove: () {
          if (!removedEntry) {
            _removeCurrentBottomSheet();
          }
        });

    bottomSheet = _StandardBottomSheet(
      key: bottomSheetKey,
      animationController: animationController,
      enableDrag: !isPersistent,
      onClosing: () {
        if (_currentBottomSheet == null) {
          return;
        }
        assert(_currentBottomSheet._widget == bottomSheet);
        if (!isPersistent && !removedEntry) {
          assert(entry != null);
          entry.remove();
          removedEntry = true;
        }
      },
      onDismissed: () {
        if (_dismissedBottomSheets.contains(bottomSheet)) {
          setState(() {
            _dismissedBottomSheets.remove(bottomSheet);
          });
        }
      },
      builder: builder,
      isPersistent: isPersistent,
      backgroundColor: backgroundColor,
      elevation: elevation,
      shape: shape,
      clipBehavior: clipBehavior,
    );

    if (!isPersistent)
      ModalRoute.of(context).addLocalHistoryEntry(entry);

    return PersistentBottomSheetController<T>._(
      bottomSheet,
      completer,
      entry != null
        ? entry.remove
        : _removeCurrentBottomSheet,
      (VoidCallback fn) { bottomSheetKey.currentState?.setState(fn); },
      !isPersistent,
    );
  }

  /// Shows a material design bottom sheet in the nearest [Scaffold]. To show
  /// a persistent bottom sheet, use the [Scaffold.bottomSheet].
  ///
  /// Returns a controller that can be used to close and otherwise manipulate the
  /// bottom sheet.
  ///
  /// To rebuild the bottom sheet (e.g. if it is stateful), call
  /// [PersistentBottomSheetController.setState] on the controller returned by
  /// this method.
  ///
  /// The new bottom sheet becomes a [LocalHistoryEntry] for the enclosing
  /// [ModalRoute] and a back button is added to the app bar of the [Scaffold]
  /// that closes the bottom sheet.
  ///
  /// To create a persistent bottom sheet that is not a [LocalHistoryEntry] and
  /// does not add a back button to the enclosing Scaffold's app bar, use the
  /// [Scaffold.bottomSheet] constructor parameter.
  ///
  /// A persistent bottom sheet shows information that supplements the primary
  /// content of the app. A persistent bottom sheet remains visible even when
  /// the user interacts with other parts of the app.
  ///
  /// A closely related widget is a modal bottom sheet, which is an alternative
  /// to a menu or a dialog and prevents the user from interacting with the rest
  /// of the app. Modal bottom sheets can be created and displayed with the
  /// [showModalBottomSheet] function.
  ///
  /// {@animation 350 622 https://flutter.github.io/assets-for-api-docs/assets/material/show_bottom_sheet.mp4}
  ///
  /// {@tool sample --template=stateless_widget_scaffold}
  ///
  /// This example demonstrates how to use `showBottomSheet` to display a
  /// bottom sheet when a user taps a button. It also demonstrates how to
  /// close a bottom sheet using the Navigator.
  ///
  /// ```dart
  /// Widget build(BuildContext context) {
  ///   return Center(
  ///     child: RaisedButton(
  ///       child: const Text('showBottomSheet'),
  ///       onPressed: () {
  ///         Scaffold.of(context).showBottomSheet<void>(
  ///           (BuildContext context) {
  ///             return Container(
  ///               height: 200,
  ///               color: Colors.amber,
  ///               child: Center(
  ///                 child: Column(
  ///                   mainAxisAlignment: MainAxisAlignment.center,
  ///                   mainAxisSize: MainAxisSize.min,
  ///                   children: <Widget>[
  ///                     const Text('BottomSheet'),
  ///                     RaisedButton(
  ///                       child: const Text('Close BottomSheet'),
  ///                       onPressed: () => Navigator.pop(context),
  ///                     )
  ///                   ],
  ///                 ),
  ///               ),
  ///             );
  ///           },
  ///         );
  ///       },
  ///     ),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  /// See also:
  ///
  ///  * [BottomSheet], which becomes the parent of the widget returned by the
  ///    `builder`.
  ///  * [showBottomSheet], which calls this method given a [BuildContext].
  ///  * [showModalBottomSheet], which can be used to display a modal bottom
  ///    sheet.
  ///  * [Scaffold.of], for information about how to obtain the [ScaffoldState].
  ///  * <https://material.io/design/components/sheets-bottom.html#standard-bottom-sheet>
  PersistentBottomSheetController<T> showBottomSheet<T>(
    WidgetBuilder builder, {
    Color backgroundColor,
    double elevation,
    ShapeBorder shape,
    Clip clipBehavior,
  }) {
    assert(() {
      if (widget.bottomSheet != null) {
        throw FlutterError(
          'Scaffold.bottomSheet cannot be specified while a bottom sheet'
          'displayed with showBottomSheet() is still visible.\n'
          'Rebuild the Scaffold with a null bottomSheet before calling showBottomSheet().'
        );
      }
      return true;
    }());
    assert(debugCheckHasMediaQuery(context));

