// Copyright 2016 The Chromium 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:math' as math;
import 'dart:ui' as ui show Color;

import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart';
import 'package:vector_math/vector_math_64.dart';

import 'binding.dart';
import 'box.dart';
import 'debug.dart';
import 'object.dart';
import 'viewport.dart';
import 'viewport_offset.dart';

// CORE TYPES FOR SLIVERS
// The RenderSliver base class and its helper types.

/// The direction in which a sliver's contents are ordered, relative to the
/// scroll offset axis.
///
/// For example, a vertical alphabetical list that is going [AxisDirection.down]
/// with a [GrowthDirection.forward] would have the A at the top and the Z at
/// the bottom, with the A adjacent to the origin, as would such a list going
/// [AxisDirection.up] with a [GrowthDirection.reverse]. On the other hand, a
/// vertical alphabetical list that is going [AxisDirection.down] with a
/// [GrowthDirection.reverse] would have the Z at the top (at scroll offset
/// zero) and the A below it.
///
/// The direction in which the scroll offset increases is given by
/// [applyGrowthDirectionToAxisDirection].
enum GrowthDirection {
  /// This sliver's contents are ordered in the same direction as the
  /// [AxisDirection].
  forward,

  /// This sliver's contents are ordered in the opposite direction of the
  /// [AxisDirection].
  reverse,
}

/// Flips the [AxisDirection] if the [GrowthDirection] is [GrowthDirection.reverse].
///
/// Specifically, returns `axisDirection` if `growthDirection` is
/// [GrowthDirection.forward], otherwise returns [flipAxisDirection] applied to
/// `axisDirection`.
///
/// This function is useful in [RenderSliver] subclasses that are given both an
/// [AxisDirection] and a [GrowthDirection] and wish to compute the
/// [AxisDirection] in which growth will occur.
AxisDirection applyGrowthDirectionToAxisDirection(AxisDirection axisDirection, GrowthDirection growthDirection) {
  assert(axisDirection != null);
  assert(growthDirection != null);
  switch (growthDirection) {
    case GrowthDirection.forward:
      return axisDirection;
    case GrowthDirection.reverse:
      return flipAxisDirection(axisDirection);
  }
  return null;
}

/// Flips the [ScrollDirection] if the [GrowthDirection] is [GrowthDirection.reverse].
///
/// Specifically, returns `scrollDirection` if `scrollDirection` is
/// [GrowthDirection.forward], otherwise returns [flipScrollDirection] applied to
/// `scrollDirection`.
///
/// This function is useful in [RenderSliver] subclasses that are given both an
/// [ScrollDirection] and a [GrowthDirection] and wish to compute the
/// [ScrollDirection] in which growth will occur.
ScrollDirection applyGrowthDirectionToScrollDirection(ScrollDirection scrollDirection, GrowthDirection growthDirection) {
  assert(scrollDirection != null);
  assert(growthDirection != null);
  switch (growthDirection) {
    case GrowthDirection.forward:
      return scrollDirection;
    case GrowthDirection.reverse:
      return flipScrollDirection(scrollDirection);
  }
  return null;
}

/// Immutable layout constraints for [RenderSliver] layout.
///
/// The [SliverConstraints] describe the current scroll state of the viewport
/// from the point of view of the sliver receiving the constraints. For example,
/// a [scrollOffset] of zero means that the leading edge of the sliver is
/// visible in the viewport, not that the viewport itself has a zero scroll
/// offset.
class SliverConstraints extends Constraints {
  /// Creates sliver constraints with the given information.
  ///
  /// All of the argument must not be null.
  const SliverConstraints({
    @required this.axisDirection,
    @required this.growthDirection,
    @required this.userScrollDirection,
    @required this.scrollOffset,
    @required this.precedingScrollExtent,
    @required this.overlap,
    @required this.remainingPaintExtent,
    @required this.crossAxisExtent,
    @required this.crossAxisDirection,
    @required this.viewportMainAxisExtent,
    @required this.remainingCacheExtent,
    @required this.cacheOrigin,
  }) : assert(axisDirection != null),
       assert(growthDirection != null),
       assert(userScrollDirection != null),
       assert(scrollOffset != null),
       assert(precedingScrollExtent != null),
       assert(overlap != null),
       assert(remainingPaintExtent != null),
       assert(crossAxisExtent != null),
       assert(crossAxisDirection != null),
       assert(viewportMainAxisExtent != null),
       assert(remainingCacheExtent != null),
       assert(cacheOrigin != null);

  /// Creates a copy of this object but with the given fields replaced with the
  /// new values.
  SliverConstraints copyWith({
    AxisDirection axisDirection,
    GrowthDirection growthDirection,
    ScrollDirection userScrollDirection,
    double scrollOffset,
    double precedingScrollExtent,
    double overlap,
    double remainingPaintExtent,
    double crossAxisExtent,
    AxisDirection crossAxisDirection,
    double viewportMainAxisExtent,
    double remainingCacheExtent,
    double cacheOrigin,
  }) {
    return SliverConstraints(
      axisDirection: axisDirection ?? this.axisDirection,
      growthDirection: growthDirection ?? this.growthDirection,
      userScrollDirection: userScrollDirection ?? this.userScrollDirection,
      scrollOffset: scrollOffset ?? this.scrollOffset,
      precedingScrollExtent: precedingScrollExtent ?? this.precedingScrollExtent,
      overlap: overlap ?? this.overlap,
      remainingPaintExtent: remainingPaintExtent ?? this.remainingPaintExtent,
      crossAxisExtent: crossAxisExtent ?? this.crossAxisExtent,
      crossAxisDirection: crossAxisDirection ?? this.crossAxisDirection,
      viewportMainAxisExtent: viewportMainAxisExtent ?? this.viewportMainAxisExtent,
      remainingCacheExtent: remainingCacheExtent ?? this.remainingCacheExtent,
      cacheOrigin: cacheOrigin ?? this.cacheOrigin,
    );
  }

  /// The direction in which the [scrollOffset] and [remainingPaintExtent]
  /// increase.
  final AxisDirection axisDirection;

  /// The direction in which the contents of slivers are ordered, relative to
  /// the [axisDirection].
  ///
  /// For example, if the [axisDirection] is [AxisDirection.up], and the
  /// [growthDirection] is [GrowthDirection.forward], then an alphabetical list
  /// will have A at the bottom, then B, then C, and so forth, with Z at the
  /// top, with the bottom of the A at scroll offset zero, and the top of the Z
  /// at the highest scroll offset.
  ///
  /// If a viewport has an overall [AxisDirection] of [AxisDirection.down], then
  /// slivers above the absolute zero offset will have an axis of
  /// [AxisDirection.up] and a growth direction of [GrowthDirection.reverse],
  /// while slivers below the absolute zero offset will have the same axis
  /// direction as the viewport and a growth direction of
  /// [GrowthDirection.forward]. (The slivers with a reverse growth direction
  /// still see only positive scroll offsets; the scroll offsets are reversed as
  /// well, with zero at the absolute zero point, and positive numbers going
  /// away from there.)
  ///
  /// Normally, the absolute zero offset is determined by the viewport's
  /// [RenderViewport.center] and [RenderViewport.anchor] properties.
  final GrowthDirection growthDirection;

  /// The direction in which the user is attempting to scroll, relative to the
  /// [axisDirection] and [growthDirection].
  ///
  /// For example, if [growthDirection] is [GrowthDirection.reverse] and
  /// [axisDirection] is [AxisDirection.down], then a
  /// [ScrollDirection.forward] means that the user is scrolling up, in the
  /// positive [scrollOffset] direction.
  ///
  /// If the _user_ is not scrolling, this will return [ScrollDirection.idle]
  /// even if there is (for example) a [ScrollActivity] currently animating the
  /// position.
  ///
  /// This is used by some slivers to determine how to react to a change in
  /// scroll offset. For example, [RenderSliverFloatingPersistentHeader] will
  /// only expand a floating app bar when the [userScrollDirection] is in the
  /// positive scroll offset direction.
  final ScrollDirection userScrollDirection;

  /// The scroll offset, in this sliver's coordinate system, that corresponds to
  /// the earliest visible part of this sliver in the [AxisDirection] if
  /// [growthDirection] is [GrowthDirection.forward] or in the opposite
  /// [AxisDirection] direction if [growthDirection] is [GrowthDirection.reverse].
  ///
  /// For example, if [AxisDirection] is [AxisDirection.down] and [growthDirection]
  /// is [GrowthDirection.forward], then scroll offset is the amount the top of
  /// the sliver has been scrolled past the top of the viewport.
  ///
  /// This value is typically used to compute whether this sliver should still
  /// protrude into the viewport via [SliverGeometry.paintExtent] and
  /// [SliverGeometry.layoutExtent] considering how far the beginning of the
  /// sliver is above the beginning of the viewport.
  ///
  /// For slivers whose top is not past the top of the viewport, the
  /// [scrollOffset] is `0` when [AxisDirection] is [AxisDirection.down] and
  /// [growthDirection] is [GrowthDirection.forward]. The set of slivers with
  /// [scrollOffset] `0` includes all the slivers that are below the bottom of the
  /// viewport.
  ///
  /// [SliverConstraints.remainingPaintExtent] is typically used to accomplish
  /// the same goal of computing whether scrolled out slivers should still
  /// partially 'protrude in' from the bottom of the viewport.
  ///
  /// Whether this corresponds to the beginning or the end of the sliver's
  /// contents depends on the [growthDirection].
  final double scrollOffset;

  /// The scroll distance that has been consumed by all [Sliver]s that came
  /// before this [Sliver].
  ///
  /// # Edge Cases
  ///
  /// [Sliver]s often lazily create their internal content as layout occurs,
  /// e.g., [SliverList]. In this case, when [Sliver]s exceed the viewport,
  /// their children are built lazily, and the [Sliver] does not have enough
  /// information to estimate its total extent, [precedingScrollExtent] will be
  /// [double.infinity] for all [Sliver]s that appear after the lazily
  /// constructed child. This is because a total [scrollExtent] cannot be
  /// calculated unless all inner children have been created and sized, or the
  /// number of children and estimated extents are provided. The infinite
  /// [scrollExtent] will become finite as soon as enough information is
  /// available to estimate the overall extent of all children within the given
  /// [Sliver].
  ///
  /// [Sliver]s may legitimately be infinite, meaning that they can scroll
  /// content forever without reaching the end. For any [Sliver]s that appear
  /// after the infinite [Sliver], the [precedingScrollExtent] will be
  /// [double.infinity].
  final double precedingScrollExtent;

  /// The number of pixels from where the pixels corresponding to the
  /// [scrollOffset] will be painted up to the first pixel that has not yet been
  /// painted on by an earlier sliver, in the [axisDirection].
  ///
  /// For example, if the previous sliver had a [SliverGeometry.paintExtent] of
  /// 100.0 pixels but a [SliverGeometry.layoutExtent] of only 50.0 pixels,
  /// then the [overlap] of this sliver will be 50.0.
  ///
  /// This is typically ignored unless the sliver is itself going to be pinned
  /// or floating and wants to avoid doing so under the previous sliver.
  final double overlap;

  /// The number of pixels of content that the sliver should consider providing.
  /// (Providing more pixels than this is inefficient.)
  ///
  /// The actual number of pixels provided should be specified in the
  /// [RenderSliver.geometry] as [SliverGeometry.paintExtent].
  ///
  /// This value may be infinite, for example if the viewport is an
  /// unconstrained [RenderShrinkWrappingViewport].
  ///
  /// This value may be 0.0, for example if the sliver is scrolled off the
  /// bottom of a downwards vertical viewport.
  final double remainingPaintExtent;

  /// The number of pixels in the cross-axis.
  ///
  /// For a vertical list, this is the width of the sliver.
  final double crossAxisExtent;

  /// The direction in which children should be placed in the cross axis.
  ///
  /// Typically used in vertical lists to describe whether the ambient
  /// [TextDirection] is [TextDirection.rtl] or [TextDirection.ltr].
  final AxisDirection crossAxisDirection;

  /// The number of pixels the viewport can display in the main axis.
  ///
  /// For a vertical list, this is the height of the viewport.
  final double viewportMainAxisExtent;

  /// Where the cache area starts relative to the [scrollOffset].
  ///
  /// Slivers that fall into the cache area located before the leading edge and
  /// after the trailing edge of the viewport should still render content
  /// because they are about to become visible when the user scrolls.
  ///
  /// The [cacheOrigin] describes where the [remainingCacheExtent] starts relative
  /// to the [scrollOffset]. A cache origin of 0 means that the sliver does not
  /// have to provide any content before the current [scrollOffset]. A
  /// [cacheOrigin] of -250.0 means that even though the first visible part of
  /// the sliver will be at the provided [scrollOffset], the sliver should
  /// render content starting 250.0 before the [scrollOffset] to fill the
  /// cache area of the viewport.
  ///
  /// The [cacheOrigin] is always negative or zero and will never exceed
  /// -[scrollOffset]. In other words, a sliver is never asked to provide
  /// content before its zero [scrollOffset].
  ///
  /// See also:
  ///
  ///  * [RenderViewport.cacheExtent] for a description of a viewport's cache area.
  final double cacheOrigin;


  /// Describes how much content the sliver should provide starting from the
  /// [cacheOrigin].
  ///
  /// Not all content in the [remainingCacheExtent] will be visible as some
  /// of it might fall into the cache area of the viewport.
  ///
  /// Each sliver should start laying out content at the [cacheOrigin] and
  /// try to provide as much content as the [remainingCacheExtent] allows.
  ///
  /// The [remainingCacheExtent] is always larger or equal to the
  /// [remainingPaintExtent]. Content, that falls in the [remainingCacheExtent],
  /// but is outside of the [remainingPaintExtent] is currently not visible
  /// in the viewport.
  ///
  /// See also:
  ///
  ///  * [RenderViewport.cacheExtent] for a description of a viewport's cache area.
  final double remainingCacheExtent;

  /// The axis along which the [scrollOffset] and [remainingPaintExtent] are measured.
  Axis get axis => axisDirectionToAxis(axisDirection);

  /// Return what the [growthDirection] would be if the [axisDirection] was
  /// either [AxisDirection.down] or [AxisDirection.right].
  ///
  /// This is the same as [growthDirection] unless the [axisDirection] is either
  /// [AxisDirection.up] or [AxisDirection.left], in which case it is the
  /// opposite growth direction.
  ///
  /// This can be useful in combination with [axis] to view the [axisDirection]
  /// and [growthDirection] in different terms.
  GrowthDirection get normalizedGrowthDirection {
    assert(axisDirection != null);
    switch (axisDirection) {
      case AxisDirection.down:
      case AxisDirection.right:
        return growthDirection;
      case AxisDirection.up:
      case AxisDirection.left:
        switch (growthDirection) {
          case GrowthDirection.forward:
            return GrowthDirection.reverse;
          case GrowthDirection.reverse:
            return GrowthDirection.forward;
        }
        return null;
    }
    return null;
  }

  @override
  bool get isTight => false;

  @override
  bool get isNormalized {
    return scrollOffset >= 0.0
        && crossAxisExtent >= 0.0
        && axisDirectionToAxis(axisDirection) != axisDirectionToAxis(crossAxisDirection)
        && viewportMainAxisExtent >= 0.0
        && remainingPaintExtent >= 0.0;
  }

  /// Returns [BoxConstraints] that reflects the sliver constraints.
  ///
  /// The `minExtent` and `maxExtent` are used as the constraints in the main
  /// axis. If non-null, the given `crossAxisExtent` is used as a tight
  /// constraint in the cross axis. Otherwise, the [crossAxisExtent] from this
  /// object is used as a constraint in the cross axis.
  ///
  /// Useful for slivers that have [RenderBox] children.
  BoxConstraints asBoxConstraints({
    double minExtent = 0.0,
    double maxExtent = double.infinity,
    double crossAxisExtent,
  }) {
    crossAxisExtent ??= this.crossAxisExtent;
    switch (axis) {
      case Axis.horizontal:
        return BoxConstraints(
          minHeight: crossAxisExtent,
          maxHeight: crossAxisExtent,
          minWidth: minExtent,
          maxWidth: maxExtent,
        );
      case Axis.vertical:
        return BoxConstraints(
          minWidth: crossAxisExtent,
          maxWidth: crossAxisExtent,
          minHeight: minExtent,
          maxHeight: maxExtent,
        );
    }
    return null;
  }

  @override
  bool debugAssertIsValid({
    bool isAppliedConstraint = false,
    InformationCollector informationCollector,
  }) {
    assert(() {
      void verify(bool check, String message) {
        if (check)
          return;
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('$runtimeType is not valid: $message'),
          if (informationCollector != null)
            ...informationCollector(),
          DiagnosticsProperty<SliverConstraints>('The offending constraints were', this, style: DiagnosticsTreeStyle.errorProperty),
        ]);
      }
      verify(axis != null, 'The "axis" is null.');
      verify(growthDirection != null, 'The "growthDirection" is null.');
      verify(scrollOffset != null, 'The "scrollOffset" is null.');
      verify(overlap != null, 'The "overlap" is null.');
      verify(remainingPaintExtent != null, 'The "remainingPaintExtent" is null.');
      verify(crossAxisExtent != null, 'The "crossAxisExtent" is null.');
      verify(viewportMainAxisExtent != null, 'The "viewportMainAxisExtent" is null.');
      verify(scrollOffset >= 0.0, 'The "scrollOffset" is negative.');
      verify(crossAxisExtent >= 0.0, 'The "crossAxisExtent" is negative.');
      verify(crossAxisDirection != null, 'The "crossAxisDirection" is null.');
      verify(axisDirectionToAxis(axisDirection) != axisDirectionToAxis(crossAxisDirection), 'The "axisDirection" and the "crossAxisDirection" are along the same axis.');
      verify(viewportMainAxisExtent >= 0.0, 'The "viewportMainAxisExtent" is negative.');
      verify(remainingPaintExtent >= 0.0, 'The "remainingPaintExtent" is negative.');
      verify(remainingCacheExtent >= 0.0, 'The "remainingCacheExtent" is negative.');
      verify(cacheOrigin <= 0.0, 'The "cacheOrigin" is positive.');
      verify(isNormalized, 'The constraints are not normalized.'); // should be redundant with earlier checks
      return true;
    }());
    return true;
  }

  @override
  bool operator ==(dynamic other) {
    if (identical(this, other))
      return true;
    if (other is! SliverConstraints)
      return false;
    final SliverConstraints typedOther = other;
    assert(typedOther.debugAssertIsValid());
    return typedOther.axisDirection == axisDirection
        && typedOther.growthDirection == growthDirection
        && typedOther.scrollOffset == scrollOffset
        && typedOther.overlap == overlap
        && typedOther.remainingPaintExtent == remainingPaintExtent
        && typedOther.crossAxisExtent == crossAxisExtent
        && typedOther.crossAxisDirection == crossAxisDirection
        && typedOther.viewportMainAxisExtent == viewportMainAxisExtent
        && typedOther.remainingCacheExtent == remainingCacheExtent
        && typedOther.cacheOrigin == cacheOrigin;
  }

  @override
  int get hashCode {
    return hashValues(
      axisDirection,
      growthDirection,
      scrollOffset,
      overlap,
      remainingPaintExtent,
      crossAxisExtent,
      crossAxisDirection,
      viewportMainAxisExtent,
      remainingCacheExtent,
      cacheOrigin,
    );
  }

  @override
  String toString() {
    return 'SliverConstraints('
             '$axisDirection, '
             '$growthDirection, '
             '$userScrollDirection, '
             'scrollOffset: ${scrollOffset.toStringAsFixed(1)}, '
             'remainingPaintExtent: ${remainingPaintExtent.toStringAsFixed(1)}, ' +
             (overlap != 0.0 ? 'overlap: ${overlap.toStringAsFixed(1)}, ' : '') +
             'crossAxisExtent: ${crossAxisExtent.toStringAsFixed(1)}, '
             'crossAxisDirection: $crossAxisDirection, '
             'viewportMainAxisExtent: ${viewportMainAxisExtent.toStringAsFixed(1)}, '
             'remainingCacheExtent: ${remainingCacheExtent.toStringAsFixed(1)} '
             'cacheOrigin: ${cacheOrigin.toStringAsFixed(1)} '
           ')';
  }
}