    _closeCurrentBottomSheet();
    final AnimationController controller = BottomSheet.createAnimationController(this)..forward();
    setState(() {
      _currentBottomSheet = _buildBottomSheet<T>(
        builder,
        false,
        animationController: controller,
        backgroundColor: backgroundColor,
        elevation: elevation,
        shape: shape,
        clipBehavior: clipBehavior,
      );
    });
    return _currentBottomSheet as PersistentBottomSheetController<T>;
  }

  // Floating Action Button API
  AnimationController _floatingActionButtonMoveController;
  FloatingActionButtonAnimator _floatingActionButtonAnimator;
  FloatingActionButtonLocation _previousFloatingActionButtonLocation;
  FloatingActionButtonLocation _floatingActionButtonLocation;

  AnimationController _floatingActionButtonVisibilityController;

  /// Gets the current value of the visibility animation for the
  /// [Scaffold.floatingActionButton].
  double get _floatingActionButtonVisibilityValue => _floatingActionButtonVisibilityController.value;

  /// Sets the current value of the visibility animation for the
  /// [Scaffold.floatingActionButton].  This value must not be null.
  set _floatingActionButtonVisibilityValue(double newValue) {
    assert(newValue != null);
    _floatingActionButtonVisibilityController.value = newValue.clamp(
      _floatingActionButtonVisibilityController.lowerBound,
      _floatingActionButtonVisibilityController.upperBound,
    ) as double;
  }

  /// Shows the [Scaffold.floatingActionButton].
  TickerFuture _showFloatingActionButton() {
    return _floatingActionButtonVisibilityController.forward();
  }

  // Moves the Floating Action Button to the new Floating Action Button Location.
  void _moveFloatingActionButton(final FloatingActionButtonLocation newLocation) {
    FloatingActionButtonLocation previousLocation = _floatingActionButtonLocation;
    double restartAnimationFrom = 0.0;
    // If the Floating Action Button is moving right now, we need to start from a snapshot of the current transition.
    if (_floatingActionButtonMoveController.isAnimating) {
      previousLocation = _TransitionSnapshotFabLocation(_previousFloatingActionButtonLocation, _floatingActionButtonLocation, _floatingActionButtonAnimator, _floatingActionButtonMoveController.value);
      restartAnimationFrom = _floatingActionButtonAnimator.getAnimationRestart(_floatingActionButtonMoveController.value);
    }

    setState(() {
      _previousFloatingActionButtonLocation = previousLocation;
      _floatingActionButtonLocation = newLocation;
    });

    // Animate the motion even when the fab is null so that if the exit animation is running,
    // the old fab will start the motion transition while it exits instead of jumping to the
    // new position.
    _floatingActionButtonMoveController.forward(from: restartAnimationFrom);
  }

  // iOS FEATURES - status bar tap, back gesture

  // On iOS, tapping the status bar scrolls the app's primary scrollable to the
  // top. We implement this by providing a primary scroll controller and
  // scrolling it to the top when tapped.

  final ScrollController _primaryScrollController = ScrollController();

  void _handleStatusBarTap() {
    if (_primaryScrollController.hasClients) {
      _primaryScrollController.animateTo(
        0.0,
        duration: const Duration(milliseconds: 300),
        curve: Curves.linear, // TODO(ianh): Use a more appropriate curve.
      );
    }
  }