/// Describes the amount of space occupied by a [RenderSliver].
///
/// A sliver can occupy space in several different ways, which is why this class
/// contains multiple values.
@immutable
class SliverGeometry extends Diagnosticable {
  /// Creates an object that describes the amount of space occupied by a sliver.
  ///
  /// If the [layoutExtent] argument is null, [layoutExtent] defaults to the
  /// [paintExtent]. If the [hitTestExtent] argument is null, [hitTestExtent]
  /// defaults to the [paintExtent]. If [visible] is null, [visible] defaults to
  /// whether [paintExtent] is greater than zero.
  ///
  /// The other arguments must not be null.
  const SliverGeometry({
    this.scrollExtent = 0.0,
    this.paintExtent = 0.0,
    this.paintOrigin = 0.0,
    double layoutExtent,
    this.maxPaintExtent = 0.0,
    this.maxScrollObstructionExtent = 0.0,
    double hitTestExtent,
    bool visible,
    this.hasVisualOverflow = false,
    this.scrollOffsetCorrection,
    double cacheExtent,
  }) : assert(scrollExtent != null),
       assert(paintExtent != null),
       assert(paintOrigin != null),
       assert(maxPaintExtent != null),
       assert(hasVisualOverflow != null),
       assert(scrollOffsetCorrection != 0.0),
       layoutExtent = layoutExtent ?? paintExtent,
       hitTestExtent = hitTestExtent ?? paintExtent,
       cacheExtent = cacheExtent ?? layoutExtent ?? paintExtent,
       visible = visible ?? paintExtent > 0.0;

  /// A sliver that occupies no space at all.
  static const SliverGeometry zero = SliverGeometry();

  /// The (estimated) total scrollable extent that this sliver has content for.
  ///
  /// This is the amount of scrolling the user needs to do to get from the
  /// beginning of this sliver to the end of this sliver.
  ///
  /// The value is used to calculate the [SliverConstraints.scrollOffset] of
  /// all slivers in the scrollable and thus should be provided whether the
  /// sliver is currently in the viewport or not.
  ///
  /// In a typical scrolling scenario, the [scrollExtent] is constant for a
  /// sliver throughout the scrolling while [paintExtent] and [layoutExtent]
  /// will progress from `0` when offscreen to between `0` and [scrollExtent]
  /// as the sliver scrolls partially into and out of the screen and is
  /// equal to [scrollExtent] while the sliver is entirely on screen. However,
  /// these relationships can be customized to achieve more special effects.
  ///
  /// This value must be accurate if the [paintExtent] is less than the
  /// [SliverConstraints.remainingPaintExtent] provided during layout.
  final double scrollExtent;

  /// The visual location of the first visible part of this sliver relative to
  /// its layout position.
  ///
  /// For example, if the sliver wishes to paint visually before its layout
  /// position, the [paintOrigin] is negative. The coordinate system this sliver
  /// uses for painting is relative to this [paintOrigin]. In other words,
  /// when [RenderSliver.paint] is called, the (0, 0) position of the [Offset]
  /// given to it is at this [paintOrigin].
  ///
  /// The coordinate system used for the [paintOrigin] itself is relative
  /// to the start of this sliver's layout position rather than relative to
  /// its current position on the viewport. In other words, in a typical
  /// scrolling scenario, [paintOrigin] remains constant at 0.0 rather than
  /// tracking from 0.0 to [SliverConstraints.viewportMainAxisExtent] as the
  /// sliver scrolls past the viewport.
  ///
  /// This value does not affect the layout of subsequent slivers. The next
  /// sliver is still placed at [layoutExtent] after this sliver's layout
  /// position. This value does affect where the [paintExtent] extent is
  /// measured from when computing the [SliverConstraints.overlap] for the next
  /// sliver.
  ///
  /// Defaults to 0.0, which means slivers start painting at their layout
  /// position by default.
  final double paintOrigin;

  /// The amount of currently visible visual space that was taken by the sliver
  /// to render the subset of the sliver that covers all or part of the
  /// [SliverConstraints.remainingPaintExtent] in the current viewport.
  ///
  /// This value does not affect how the next sliver is positioned. In other
  /// words, if this value was 100 and [layoutExtent] was 0, typical slivers
  /// placed after it would end up drawing in the same 100 pixel space while
  /// painting.
  ///
  /// This must be between zero and [SliverConstraints.remainingPaintExtent].
  ///
  /// This value is typically 0 when outside of the viewport and grows or
  /// shrinks from 0 or to 0 as the sliver is being scrolled into and out of the
  /// viewport unless the sliver wants to achieve a special effect and paint
  /// even when scrolled away.
  ///
  /// This contributes to the calculation for the next sliver's
  /// [SliverConstraints.overlap].
  final double paintExtent;

  /// The distance from the first visible part of this sliver to the first
  /// visible part of the next sliver, assuming the next sliver's
  /// [SliverConstraints.scrollOffset] is zero.
  ///
  /// This must be between zero and [paintExtent]. It defaults to [paintExtent].
  ///
  /// This value is typically 0 when outside of the viewport and grows or
  /// shrinks from 0 or to 0 as the sliver is being scrolled into and out of the
  /// viewport unless then sliver wants to achieve a special effect and push
  /// down the layout start position of subsequent slivers before the sliver is
  /// even scrolled into the viewport.
  final double layoutExtent;

  /// The (estimated) total paint extent that this sliver would be able to
  /// provide if the [SliverConstraints.remainingPaintExtent] was infinite.
  ///
  /// This is used by viewports that implement shrink-wrapping.
  ///
  /// By definition, this cannot be less than [paintExtent].
  final double maxPaintExtent;

  /// The maximum extent by which this sliver can reduce the area in which
  /// content can scroll if the sliver were pinned at the edge.
  ///
  /// Slivers that never get pinned at the edge, should return zero.
  ///
  /// A pinned app bar is an example for a sliver that would use this setting:
  /// When the app bar is pinned to the top, the area in which content can
  /// actually scroll is reduced by the height of the app bar.
  final double maxScrollObstructionExtent;

  /// The distance from where this sliver started painting to the bottom of
  /// where it should accept hits.
  ///
  /// This must be between zero and [paintExtent]. It defaults to [paintExtent].
  final double hitTestExtent;

  /// Whether this sliver should be painted.
  ///
  /// By default, this is true if [paintExtent] is greater than zero, and
  /// false if [paintExtent] is zero.
  final bool visible;

  /// Whether this sliver has visual overflow.
  ///
  /// By default, this is false, which means the viewport does not need to clip
  /// its children. If any slivers have visual overflow, the viewport will apply
  /// a clip to its children.
  final bool hasVisualOverflow;

  /// If this is non-zero after [RenderSliver.performLayout] returns, the scroll
  /// offset will be adjusted by the parent and then the entire layout of the
  /// parent will be rerun.
  ///
  /// When the value is non-zero, the [RenderSliver] does not need to compute
  /// the rest of the values when constructing the [SliverGeometry] or call
  /// [RenderObject.layout] on its children since [RenderSliver.performLayout]
  /// will be called again on this sliver in the same frame after the
  /// [SliverConstraints.scrollOffset] correction has been applied, when the
  /// proper [SliverGeometry] and layout of its children can be computed.
  ///
  /// If the parent is also a [RenderSliver], it must propagate this value
  /// in its own [RenderSliver.geometry] property until a viewport which adjusts
  /// its offset based on this value.
  final double scrollOffsetCorrection;

  /// How many pixels the sliver has consumed in the
  /// [SliverConstraints.remainingCacheExtent].
  ///
  /// This value should be equal to or larger than the [layoutExtent] because
  /// the sliver always consumes at least the [layoutExtent] from the
  /// [SliverConstraints.remainingCacheExtent] and possibly more if it falls
  /// into the cache area of the viewport.
  ///
  /// See also:
  ///
  ///  * [RenderViewport.cacheExtent] for a description of a viewport's cache area.
  final double cacheExtent;

  /// Asserts that this geometry is internally consistent.
  ///
  /// Does nothing if asserts are disabled. Always returns true.
  bool debugAssertIsValid({
    InformationCollector informationCollector,
  }) {
    assert(() {
      void verify(bool check, String summary, {List<DiagnosticsNode> details}) {
        if (check)
          return;
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('$runtimeType is not valid: $summary'),
          ...?details,
          if (informationCollector != null)
            ...informationCollector(),
        ]);
      }

      verify(scrollExtent != null, 'The "scrollExtent" is null.');
      verify(scrollExtent >= 0.0, 'The "scrollExtent" is negative.');
      verify(paintExtent != null, 'The "paintExtent" is null.');
      verify(paintExtent >= 0.0, 'The "paintExtent" is negative.');
      verify(paintOrigin != null, 'The "paintOrigin" is null.');
      verify(layoutExtent != null, 'The "layoutExtent" is null.');
      verify(layoutExtent >= 0.0, 'The "layoutExtent" is negative.');
      verify(cacheExtent >= 0.0, 'The "cacheExtent" is negative.');
      if (layoutExtent > paintExtent) {
        verify(false,
          'The "layoutExtent" exceeds the "paintExtent".',
          details: _debugCompareFloats('paintExtent', paintExtent, 'layoutExtent', layoutExtent),
        );
      }
      verify(maxPaintExtent != null, 'The "maxPaintExtent" is null.');
      // If the paintExtent is slightly more than the maxPaintExtent, but the difference is still less
      // than precisionErrorTolerance, we will not throw the assert below.
      if (paintExtent - maxPaintExtent > precisionErrorTolerance) {
        verify(false,
          'The "maxPaintExtent" is less than the "paintExtent".',
          details:
            _debugCompareFloats('maxPaintExtent', maxPaintExtent, 'paintExtent', paintExtent)
              ..add(ErrorDescription('By definition, a sliver can\'t paint more than the maximum that it can paint!')),
        );
      }
      verify(hitTestExtent != null, 'The "hitTestExtent" is null.');
      verify(hitTestExtent >= 0.0, 'The "hitTestExtent" is negative.');
      verify(visible != null, 'The "visible" property is null.');
      verify(hasVisualOverflow != null, 'The "hasVisualOverflow" is null.');
      verify(scrollOffsetCorrection != 0.0, 'The "scrollOffsetCorrection" is zero.');
      return true;
    }());
    return true;
  }