  // INTERNALS

  _ScaffoldGeometryNotifier _geometryNotifier;

  // Backwards compatibility for deprecated resizeToAvoidBottomPadding property
  bool get _resizeToAvoidBottomInset {
    // ignore: deprecated_member_use_from_same_package
    return widget.resizeToAvoidBottomInset ?? widget.resizeToAvoidBottomPadding ?? true;
  }

  @override
  void initState() {
    super.initState();
    _geometryNotifier = _ScaffoldGeometryNotifier(const ScaffoldGeometry(), context);
    _floatingActionButtonLocation = widget.floatingActionButtonLocation ?? _kDefaultFloatingActionButtonLocation;
    _floatingActionButtonAnimator = widget.floatingActionButtonAnimator ?? _kDefaultFloatingActionButtonAnimator;
    _previousFloatingActionButtonLocation = _floatingActionButtonLocation;
    _floatingActionButtonMoveController = AnimationController(
      vsync: this,
      lowerBound: 0.0,
      upperBound: 1.0,
      value: 1.0,
      duration: kFloatingActionButtonSegue * 2,
    );

    _floatingActionButtonVisibilityController = AnimationController(
      duration: kFloatingActionButtonSegue,
      vsync: this,
    );
  }

  @override
  void didUpdateWidget(Scaffold oldWidget) {
    // Update the Floating Action Button Animator, and then schedule the Floating Action Button for repositioning.
    if (widget.floatingActionButtonAnimator != oldWidget.floatingActionButtonAnimator) {
      _floatingActionButtonAnimator = widget.floatingActionButtonAnimator ?? _kDefaultFloatingActionButtonAnimator;
    }
    if (widget.floatingActionButtonLocation != oldWidget.floatingActionButtonLocation) {
      _moveFloatingActionButton(widget.floatingActionButtonLocation ?? _kDefaultFloatingActionButtonLocation);
    }
    if (widget.bottomSheet != oldWidget.bottomSheet) {
      assert(() {
        if (widget.bottomSheet != null && _currentBottomSheet?._isLocalHistoryEntry == true) {
          throw FlutterError.fromParts(<DiagnosticsNode>[
            ErrorSummary(
              'Scaffold.bottomSheet cannot be specified while a bottom sheet displayed '
              'with showBottomSheet() is still visible.'
            ),
            ErrorHint(
              'Use the PersistentBottomSheetController '
              'returned by showBottomSheet() to close the old bottom sheet before creating '
              'a Scaffold with a (non null) bottomSheet.'
            ),
          ]);
        }
        return true;
      }());
      _closeCurrentBottomSheet();
      _maybeBuildPersistentBottomSheet();
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  void didChangeDependencies() {
    final MediaQueryData mediaQuery = MediaQuery.of(context);
    // If we transition from accessible navigation to non-accessible navigation
    // and there is a SnackBar that would have timed out that has already
    // completed its timer, dismiss that SnackBar. If the timer hasn't finished
    // yet, let it timeout as normal.
    if (_accessibleNavigation == true
      && !mediaQuery.accessibleNavigation
      && _snackBarTimer != null
      && !_snackBarTimer.isActive) {
      hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
    }
    _accessibleNavigation = mediaQuery.accessibleNavigation;
    _maybeBuildPersistentBottomSheet();
    super.didChangeDependencies();
  }

  @override
  void dispose() {
    _snackBarController?.dispose();
    _snackBarTimer?.cancel();
    _snackBarTimer = null;
    _geometryNotifier.dispose();
    for (final _StandardBottomSheet bottomSheet in _dismissedBottomSheets) {
      bottomSheet.animationController?.dispose();
    }
    if (_currentBottomSheet != null) {
      _currentBottomSheet._widget.animationController?.dispose();
    }
    _floatingActionButtonMoveController.dispose();
    _floatingActionButtonVisibilityController.dispose();
    super.dispose();
  }

  void _addIfNonNull(
    List<LayoutId> children,
    Widget child,
    Object childId, {
    @required bool removeLeftPadding,
    @required bool removeTopPadding,
    @required bool removeRightPadding,
    @required bool removeBottomPadding,
    bool removeBottomInset = false,
    bool maintainBottomViewPadding = false,
  }) {
    MediaQueryData data = MediaQuery.of(context).removePadding(
      removeLeft: removeLeftPadding,
      removeTop: removeTopPadding,
      removeRight: removeRightPadding,
      removeBottom: removeBottomPadding,
    );
    if (removeBottomInset)
      data = data.removeViewInsets(removeBottom: true);

    if (maintainBottomViewPadding && data.viewInsets.bottom != 0.0) {
      data = data.copyWith(
        padding: data.padding.copyWith(bottom: data.viewPadding.bottom)
      );
    }

    if (child != null) {
      children.add(
        LayoutId(
          id: childId,
          child: MediaQuery(data: data, child: child),
        ),
      );
    }
  }

  void _buildEndDrawer(List<LayoutId> children, TextDirection textDirection) {
    if (widget.endDrawer != null) {
      assert(hasEndDrawer);
      _addIfNonNull(
        children,
        DrawerController(
          key: _endDrawerKey,
          alignment: DrawerAlignment.end,
          child: widget.endDrawer,
          drawerCallback: _endDrawerOpenedCallback,
          dragStartBehavior: widget.drawerDragStartBehavior,
          scrimColor: widget.drawerScrimColor,
          edgeDragWidth: widget.drawerEdgeDragWidth,
        ),
        _ScaffoldSlot.endDrawer,
        // remove the side padding from the side we're not touching
        removeLeftPadding: textDirection == TextDirection.ltr,
        removeTopPadding: false,
        removeRightPadding: textDirection == TextDirection.rtl,
        removeBottomPadding: false,
      );
    }
  }

  void _buildDrawer(List<LayoutId> children, TextDirection textDirection) {
    if (widget.drawer != null) {
      assert(hasDrawer);
      _addIfNonNull(
        children,
        DrawerController(
          key: _drawerKey,
          alignment: DrawerAlignment.start,
          child: widget.drawer,
          drawerCallback: _drawerOpenedCallback,
          dragStartBehavior: widget.drawerDragStartBehavior,
          scrimColor: widget.drawerScrimColor,
          edgeDragWidth: widget.drawerEdgeDragWidth,
        ),
        _ScaffoldSlot.drawer,
        // remove the side padding from the side we're not touching
        removeLeftPadding: textDirection == TextDirection.rtl,
        removeTopPadding: false,
        removeRightPadding: textDirection == TextDirection.ltr,
        removeBottomPadding: false,
      );
    }
  }

  bool _showBodyScrim = false;
  Color _bodyScrimColor = Colors.black;

  /// Whether to show a [ModalBarrier] over the body of the scaffold.
  ///
  /// The `value` parameter must not be null.
  void showBodyScrim(bool value, double opacity) {
    assert(value != null);
    if (_showBodyScrim == value && _bodyScrimColor.opacity == opacity) {
      return;
    }
    setState(() {
      _showBodyScrim = value;
      _bodyScrimColor = Colors.black.withOpacity(opacity);
    });
  }

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMediaQuery(context));
    assert(debugCheckHasDirectionality(context));
    final MediaQueryData mediaQuery = MediaQuery.of(context);
    final ThemeData themeData = Theme.of(context);
    final TextDirection textDirection = Directionality.of(context);
    _accessibleNavigation = mediaQuery.accessibleNavigation;

    if (_snackBars.isNotEmpty) {
      final ModalRoute<dynamic> route = ModalRoute.of(context);
      if (route == null || route.isCurrent) {
        if (_snackBarController.isCompleted && _snackBarTimer == null) {
          final SnackBar snackBar = _snackBars.first._widget;
          _snackBarTimer = Timer(snackBar.duration, () {
            assert(_snackBarController.status == AnimationStatus.forward ||
                   _snackBarController.status == AnimationStatus.completed);
            // Look up MediaQuery again in case the setting changed.
            final MediaQueryData mediaQuery = MediaQuery.of(context);
            if (mediaQuery.accessibleNavigation && snackBar.action != null)
              return;
            hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
          });
        }
      } else {
        _snackBarTimer?.cancel();
        _snackBarTimer = null;
      }
    }