  @override
  String toStringShort() => '$runtimeType';

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DoubleProperty('scrollExtent', scrollExtent));
    if (paintExtent > 0.0) {
      properties.add(DoubleProperty('paintExtent', paintExtent, unit : visible ? null : ' but not painting'));
    } else if (paintExtent == 0.0) {
      if (visible) {
        properties.add(DoubleProperty('paintExtent', paintExtent, unit: visible ? null : ' but visible'));
      }
      properties.add(FlagProperty('visible', value: visible, ifFalse: 'hidden'));
    } else {
      // Negative paintExtent!
      properties.add(DoubleProperty('paintExtent', paintExtent, tooltip: '!'));
    }
    properties.add(DoubleProperty('paintOrigin', paintOrigin, defaultValue: 0.0));
    properties.add(DoubleProperty('layoutExtent', layoutExtent, defaultValue: paintExtent));
    properties.add(DoubleProperty('maxPaintExtent', maxPaintExtent));
    properties.add(DoubleProperty('hitTestExtent', hitTestExtent, defaultValue: paintExtent));
    properties.add(DiagnosticsProperty<bool>('hasVisualOverflow', hasVisualOverflow, defaultValue: false));
    properties.add(DoubleProperty('scrollOffsetCorrection', scrollOffsetCorrection, defaultValue: null));
    properties.add(DoubleProperty('cacheExtent', cacheExtent, defaultValue: 0.0));
  }
}

/// Method signature for hit testing a [RenderSLiver].
///
/// Used by [SliverHitTestResult.addWithAxisOffset] to hit test [RenderSliver]
/// children.
///
/// See also:
///
///  * [RenderSliver.hitTest], which documents more details around hit testing
///    [RenderSliver]s.
typedef SliverHitTest = bool Function(SliverHitTestResult result, { @required double mainAxisPosition, @required double crossAxisPosition });

/// The result of performing a hit test on [RenderSliver]s.
///
/// An instance of this class is provided to [RenderSliver.hitTest] to record
/// the result of the hit test.
class SliverHitTestResult extends HitTestResult {
  /// Creates an empty hit test result for hit testing on [RenderSliver].
  SliverHitTestResult() : super();

  /// Wraps `result` to create a [HitTestResult] that implements the
  /// [SliverHitTestResult] protocol for hit testing on [RenderSliver]s.
  ///
  /// This method is used by [RenderObject]s that adapt between the
  /// [RenderSliver]-world and the non-[RenderSliver]-world to convert a
  /// (subtype of) [HitTestResult] to a [SliverHitTestResult] for hit testing on
  /// [RenderSliver]s.
  ///
  /// The [HitTestEntry] instances added to the returned [SliverHitTestResult]
  /// are also added to the wrapped `result` (both share the same underlying
  /// data structure to store [HitTestEntry] instances).
  ///
  /// See also:
  ///
  ///  * [HitTestResult.wrap], which turns a [SliverHitTestResult] back into a
  ///    generic [HitTestResult].
  ///  * [BoxHitTestResult.wrap], which turns a [SliverHitTestResult] into a
  ///    [BoxHitTestResult] for hit testing on [RenderBox] children.
  SliverHitTestResult.wrap(HitTestResult result) : super.wrap(result);

  /// Transforms `mainAxisPosition` and `crossAxisPosition` to the local
  /// coordinate system of a child for hit-testing the child.
  ///
  /// The actual hit testing of the child needs to be implemented in the
  /// provided `hitTest` callback, which is invoked with the transformed
  /// `position` as argument.
  ///
  /// For the transform `mainAxisOffset` is subtracted from `mainAxisPosition`
  /// and `crossAxisOffset` is subtracted from `crossAxisPosition`.
  ///
  /// The `paintOffset` describes how the paint position of a point painted at
  /// the provided `mainAxisPosition` and `crossAxisPosition` would change after
  /// `mainAxisOffset` and `crossAxisOffset` have been applied. This
  /// `paintOffset` is used to properly convert [PointerEvent]s to the local
  /// coordinate system of the event receiver.
  ///
  /// The `paintOffset` may be null if `mainAxisOffset` and `crossAxisOffset` are
  /// both zero.
  ///
  /// The function returns the return value of `hitTest`.
  bool addWithAxisOffset({
    @required Offset paintOffset,
    @required double mainAxisOffset,
    @required double crossAxisOffset,
    @required double mainAxisPosition,
    @required double crossAxisPosition,
    @required SliverHitTest hitTest,
  }) {
    assert(mainAxisOffset != null);
    assert(crossAxisOffset != null);
    assert(mainAxisPosition != null);
    assert(crossAxisPosition != null);
    assert(hitTest != null);
    if (paintOffset != null) {
      pushTransform(Matrix4.translationValues(paintOffset.dx, paintOffset.dy, 0));
    }
    final bool isHit = hitTest(
      this,
      mainAxisPosition: mainAxisPosition - mainAxisOffset,
      crossAxisPosition: crossAxisPosition - crossAxisOffset,
    );
    if (paintOffset != null) {
      popTransform();
    }
    return isHit;
  }
}

/// A hit test entry used by [RenderSliver].
///
/// The coordinate system used by this hit test entry is relative to the
/// [AxisDirection] of the target sliver.
class SliverHitTestEntry extends HitTestEntry {
  /// Creates a sliver hit test entry.
  ///
  /// The [mainAxisPosition] and [crossAxisPosition] arguments must not be null.
  SliverHitTestEntry(
    RenderSliver target, {
    @required this.mainAxisPosition,
    @required this.crossAxisPosition,
  }) : assert(mainAxisPosition != null),
       assert(crossAxisPosition != null),
       super(target);

  @override
  RenderSliver get target => super.target;

  /// The distance in the [AxisDirection] from the edge of the sliver's painted
  /// area (as given by the [SliverConstraints.scrollOffset]) to the hit point.
  /// This can be an unusual direction, for example in the [AxisDirection.up]
  /// case this is a distance from the _bottom_ of the sliver's painted area.
  final double mainAxisPosition;

  /// The distance to the hit point in the axis opposite the
  /// [SliverConstraints.axis].
  ///
  /// If the cross axis is horizontal (i.e. the
  /// [SliverConstraints.axisDirection] is either [AxisDirection.down] or
  /// [AxisDirection.up]), then the `crossAxisPosition` is a distance from the
  /// left edge of the sliver. If the cross axis is vertical (i.e. the
  /// [SliverConstraints.axisDirection] is either [AxisDirection.right] or
  /// [AxisDirection.left]), then the `crossAxisPosition` is a distance from the
  /// top edge of the sliver.
  ///
  /// This is always a distance from the left or top of the parent, never a
  /// distance from the right or bottom.
  final double crossAxisPosition;

  @override
  String toString() => '${target.runtimeType}@(mainAxis: $mainAxisPosition, crossAxis: $crossAxisPosition)';
}

/// Parent data structure used by parents of slivers that position their
/// children using layout offsets.
///
/// This data structure is optimized for fast layout. It is best used by parents
/// that expect to have many children whose relative positions don't change even
/// when the scroll offset does.
class SliverLogicalParentData extends ParentData {
  /// The position of the child relative to the zero scroll offset.
  ///
  /// The number of pixels from from the zero scroll offset of the parent sliver
  /// (the line at which its [SliverConstraints.scrollOffset] is zero) to the
  /// side of the child closest to that offset.
  ///
  /// In a typical list, this does not change as the parent is scrolled.
  double layoutOffset = 0.0;

  @override
  String toString() => 'layoutOffset=${layoutOffset.toStringAsFixed(1)}';
}

/// Parent data for slivers that have multiple children and that position their
/// children using layout offsets.
class SliverLogicalContainerParentData extends SliverLogicalParentData with ContainerParentDataMixin<RenderSliver> { }

/// Parent data structure used by parents of slivers that position their
/// children using absolute coordinates.
///
/// For example, used by [RenderViewport].
///
/// This data structure is optimized for fast painting, at the cost of requiring
/// additional work during layout when the children change their offsets. It is
/// best used by parents that expect to have few children, especially if those
/// children will themselves be very tall relative to the parent.
class SliverPhysicalParentData extends ParentData {
  /// The position of the child relative to the parent.
  ///
  /// This is the distance from the top left visible corner of the parent to the
  /// top left visible corner of the sliver.
  Offset paintOffset = Offset.zero;

  /// Apply the [paintOffset] to the given [transform].
  ///
  /// Used to implement [RenderObject.applyPaintTransform] by slivers that use
  /// [SliverPhysicalParentData].
  void applyPaintTransform(Matrix4 transform) {
    transform.translate(paintOffset.dx, paintOffset.dy);
  }

  @override
  String toString() => 'paintOffset=$paintOffset';
}

/// Parent data for slivers that have multiple children and that position their
/// children using absolute coordinates.
class SliverPhysicalContainerParentData extends SliverPhysicalParentData with ContainerParentDataMixin<RenderSliver> { }

List<DiagnosticsNode> _debugCompareFloats(String labelA, double valueA, String labelB, double valueB) {
  return <DiagnosticsNode>[
    if (valueA.toStringAsFixed(1) != valueB.toStringAsFixed(1))
      ErrorDescription(
        'The $labelA is ${valueA.toStringAsFixed(1)}, but '
        'the $labelB is ${valueB.toStringAsFixed(1)}.'
      )
    else ...<DiagnosticsNode>[
      ErrorDescription('The $labelA is $valueA, but the $labelB is $valueB.'),
      ErrorHint(
        'Maybe you have fallen prey to floating point rounding errors, and should explicitly '
        'apply the min() or max() functions, or the clamp() method, to the $labelB?'
      ),
    ],
  ];
}