    final List<LayoutId> children = <LayoutId>[];
    _addIfNonNull(
      children,
      widget.body == null ? null : _BodyBuilder(
        extendBody: widget.extendBody,
        extendBodyBehindAppBar: widget.extendBodyBehindAppBar,
        body: widget.body,
      ),
      _ScaffoldSlot.body,
      removeLeftPadding: false,
      removeTopPadding: widget.appBar != null,
      removeRightPadding: false,
      removeBottomPadding: widget.bottomNavigationBar != null || widget.persistentFooterButtons != null,
      removeBottomInset: _resizeToAvoidBottomInset,
    );
    if (_showBodyScrim) {
      _addIfNonNull(
        children,
        ModalBarrier(
          dismissible: false,
          color: _bodyScrimColor,
        ),
        _ScaffoldSlot.bodyScrim,
        removeLeftPadding: true,
        removeTopPadding: true,
        removeRightPadding: true,
        removeBottomPadding: true,
      );
    }

    if (widget.appBar != null) {
      final double topPadding = widget.primary ? mediaQuery.padding.top : 0.0;
      _appBarMaxHeight = widget.appBar.preferredSize.height + topPadding;
      assert(_appBarMaxHeight >= 0.0 && _appBarMaxHeight.isFinite);
      _addIfNonNull(
        children,
        ConstrainedBox(
          constraints: BoxConstraints(maxHeight: _appBarMaxHeight),
          child: FlexibleSpaceBar.createSettings(
            currentExtent: _appBarMaxHeight,
            child: widget.appBar,
          ),
        ),
        _ScaffoldSlot.appBar,
        removeLeftPadding: false,
        removeTopPadding: false,
        removeRightPadding: false,
        removeBottomPadding: true,
      );
    }

    bool isSnackBarFloating = false;
    if (_snackBars.isNotEmpty) {
      final SnackBarBehavior snackBarBehavior = _snackBars.first._widget.behavior
        ?? themeData.snackBarTheme.behavior
        ?? SnackBarBehavior.fixed;
      isSnackBarFloating = snackBarBehavior == SnackBarBehavior.floating;

      _addIfNonNull(
        children,
        _snackBars.first._widget,
        _ScaffoldSlot.snackBar,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: widget.bottomNavigationBar != null || widget.persistentFooterButtons != null,
        maintainBottomViewPadding: !_resizeToAvoidBottomInset,
      );
    }

    if (widget.persistentFooterButtons != null) {
      _addIfNonNull(
        children,
        Container(
          decoration: BoxDecoration(
            border: Border(
              top: Divider.createBorderSide(context, width: 1.0),
            ),
          ),
          child: SafeArea(
            top: false,
            child: ButtonBar(
              children: widget.persistentFooterButtons,
            ),
          ),
        ),
        _ScaffoldSlot.persistentFooter,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: false,
        maintainBottomViewPadding: !_resizeToAvoidBottomInset,
      );
    }

    if (widget.bottomNavigationBar != null) {
      _addIfNonNull(
        children,
        widget.bottomNavigationBar,
        _ScaffoldSlot.bottomNavigationBar,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: false,
        maintainBottomViewPadding: !_resizeToAvoidBottomInset,
      );
    }

    if (_currentBottomSheet != null || _dismissedBottomSheets.isNotEmpty) {
      final Widget stack = Stack(
        alignment: Alignment.bottomCenter,
        children: <Widget>[
          ..._dismissedBottomSheets,
          if (_currentBottomSheet != null) _currentBottomSheet._widget,
        ],
      );
      _addIfNonNull(
        children,
        stack,
        _ScaffoldSlot.bottomSheet,
        removeLeftPadding: false,
        removeTopPadding: true,
        removeRightPadding: false,
        removeBottomPadding: _resizeToAvoidBottomInset,
      );
    }