/// Base class for the render objects that implement scroll effects in viewports.
///
/// A [RenderViewport] has a list of child slivers. Each sliver — literally a
/// slice of the viewport's contents — is laid out in turn, covering the
/// viewport in the process. (Every sliver is laid out each time, including
/// those that have zero extent because they are "scrolled off" or are beyond
/// the end of the viewport.)
///
/// Slivers participate in the _sliver protocol_, wherein during [layout] each
/// sliver receives a [SliverConstraints] object and computes a corresponding
/// [SliverGeometry] that describes where it fits in the viewport. This is
/// analogous to the box protocol used by [RenderBox], which gets a
/// [BoxConstraints] as input and computes a [Size].
///
/// Slivers have a leading edge, which is where the position described by
/// [SliverConstraints.scrollOffset] for this sliver begins. Slivers have
/// several dimensions, the primary of which is [SliverGeometry.paintExtent],
/// which describes the extent of the sliver along the main axis, starting from
/// the leading edge, reaching either the end of the viewport or the end of the
/// sliver, whichever comes first.
///
/// Slivers can change dimensions based on the changing constraints in a
/// non-linear fashion, to achieve various scroll effects. For example, the
/// various [RenderSliverPersistentHeader] subclasses, on which [SliverAppBar]
/// is based, achieve effects such as staying visible despite the scroll offset,
/// or reappearing at different offsets based on the user's scroll direction
/// ([SliverConstraints.userScrollDirection]).
///
/// ## Writing a RenderSliver subclass
///
/// Slivers can have sliver children, or children from another coordinate
/// system, typically box children. (For details on the box protocol, see
/// [RenderBox].) Slivers can also have different child models, typically having
/// either one child, or a list of children.
///
/// ### Examples of slivers
///
/// A good example of a sliver with a single child that is also itself a sliver
/// is [RenderSliverPadding], which indents its child. A sliver-to-sliver render
/// object such as this must construct a [SliverConstraints] object for its
/// child, then must take its child's [SliverGeometry] and use it to form its
/// own [geometry].
///
/// The other common kind of one-child sliver is a sliver that has a single
/// [RenderBox] child. An example of that would be [RenderSliverToBoxAdapter],
/// which lays out a single box and sizes itself around the box. Such a sliver
/// must use its [SliverConstraints] to create a [BoxConstraints] for the
/// child, lay the child out (using the child's [layout] method), and then use
/// the child's [RenderBox.size] to generate the sliver's [SliverGeometry].
///
/// The most common kind of sliver though is one with multiple children. The
/// most straight-forward example of this is [RenderSliverList], which arranges
/// its children one after the other in the main axis direction. As with the
/// one-box-child sliver case, it uses its [constraints] to create a
/// [BoxConstraints] for the children, and then it uses the aggregate
/// information from all its children to generate its [geometry]. Unlike the
/// one-child cases, however, it is judicious in which children it actually lays
/// out (and later paints). If the scroll offset is 1000 pixels, and it
/// previously determined that the first three children are each 400 pixels
/// tall, then it will skip the first two and start the layout with its third
/// child.
///
/// ### Layout
///
/// As they are laid out, slivers decide their [geometry], which includes their
/// size ([SliverGeometry.paintExtent]) and the position of the next sliver
/// ([SliverGeometry.layoutExtent]), as well as the position of each of their
/// children, based on the input [constraints] from the viewport such as the
/// scroll offset ([SliverConstraints.scrollOffset]).
///
/// For example, a sliver that just paints a box 100 pixels high would say its
/// [SliverGeometry.paintExtent] was 100 pixels when the scroll offset was zero,
/// but would say its [SliverGeometry.paintExtent] was 25 pixels when the scroll
/// offset was 75 pixels, and would say it was zero when the scroll offset was
/// 100 pixels or more. (This is assuming that
/// [SliverConstraints.remainingPaintExtent] was more than 100 pixels.)
///
/// The various dimensions that are provided as input to this system are in the
/// [constraints]. They are described in detail in the documentation for the
/// [SliverConstraints] class.
///
/// The [performLayout] function must take these [constraints] and create a
/// [SliverGeometry] object that it must then assign to the [geometry] property.
/// The different dimensions of the geometry that can be configured are
/// described in detail in the documentation for the [SliverGeometry] class.
///
/// ### Painting
///
/// In addition to implementing layout, a sliver must also implement painting.
/// This is achieved by overriding the [paint] method.
///
/// The [paint] method is called with an [Offset] from the [Canvas] origin to
/// the top-left corner of the sliver, _regardless of the axis direction_.
///
/// Subclasses should also override [applyPaintTransform] to provide the
/// [Matrix4] describing the position of each child relative to the sliver.
/// (This is used by, among other things, the accessibility layer, to determine
/// the bounds of the child.)
///
/// ### Hit testing
///
/// To implement hit testing, either override the [hitTestSelf] and
/// [hitTestChildren] methods, or, for more complex cases, instead override the
/// [hitTest] method directly.
///
/// To actually react to pointer events, the [handleEvent] method may be
/// implemented. By default it does nothing. (Typically gestures are handled by
/// widgets in the box protocol, not by slivers directly.)
///
/// ### Helper methods
///
/// There are a number of methods that a sliver should implement which will make
/// the other methods easier to implement. Each method listed below has detailed
/// documentation. In addition, the [RenderSliverHelpers] class can be used to
/// mix in some helpful methods.
///
/// #### childScrollOffset
///
/// If the subclass positions children anywhere other than at scroll offset
/// zero, it should override [childScrollOffset]. For example,
/// [RenderSliverList] and [RenderSliverGrid] override this method, but
/// [RenderSliverToBoxAdapter] does not.
///
/// This is used by, among other things, [Scrollable.ensureVisible].
///
/// #### childMainAxisPosition
///
/// Subclasses should implement [childMainAxisPosition] to describe where their
/// children are positioned.
///
/// #### childCrossAxisPosition
///
/// If the subclass positions children in the cross-axis at a position other
/// than zero, then it should override [childCrossAxisPosition]. For example
/// [RenderSliverGrid] overrides this method.
abstract class RenderSliver extends RenderObject {
  // layout input
  @override
  SliverConstraints get constraints => super.constraints;

  /// The amount of space this sliver occupies.
  ///
  /// This value is stale whenever this object is marked as needing layout.
  /// During [performLayout], do not read the [geometry] of a child unless you
  /// pass true for parentUsesSize when calling the child's [layout] function.
  ///
  /// The geometry of a sliver should be set only during the sliver's
  /// [performLayout] or [performResize] functions. If you wish to change the
  /// geometry of a sliver outside of those functions, call [markNeedsLayout]
  /// instead to schedule a layout of the sliver.
  SliverGeometry get geometry => _geometry;
  SliverGeometry _geometry;
  set geometry(SliverGeometry value) {
    assert(!(debugDoingThisResize && debugDoingThisLayout));
    assert(sizedByParent || !debugDoingThisResize);
    assert(() {
      if ((sizedByParent && debugDoingThisResize) ||
          (!sizedByParent && debugDoingThisLayout))
        return true;
      assert(!debugDoingThisResize);
      DiagnosticsNode contract, violation, hint;
      if (debugDoingThisLayout) {
        assert(sizedByParent);
        violation = ErrorDescription('It appears that the geometry setter was called from performLayout().');
      } else {
        violation = ErrorDescription('The geometry setter was called from outside layout (neither performResize() nor performLayout() were being run for this object).');
        if (owner != null && owner.debugDoingLayout)
          hint = ErrorDescription('Only the object itself can set its geometry. It is a contract violation for other objects to set it.');
      }
      if (sizedByParent)
        contract = ErrorDescription('Because this RenderSliver has sizedByParent set to true, it must set its geometry in performResize().');
      else
        contract = ErrorDescription('Because this RenderSliver has sizedByParent set to false, it must set its geometry in performLayout().');

      final List<DiagnosticsNode> information = <DiagnosticsNode>[
        ErrorSummary('RenderSliver geometry setter called incorrectly.'),
        violation,
        if (hint != null) hint,
        contract,
        describeForError('The RenderSliver in question is'),
      ];
      throw FlutterError.fromParts(information);
    }());
    _geometry = value;
  }

  @override
  Rect get semanticBounds => paintBounds;

  @override
  Rect get paintBounds {
    assert(constraints.axis != null);
    switch (constraints.axis) {
      case Axis.horizontal:
        return Rect.fromLTWH(
          0.0, 0.0,
          geometry.paintExtent,
          constraints.crossAxisExtent,
        );
      case Axis.vertical:
        return Rect.fromLTWH(
          0.0, 0.0,
          constraints.crossAxisExtent,
          geometry.paintExtent,
        );
    }
    return null;
  }

  @override
  void debugResetSize() { }

  @override
  void debugAssertDoesMeetConstraints() {
    assert(geometry.debugAssertIsValid(
      informationCollector: () sync* {
        yield describeForError('The RenderSliver that returned the offending geometry was');
      }
    ));
    assert(() {
      if (geometry.paintExtent > constraints.remainingPaintExtent) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('SliverGeometry has a paintOffset that exceeds the remainingPaintExtent from the constraints.'),
          describeForError('The render object whose geometry violates the constraints is the following'),
          ..._debugCompareFloats(
            'remainingPaintExtent', constraints.remainingPaintExtent,
            'paintExtent', geometry.paintExtent,
          ),
          ErrorDescription(
            'The paintExtent must cause the child sliver to paint within the viewport, and so '
            'cannot exceed the remainingPaintExtent.',
          ),
        ]);
      }
      return true;
    }());
  }

  @override
  void performResize() {
    assert(false);
  }

  /// For a center sliver, the distance before the absolute zero scroll offset
  /// that this sliver can cover.
  ///
  /// For example, if an [AxisDirection.down] viewport with an
  /// [RenderViewport.anchor] of 0.5 has a single sliver with a height of 100.0
  /// and its [centerOffsetAdjustment] returns 50.0, then the sliver will be
  /// centered in the viewport when the scroll offset is 0.0.
  ///
  /// The distance here is in the opposite direction of the
  /// [RenderViewport.axisDirection], so values will typically be positive.
  double get centerOffsetAdjustment => 0.0;