    _addIfNonNull(
      children,
      _FloatingActionButtonTransition(
        child: widget.floatingActionButton,
        fabMoveAnimation: _floatingActionButtonMoveController,
        fabMotionAnimator: _floatingActionButtonAnimator,
        geometryNotifier: _geometryNotifier,
        currentController: _floatingActionButtonVisibilityController,
      ),
      _ScaffoldSlot.floatingActionButton,
      removeLeftPadding: true,
      removeTopPadding: true,
      removeRightPadding: true,
      removeBottomPadding: true,
    );

    switch (themeData.platform) {
      case TargetPlatform.iOS:
      case TargetPlatform.macOS:
        _addIfNonNull(
          children,
          GestureDetector(
            behavior: HitTestBehavior.opaque,
            onTap: _handleStatusBarTap,
            // iOS accessibility automatically adds scroll-to-top to the clock in the status bar
            excludeFromSemantics: true,
          ),
          _ScaffoldSlot.statusBar,
          removeLeftPadding: false,
          removeTopPadding: true,
          removeRightPadding: false,
          removeBottomPadding: true,
        );
        break;
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        break;
    }

    if (_endDrawerOpened) {
      _buildDrawer(children, textDirection);
      _buildEndDrawer(children, textDirection);
    } else {
      _buildEndDrawer(children, textDirection);
      _buildDrawer(children, textDirection);
    }

    // The minimum insets for contents of the Scaffold to keep visible.
    final EdgeInsets minInsets = mediaQuery.padding.copyWith(
      bottom: _resizeToAvoidBottomInset ? mediaQuery.viewInsets.bottom : 0.0,
    );

    // extendBody locked when keyboard is open
    final bool _extendBody = minInsets.bottom <= 0 && widget.extendBody;

    return _ScaffoldScope(
      hasDrawer: hasDrawer,
      geometryNotifier: _geometryNotifier,
      child: PrimaryScrollController(
        controller: _primaryScrollController,
        child: Material(
          color: widget.backgroundColor ?? themeData.scaffoldBackgroundColor,
          child: AnimatedBuilder(animation: _floatingActionButtonMoveController, builder: (BuildContext context, Widget child) {
            return CustomMultiChildLayout(
              children: children,
              delegate: _ScaffoldLayout(
                extendBody: _extendBody,
                extendBodyBehindAppBar: widget.extendBodyBehindAppBar,
                minInsets: minInsets,
                currentFloatingActionButtonLocation: _floatingActionButtonLocation,
                floatingActionButtonMoveAnimationProgress: _floatingActionButtonMoveController.value,
                floatingActionButtonMotionAnimator: _floatingActionButtonAnimator,
                geometryNotifier: _geometryNotifier,
                previousFloatingActionButtonLocation: _previousFloatingActionButtonLocation,
                textDirection: textDirection,
                isSnackBarFloating: isSnackBarFloating,
              ),
            );
          }),
        ),
      ),
    );
  }
}

/// An interface for controlling a feature of a [Scaffold].
///
/// Commonly obtained from [ScaffoldState.showSnackBar] or [ScaffoldState.showBottomSheet].
class ScaffoldFeatureController<T extends Widget, U> {
  const ScaffoldFeatureController._(this._widget, this._completer, this.close, this.setState);
  final T _widget;
  final Completer<U> _completer;

  /// Completes when the feature controlled by this object is no longer visible.
  Future<U> get closed => _completer.future;

  /// Remove the feature (e.g., bottom sheet or snack bar) from the scaffold.
  final VoidCallback close;

  /// Mark the feature (e.g., bottom sheet or snack bar) as needing to rebuild.
  final StateSetter setState;
}

class _StandardBottomSheet extends StatefulWidget {
  const _StandardBottomSheet({
    Key key,
    this.animationController,
    this.enableDrag = true,
    this.onClosing,
    this.onDismissed,
    this.builder,
    this.isPersistent = false,
    this.backgroundColor,
    this.elevation,
    this.shape,
    this.clipBehavior,
  }) : super(key: key);

  final AnimationController animationController; // we control it, but it must be disposed by whoever created it.
  final bool enableDrag;
  final VoidCallback onClosing;
  final VoidCallback onDismissed;
  final WidgetBuilder builder;
  final bool isPersistent;
  final Color backgroundColor;
  final double elevation;
  final ShapeBorder shape;
  final Clip clipBehavior;