  /// Determines the set of render objects located at the given position.
  ///
  /// Returns true if the given point is contained in this render object or one
  /// of its descendants. Adds any render objects that contain the point to the
  /// given hit test result.
  ///
  /// The caller is responsible for providing the position in the local
  /// coordinate space of the callee. The callee is responsible for checking
  /// whether the given position is within its bounds.
  ///
  /// Hit testing requires layout to be up-to-date but does not require painting
  /// to be up-to-date. That means a render object can rely upon [performLayout]
  /// having been called in [hitTest] but cannot rely upon [paint] having been
  /// called. For example, a render object might be a child of a [RenderOpacity]
  /// object, which calls [hitTest] on its children when its opacity is zero
  /// even through it does not [paint] its children.
  ///
  /// ## Coordinates for RenderSliver objects
  ///
  /// The `mainAxisPosition` is the distance in the [AxisDirection] (after
  /// applying the [GrowthDirection]) from the edge of the sliver's painted
  /// area. This can be an unusual direction, for example in the
  /// [AxisDirection.up] case this is a distance from the _bottom_ of the
  /// sliver's painted area.
  ///
  /// The `crossAxisPosition` is the distance in the other axis. If the cross
  /// axis is horizontal (i.e. the [SliverConstraints.axisDirection] is either
  /// [AxisDirection.down] or [AxisDirection.up]), then the `crossAxisPosition`
  /// is a distance from the left edge of the sliver. If the cross axis is
  /// vertical (i.e. the [SliverConstraints.axisDirection] is either
  /// [AxisDirection.right] or [AxisDirection.left]), then the
  /// `crossAxisPosition` is a distance from the top edge of the sliver.
  ///
  /// ## Implementing hit testing for slivers
  ///
  /// The most straight-forward way to implement hit testing for a new sliver
  /// render object is to override its [hitTestSelf] and [hitTestChildren]
  /// methods.
  bool hitTest(SliverHitTestResult result, { @required double mainAxisPosition, @required double crossAxisPosition }) {
    if (mainAxisPosition >= 0.0 && mainAxisPosition < geometry.hitTestExtent &&
        crossAxisPosition >= 0.0 && crossAxisPosition < constraints.crossAxisExtent) {
      if (hitTestChildren(result, mainAxisPosition: mainAxisPosition, crossAxisPosition: crossAxisPosition) ||
          hitTestSelf(mainAxisPosition: mainAxisPosition, crossAxisPosition: crossAxisPosition)) {
        result.add(SliverHitTestEntry(
          this,
          mainAxisPosition: mainAxisPosition,
          crossAxisPosition: crossAxisPosition,
        ));
        return true;
      }
    }
    return false;
  }

  /// Override this method if this render object can be hit even if its
  /// children were not hit.
  ///
  /// Used by [hitTest]. If you override [hitTest] and do not call this
  /// function, then you don't need to implement this function.
  ///
  /// For a discussion of the semantics of the arguments, see [hitTest].
  @protected
  bool hitTestSelf({ @required double mainAxisPosition, @required double crossAxisPosition }) => false;

  /// Override this method to check whether any children are located at the
  /// given position.
  ///
  /// Typically children should be hit-tested in reverse paint order so that
  /// hit tests at locations where children overlap hit the child that is
  /// visually "on top" (i.e., paints later).
  ///
  /// Used by [hitTest]. If you override [hitTest] and do not call this
  /// function, then you don't need to implement this function.
  ///
  /// For a discussion of the semantics of the arguments, see [hitTest].
  @protected
  bool hitTestChildren(SliverHitTestResult result, { @required double mainAxisPosition, @required double crossAxisPosition }) => false;

  /// Computes the portion of the region from `from` to `to` that is visible,
  /// assuming that only the region from the [SliverConstraints.scrollOffset]
  /// that is [SliverConstraints.remainingPaintExtent] high is visible, and that
  /// the relationship between scroll offsets and paint offsets is linear.
  ///
  /// For example, if the constraints have a scroll offset of 100 and a
  /// remaining paint extent of 100, and the arguments to this method describe
  /// the region 50..150, then the returned value would be 50 (from scroll
  /// offset 100 to scroll offset 150).
  ///
  /// This method is not useful if there is not a 1:1 relationship between
  /// consumed scroll offset and consumed paint extent. For example, if the
  /// sliver always paints the same amount but consumes a scroll offset extent
  /// that is proportional to the [SliverConstraints.scrollOffset], then this
  /// function's results will not be consistent.
  // This could be a static method but isn't, because it would be less convenient
  // to call it from subclasses if it was.
  double calculatePaintOffset(SliverConstraints constraints, { @required double from, @required double to }) {
    assert(from <= to);
    final double a = constraints.scrollOffset;
    final double b = constraints.scrollOffset + constraints.remainingPaintExtent;
    // the clamp on the next line is to avoid floating point rounding errors
    return (to.clamp(a, b) - from.clamp(a, b)).clamp(0.0, constraints.remainingPaintExtent);
  }

  /// Computes the portion of the region from `from` to `to` that is within
  /// the cache extent of the viewport, assuming that only the region from the
  /// [SliverConstraints.cacheOrigin] that is
  /// [SliverConstraints.remainingCacheExtent] high is visible, and that
  /// the relationship between scroll offsets and paint offsets is linear.
  ///
  /// This method is not useful if there is not a 1:1 relationship between
  /// consumed scroll offset and consumed cache extent.
  double calculateCacheOffset(SliverConstraints constraints, { @required double from, @required double to }) {
    assert(from <= to);
    final double a = constraints.scrollOffset + constraints.cacheOrigin;
    final double b = constraints.scrollOffset + constraints.remainingCacheExtent;
    // the clamp on the next line is to avoid floating point rounding errors
    return (to.clamp(a, b) - from.clamp(a, b)).clamp(0.0, constraints.remainingCacheExtent);
  }

  /// Returns the distance from the leading _visible_ edge of the sliver to the
  /// side of the given child closest to that edge.
  ///
  /// For example, if the [constraints] describe this sliver as having an axis
  /// direction of [AxisDirection.down], then this is the distance from the top
  /// of the visible portion of the sliver to the top of the child. On the other
  /// hand, if the [constraints] describe this sliver as having an axis
  /// direction of [AxisDirection.up], then this is the distance from the bottom
  /// of the visible portion of the sliver to the bottom of the child. In both
  /// cases, this is the direction of increasing
  /// [SliverConstraints.scrollOffset] and
  /// [SliverLogicalParentData.layoutOffset].
  ///
  /// For children that are [RenderSliver]s, the leading edge of the _child_
  /// will be the leading _visible_ edge of the child, not the part of the child
  /// that would locally be a scroll offset 0.0. For children that are not
  /// [RenderSliver]s, for example a [RenderBox] child, it's the actual distance
  /// to the edge of the box, since those boxes do not know how to handle being
  /// scrolled.
  ///
  /// This method differs from [childScrollOffset] in that
  /// [childMainAxisPosition] gives the distance from the leading _visible_ edge
  /// of the sliver whereas [childScrollOffset] gives the distance from the
  /// sliver's zero scroll offset.
  ///
  /// Calling this for a child that is not visible is not valid.
  @protected
  double childMainAxisPosition(covariant RenderObject child) {
    assert(() {
      throw FlutterError('$runtimeType does not implement childPosition.');
    }());
    return 0.0;
  }

  /// Returns the distance along the cross axis from the zero of the cross axis
  /// in this sliver's [paint] coordinate space to the nearest side of the given
  /// child.
  ///
  /// For example, if the [constraints] describe this sliver as having an axis
  /// direction of [AxisDirection.down], then this is the distance from the left
  /// of the sliver to the left of the child. Similarly, if the [constraints]
  /// describe this sliver as having an axis direction of [AxisDirection.up],
  /// then this is value is the same. If the axis direction is
  /// [AxisDirection.left] or [AxisDirection.right], then it is the distance
  /// from the top of the sliver to the top of the child.
  ///
  /// Calling this for a child that is not visible is not valid.
  @protected
  double childCrossAxisPosition(covariant RenderObject child) => 0.0;

  /// Returns the scroll offset for the leading edge of the given child.
  ///
  /// The `child` must be a child of this sliver.
  ///
  /// This method differs from [childMainAxisPosition] in that
  /// [childMainAxisPosition] gives the distance from the leading _visible_ edge
  /// of the sliver whereas [childScrollOffset] gives the distance from sliver's
  /// zero scroll offset.
  double childScrollOffset(covariant RenderObject child) {
    assert(child.parent == this);
    return 0.0;
  }

  @override
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    assert(() {
      throw FlutterError('$runtimeType does not implement applyPaintTransform.');
    }());
  }

  /// This returns a [Size] with dimensions relative to the leading edge of the
  /// sliver, specifically the same offset that is given to the [paint] method.
  /// This means that the dimensions may be negative.
  ///
  /// This is only valid after [layout] has completed.
  ///
  /// See also:
  ///
  ///  * [getAbsoluteSize], which returns absolute size.
  @protected
  Size getAbsoluteSizeRelativeToOrigin() {
    assert(geometry != null);
    assert(!debugNeedsLayout);
    switch (applyGrowthDirectionToAxisDirection(constraints.axisDirection, constraints.growthDirection)) {
      case AxisDirection.up:
        return Size(constraints.crossAxisExtent, -geometry.paintExtent);
      case AxisDirection.right:
        return Size(geometry.paintExtent, constraints.crossAxisExtent);
      case AxisDirection.down:
        return Size(constraints.crossAxisExtent, geometry.paintExtent);
      case AxisDirection.left:
        return Size(-geometry.paintExtent, constraints.crossAxisExtent);
    }
    return null;
  }

  /// This returns the absolute [Size] of the sliver.
  ///
  /// The dimensions are always positive and calling this is only valid after
  /// [layout] has completed.
  ///
  /// See also:
  ///
  ///  * [getAbsoluteSizeRelativeToOrigin], which returns the size relative to
  ///    the leading edge of the sliver.
  @protected
  Size getAbsoluteSize() {
    assert(geometry != null);
    assert(!debugNeedsLayout);
    switch (constraints.axisDirection) {
      case AxisDirection.up:
      case AxisDirection.down:
        return Size(constraints.crossAxisExtent, geometry.paintExtent);
      case AxisDirection.right:
      case AxisDirection.left:
        return Size(geometry.paintExtent, constraints.crossAxisExtent);
    }
    return null;
  }

  void _debugDrawArrow(Canvas canvas, Paint paint, Offset p0, Offset p1, GrowthDirection direction) {
    assert(() {
      if (p0 == p1)
        return true;
      assert(p0.dx == p1.dx || p0.dy == p1.dy); // must be axis-aligned
      final double d = (p1 - p0).distance * 0.2;
      Offset temp;
      double dx1, dx2, dy1, dy2;
      switch (direction) {
        case GrowthDirection.forward:
          dx1 = dx2 = dy1 = dy2 = d;
          break;
        case GrowthDirection.reverse:
          temp = p0;
          p0 = p1;
          p1 = temp;
          dx1 = dx2 = dy1 = dy2 = -d;
          break;
      }
      if (p0.dx == p1.dx) {
        dx2 = -dx2;
      } else {
        dy2 = -dy2;
      }
      canvas.drawPath(
        Path()
          ..moveTo(p0.dx, p0.dy)
          ..lineTo(p1.dx, p1.dy)
          ..moveTo(p1.dx - dx1, p1.dy - dy1)
          ..lineTo(p1.dx, p1.dy)
          ..lineTo(p1.dx - dx2, p1.dy - dy2),
        paint,
      );
      return true;
    }());
  }

  @override
  void debugPaint(PaintingContext context, Offset offset) {
    assert(() {
      if (debugPaintSizeEnabled) {
        final double strokeWidth = math.min(4.0, geometry.paintExtent / 30.0);
        final Paint paint = Paint()
          ..color = const Color(0xFF33CC33)
          ..strokeWidth = strokeWidth
          ..style = PaintingStyle.stroke
          ..maskFilter = MaskFilter.blur(BlurStyle.solid, strokeWidth);
        final double arrowExtent = geometry.paintExtent;
        final double padding = math.max(2.0, strokeWidth);
        final Canvas canvas = context.canvas;
        canvas.drawCircle(
          offset.translate(padding, padding),
          padding * 0.5,
          paint,
        );
        switch (constraints.axis) {
          case Axis.vertical:
            canvas.drawLine(
              offset,
              offset.translate(constraints.crossAxisExtent, 0.0),
              paint,
            );
            _debugDrawArrow(
              canvas,
              paint,
              offset.translate(constraints.crossAxisExtent * 1.0 / 4.0, padding),
              offset.translate(constraints.crossAxisExtent * 1.0 / 4.0, arrowExtent - padding),
              constraints.normalizedGrowthDirection,
            );
            _debugDrawArrow(
              canvas,
              paint,
              offset.translate(constraints.crossAxisExtent * 3.0 / 4.0, padding),
              offset.translate(constraints.crossAxisExtent * 3.0 / 4.0, arrowExtent - padding),
              constraints.normalizedGrowthDirection,
            );
            break;
          case Axis.horizontal:
            canvas.drawLine(
              offset,
              offset.translate(0.0, constraints.crossAxisExtent),
              paint,
            );
            _debugDrawArrow(
              canvas,
              paint,
              offset.translate(padding, constraints.crossAxisExtent * 1.0 / 4.0),
              offset.translate(arrowExtent - padding, constraints.crossAxisExtent * 1.0 / 4.0),
              constraints.normalizedGrowthDirection,
            );
            _debugDrawArrow(
              canvas,
              paint,
              offset.translate(padding, constraints.crossAxisExtent * 3.0 / 4.0),
              offset.translate(arrowExtent - padding, constraints.crossAxisExtent * 3.0 / 4.0),
              constraints.normalizedGrowthDirection,
            );
            break;
        }
      }
      return true;
    }());
  }

  // This override exists only to change the type of the second argument.
  @override
  void handleEvent(PointerEvent event, SliverHitTestEntry entry) { }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<SliverGeometry>('geometry', geometry));
  }
}

/// Mixin for [RenderSliver] subclasses that provides some utility functions.
abstract class RenderSliverHelpers implements RenderSliver {

  bool _getRightWayUp(SliverConstraints constraints) {
    assert(constraints != null);
    assert(constraints.axisDirection != null);
    bool rightWayUp;
    switch (constraints.axisDirection) {
      case AxisDirection.up:
      case AxisDirection.left:
        rightWayUp = false;
        break;
      case AxisDirection.down:
      case AxisDirection.right:
        rightWayUp = true;
        break;
    }
    assert(constraints.growthDirection != null);
    switch (constraints.growthDirection) {
      case GrowthDirection.forward:
        break;
      case GrowthDirection.reverse:
        rightWayUp = !rightWayUp;
        break;
    }
    assert(rightWayUp != null);
    return rightWayUp;
  }

  /// Utility function for [hitTestChildren] for use when the children are
  /// [RenderBox] widgets.
  ///
  /// This function takes care of converting the position from the sliver
  /// coordinate system to the Cartesian coordinate system used by [RenderBox].
  ///
  /// This function relies on [childMainAxisPosition] to determine the position of
  /// child in question.
  ///
  /// Calling this for a child that is not visible is not valid.
  @protected
  bool hitTestBoxChild(BoxHitTestResult result, RenderBox child, { @required double mainAxisPosition, @required double crossAxisPosition }) {
    final bool rightWayUp = _getRightWayUp(constraints);
    double delta = childMainAxisPosition(child);
    final double crossAxisDelta = childCrossAxisPosition(child);
    double absolutePosition = mainAxisPosition - delta;
    final double absoluteCrossAxisPosition = crossAxisPosition - crossAxisDelta;
    Offset paintOffset, transformedPosition;
    assert(constraints.axis != null);
    switch (constraints.axis) {
      case Axis.horizontal:
        if (!rightWayUp) {
          absolutePosition = child.size.width - absolutePosition;
          delta = geometry.paintExtent - child.size.width - delta;
        }
        paintOffset = Offset(delta, crossAxisDelta);
        transformedPosition = Offset(absolutePosition, absoluteCrossAxisPosition);
        break;
      case Axis.vertical:
        if (!rightWayUp) {
          absolutePosition = child.size.height - absolutePosition;
          delta = geometry.paintExtent - child.size.height - delta;
        }
        paintOffset = Offset(crossAxisDelta, delta);
        transformedPosition = Offset(absoluteCrossAxisPosition, absolutePosition);
        break;
    }
    assert(paintOffset != null);
    assert(transformedPosition != null);
    return result.addWithPaintOffset(
      offset: paintOffset,
      position: null, // Manually adapting from sliver to box position above.
      hitTest: (BoxHitTestResult result, Offset _) {
        return child.hitTest(result, position: transformedPosition);
      },
    );
  }

  /// Utility function for [applyPaintTransform] for use when the children are
  /// [RenderBox] widgets.
  ///
  /// This function turns the value returned by [childMainAxisPosition] and
  /// [childCrossAxisPosition]for the child in question into a translation that
  /// it then applies to the given matrix.
  ///
  /// Calling this for a child that is not visible is not valid.
  @protected
  void applyPaintTransformForBoxChild(RenderBox child, Matrix4 transform) {
    final bool rightWayUp = _getRightWayUp(constraints);
    double delta = childMainAxisPosition(child);
    final double crossAxisDelta = childCrossAxisPosition(child);
    assert(constraints.axis != null);
    switch (constraints.axis) {
      case Axis.horizontal:
        if (!rightWayUp)
          delta = geometry.paintExtent - child.size.width - delta;
        transform.translate(delta, crossAxisDelta);
        break;
      case Axis.vertical:
        if (!rightWayUp)
          delta = geometry.paintExtent - child.size.height - delta;
        transform.translate(crossAxisDelta, delta);
        break;
    }
  }
}

// ADAPTER FOR RENDER BOXES INSIDE SLIVERS
// Transitions from the RenderSliver world to the RenderBox world.

/// An abstract class for [RenderSliver]s that contains a single [RenderBox].
///
/// See also:
///
///  * [RenderSliver], which explains more about the Sliver protocol.
///  * [RenderBox], which explains more about the Box protocol.
///  * [RenderSliverToBoxAdapter], which extends this class to size the child
///    according to its preferred size.
///  * [RenderSliverFillRemaining], which extends this class to size the child
///    to fill the remaining space in the viewport.
abstract class RenderSliverSingleBoxAdapter extends RenderSliver with RenderObjectWithChildMixin<RenderBox>, RenderSliverHelpers {
  /// Creates a [RenderSliver] that wraps a [RenderBox].
  RenderSliverSingleBoxAdapter({
    RenderBox child,
  }) {
    this.child = child;
  }

  @override
  void setupParentData(RenderObject child) {
    if (child.parentData is! SliverPhysicalParentData)
      child.parentData = SliverPhysicalParentData();
  }

  /// Sets the [SliverPhysicalParentData.paintOffset] for the given child
  /// according to the [SliverConstraints.axisDirection] and
  /// [SliverConstraints.growthDirection] and the given geometry.
  @protected
  void setChildParentData(RenderObject child, SliverConstraints constraints, SliverGeometry geometry) {
    final SliverPhysicalParentData childParentData = child.parentData;
    assert(constraints.axisDirection != null);
    assert(constraints.growthDirection != null);
    switch (applyGrowthDirectionToAxisDirection(constraints.axisDirection, constraints.growthDirection)) {
      case AxisDirection.up:
        childParentData.paintOffset = Offset(0.0, -(geometry.scrollExtent - (geometry.paintExtent + constraints.scrollOffset)));
        break;
      case AxisDirection.right:
        childParentData.paintOffset = Offset(-constraints.scrollOffset, 0.0);
        break;
      case AxisDirection.down:
        childParentData.paintOffset = Offset(0.0, -constraints.scrollOffset);
        break;
      case AxisDirection.left:
        childParentData.paintOffset = Offset(-(geometry.scrollExtent - (geometry.paintExtent + constraints.scrollOffset)), 0.0);
        break;
    }
    assert(childParentData.paintOffset != null);
  }