  @override
  _StandardBottomSheetState createState() => _StandardBottomSheetState();
}

class _StandardBottomSheetState extends State<_StandardBottomSheet> {
  @override
  void initState() {
    super.initState();
    assert(widget.animationController != null);
    assert(widget.animationController.status == AnimationStatus.forward
        || widget.animationController.status == AnimationStatus.completed);
    widget.animationController.addStatusListener(_handleStatusChange);
  }

  @override
  void didUpdateWidget(_StandardBottomSheet oldWidget) {
    super.didUpdateWidget(oldWidget);
    assert(widget.animationController == oldWidget.animationController);
  }

  Future<void> close() {
    assert(widget.animationController != null);
    widget.animationController.reverse();
    if (widget.onClosing != null) {
      widget.onClosing();
    }
    return null;
  }

  void _handleStatusChange(AnimationStatus status) {
    if (status == AnimationStatus.dismissed && widget.onDismissed != null) {
      widget.onDismissed();
    }
  }

  bool extentChanged(DraggableScrollableNotification notification) {
    final double extentRemaining = 1.0 - notification.extent;
    final ScaffoldState scaffold = Scaffold.of(context);
    if (extentRemaining < _kBottomSheetDominatesPercentage) {
      scaffold._floatingActionButtonVisibilityValue = extentRemaining * _kBottomSheetDominatesPercentage * 10;
      scaffold.showBodyScrim(true,  math.max(
        _kMinBottomSheetScrimOpacity,
        _kMaxBottomSheetScrimOpacity - scaffold._floatingActionButtonVisibilityValue,
      ));
    } else {
      scaffold._floatingActionButtonVisibilityValue = 1.0;
      scaffold.showBodyScrim(false, 0.0);
    }
    // If the Scaffold.bottomSheet != null, we're a persistent bottom sheet.
    if (notification.extent == notification.minExtent && scaffold.widget.bottomSheet == null) {
      close();
    }
    return false;
  }

  Widget _wrapBottomSheet(Widget bottomSheet) {
    return Semantics(
      container: true,
      onDismiss: close,
      child:  NotificationListener<DraggableScrollableNotification>(
        onNotification: extentChanged,
        child: bottomSheet,
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    if (widget.animationController != null) {
      return AnimatedBuilder(
        animation: widget.animationController,
        builder: (BuildContext context, Widget child) {
          return Align(
            alignment: AlignmentDirectional.topStart,
            heightFactor: widget.animationController.value,
            child: child,
          );
        },
        child: _wrapBottomSheet(
          BottomSheet(
            animationController: widget.animationController,
            enableDrag: widget.enableDrag,
            onClosing: widget.onClosing,
            builder: widget.builder,
            backgroundColor: widget.backgroundColor,
            elevation: widget.elevation,
            shape: widget.shape,
            clipBehavior: widget.clipBehavior,
          ),
        ),
      );
    }

    return _wrapBottomSheet(
      BottomSheet(
        onClosing: widget.onClosing,
        builder: widget.builder,
        backgroundColor: widget.backgroundColor,
      ),
    );
  }

}

/// A [ScaffoldFeatureController] for standard bottom sheets.
///
/// This is the type of objects returned by [ScaffoldState.showBottomSheet].
///
/// This controller is used to display both standard and persistent bottom
/// sheets. A bottom sheet is only persistent if it is set as the
/// [Scaffold.bottomSheet].
class PersistentBottomSheetController<T> extends ScaffoldFeatureController<_StandardBottomSheet, T> {
  const PersistentBottomSheetController._(
    _StandardBottomSheet widget,
    Completer<T> completer,
    VoidCallback close,
    StateSetter setState,
    this._isLocalHistoryEntry,
  ) : super._(widget, completer, close, setState);

  final bool _isLocalHistoryEntry;
}

class _ScaffoldScope extends InheritedWidget {
  const _ScaffoldScope({
    Key key,
    @required this.hasDrawer,
    @required this.geometryNotifier,
    @required Widget child,
  }) : assert(hasDrawer != null),
       super(key: key, child: child);

  final bool hasDrawer;
  final _ScaffoldGeometryNotifier geometryNotifier;

  @override
  bool updateShouldNotify(_ScaffoldScope oldWidget) {
    return hasDrawer != oldWidget.hasDrawer;
  }
}