  @override
  bool hitTestChildren(SliverHitTestResult result, { @required double mainAxisPosition, @required double crossAxisPosition }) {
    assert(geometry.hitTestExtent > 0.0);
    if (child != null)
      return hitTestBoxChild(BoxHitTestResult.wrap(result), child, mainAxisPosition: mainAxisPosition, crossAxisPosition: crossAxisPosition);
    return false;
  }

  @override
  double childMainAxisPosition(RenderBox child) {
    return -constraints.scrollOffset;
  }

  @override
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    assert(child != null);
    assert(child == this.child);
    final SliverPhysicalParentData childParentData = child.parentData;
    childParentData.applyPaintTransform(transform);
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (child != null && geometry.visible) {
      final SliverPhysicalParentData childParentData = child.parentData;
      context.paintChild(child, offset + childParentData.paintOffset);
    }
  }
}

/// A [RenderSliver] that contains a single [RenderBox].
///
/// The child will not be laid out if it is not visible. It is sized according
/// to the child's preferences in the main axis, and with a tight constraint
/// forcing it to the dimensions of the viewport in the cross axis.
///
/// See also:
///
///  * [RenderSliver], which explains more about the Sliver protocol.
///  * [RenderBox], which explains more about the Box protocol.
///  * [RenderViewport], which allows [RenderSliver] objects to be placed inside
///    a [RenderBox] (the opposite of this class).
class RenderSliverToBoxAdapter extends RenderSliverSingleBoxAdapter {
  /// Creates a [RenderSliver] that wraps a [RenderBox].
  RenderSliverToBoxAdapter({
    RenderBox child,
  }) : super(child: child);

  @override
  void performLayout() {
    if (child == null) {
      geometry = SliverGeometry.zero;
      return;
    }
    child.layout(constraints.asBoxConstraints(), parentUsesSize: true);
    double childExtent;
    switch (constraints.axis) {
      case Axis.horizontal:
        childExtent = child.size.width;
        break;
      case Axis.vertical:
        childExtent = child.size.height;
        break;
    }
    assert(childExtent != null);
    final double paintedChildSize = calculatePaintOffset(constraints, from: 0.0, to: childExtent);
    final double cacheExtent = calculateCacheOffset(constraints, from: 0.0, to: childExtent);

    assert(paintedChildSize.isFinite);
    assert(paintedChildSize >= 0.0);
    geometry = SliverGeometry(
      scrollExtent: childExtent,
      paintExtent: paintedChildSize,
      cacheExtent: cacheExtent,
      maxPaintExtent: childExtent,
      hitTestExtent: paintedChildSize,
      hasVisualOverflow: childExtent > constraints.remainingPaintExtent || constraints.scrollOffset > 0.0,
    );
    setChildParentData(child, constraints, geometry);
  }
}

/// Makes its sliver child partially transparent.
///
/// This class paints its sliver child into an intermediate buffer and then
/// blends the sliver child back into the scene, partially transparent.
///
/// For values of opacity other than 0.0 and 1.0, this class is relatively
/// expensive, because it requires painting the sliver child into an intermediate
/// buffer. For the value 0.0, the sliver child is simply not painted at all.
/// For the value 1.0, the sliver child is painted immediately without an
/// intermediate buffer.
class RenderSliverOpacity extends RenderSliver with RenderObjectWithChildMixin<RenderSliver> {
  /// Creates a partially transparent render object.
  ///
  /// The [opacity] argument must be between 0.0 and 1.0, inclusive.
  RenderSliverOpacity({
    double opacity = 1.0,
    bool alwaysIncludeSemantics = false,
    RenderSliver sliver,
  }) : assert(opacity != null && opacity >= 0.0 && opacity <= 1.0),
       assert(alwaysIncludeSemantics != null),
       _opacity = opacity,
       _alwaysIncludeSemantics = alwaysIncludeSemantics,
       _alpha = ui.Color.getAlphaFromOpacity(opacity) {
    child = sliver;
  }

  @override
  bool get alwaysNeedsCompositing => child != null && (_alpha != 0 && _alpha != 255);

  int _alpha;

  /// The fraction to scale the child's alpha value.
  ///
  /// An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent
  /// (i.e. invisible).
  ///
  /// The opacity must not be null.
  ///
  /// Values 1.0 and 0.0 are painted with a fast path. Other values
  /// require painting the child into an intermediate buffer, which is
  /// expensive.
  double get opacity => _opacity;
  double _opacity;
  set opacity(double value) {
    assert(value != null);
    assert(value >= 0.0 && value <= 1.0);
    if (_opacity == value)
      return;
    final bool didNeedCompositing = alwaysNeedsCompositing;
    final bool wasVisible = _alpha != 0;
    _opacity = value;
    _alpha = ui.Color.getAlphaFromOpacity(_opacity);
    if (didNeedCompositing != alwaysNeedsCompositing)
      markNeedsCompositingBitsUpdate();
    markNeedsPaint();
    if (wasVisible != (_alpha != 0) && !alwaysIncludeSemantics)
      markNeedsSemanticsUpdate();
  }

  /// Whether child semantics are included regardless of the opacity.
  ///
  /// If false, semantics are excluded when [opacity] is 0.0.
  ///
  /// Defaults to false.
  bool get alwaysIncludeSemantics => _alwaysIncludeSemantics;
  bool _alwaysIncludeSemantics;
  set alwaysIncludeSemantics(bool value) {
    if (value == _alwaysIncludeSemantics)
      return;
    _alwaysIncludeSemantics = value;
    markNeedsSemanticsUpdate();
  }

  @override
  void setupParentData(RenderObject child) {
    if (child.parentData is! SliverPhysicalParentData)
      child.parentData = SliverPhysicalParentData();
  }

  @override
  void performLayout() {
    assert(child != null);
    child.layout(constraints, parentUsesSize: true);
    geometry = child.geometry;
  }

  @override
  bool hitTestChildren(SliverHitTestResult result, {double mainAxisPosition, double crossAxisPosition}) {
    return child != null
      && child.geometry.hitTestExtent > 0
      && child.hitTest(
        result,
        mainAxisPosition: mainAxisPosition,
        crossAxisPosition: crossAxisPosition,
      );
  }

  @override
  double childMainAxisPosition(RenderSliver child) {
    assert(child != null);
    assert(child == this.child);
    return 0.0;
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    void _paintWithOpacity(PaintingContext context, Offset offset) => context.paintChild(child, offset);
    if (child != null && child.geometry.visible) {
      if (_alpha == 0) {
        // No need to keep the layer. We'll create a new one if necessary.
        layer = null;
        return;
      }
      if (_alpha == 255) {
        // No need to keep the layer. We'll create a new one if necessary.
        layer = null;
        context.paintChild(child, offset);
        return;
      }
      assert(needsCompositing);
      layer = context.pushOpacity(
        offset,
        _alpha,
        _paintWithOpacity,
        oldLayer: layer,
      );
    }
  }

  @override
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    assert(child != null);
    final SliverPhysicalParentData childParentData = child.parentData;
    childParentData.applyPaintTransform(transform);
  }

  @override
  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
    if (child != null && (_alpha != 0 || alwaysIncludeSemantics))
      visitor(child);
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DoubleProperty('opacity', opacity));
    properties.add(FlagProperty('alwaysIncludeSemantics', value: alwaysIncludeSemantics, ifTrue: 'alwaysIncludeSemantics'));
  }
}

/// A render object that is invisible during hit testing.
///
/// When [ignoring] is true, this render object (and its subtree) is invisible
/// to hit testing. It still consumes space during layout and paints its sliver
/// child as usual. It just cannot be the target of located events, because its
/// render object returns false from [hitTest].
///
/// When [ignoringSemantics] is true, the subtree will be invisible to the
/// semantics layer (and thus e.g. accessibility tools). If [ignoringSemantics]
/// is null, it uses the value of [ignoring].
class RenderSliverIgnorePointer extends RenderSliver with RenderObjectWithChildMixin<RenderSliver> {
  /// Creates a render object that is invisible to hit testing.
  ///
  /// The [ignoring] argument must not be null. If [ignoringSemantics] is null,
  /// this render object will be ignored for semantics if [ignoring] is true.
  RenderSliverIgnorePointer({
    RenderSliver sliver,
    bool ignoring = true,
    bool ignoringSemantics,
  }) : assert(ignoring != null),
       _ignoring = ignoring,
       _ignoringSemantics = ignoringSemantics {
    child = sliver;
  }

  /// Whether this render object is ignored during hit testing.
  ///
  /// Regardless of whether this render object is ignored during hit testing, it
  /// will still consume space during layout and be visible during painting.
  bool get ignoring => _ignoring;
  bool _ignoring;
  set ignoring(bool value) {
    assert(value != null);
    if (value == _ignoring)
      return;
    _ignoring = value;
    if (_ignoringSemantics == null || !_ignoringSemantics)
      markNeedsSemanticsUpdate();
  }

  /// Whether the semantics of this render object is ignored when compiling the
  /// semantics tree.
  ///
  /// If null, defaults to value of [ignoring].
  ///
  /// See [SemanticsNode] for additional information about the semantics tree.
  bool get ignoringSemantics => _ignoringSemantics;
  bool _ignoringSemantics;
  set ignoringSemantics(bool value) {
    if (value == _ignoringSemantics)
      return;
    final bool oldEffectiveValue = _effectiveIgnoringSemantics;
    _ignoringSemantics = value;
    if (oldEffectiveValue != _effectiveIgnoringSemantics)
      markNeedsSemanticsUpdate();
  }

  bool get _effectiveIgnoringSemantics => ignoringSemantics ?? ignoring;

  @override
  void setupParentData(RenderObject child) {
    if (child.parentData is! SliverPhysicalParentData)
      child.parentData = SliverPhysicalParentData();
  }

  @override
  void performLayout() {
    assert(child != null);
    child.layout(constraints, parentUsesSize: true);
    geometry = child.geometry;
  }

  @override
  bool hitTest(SliverHitTestResult result, {double mainAxisPosition, double crossAxisPosition}) {
    return !ignoring
      && super.hitTest(
        result,
        mainAxisPosition: mainAxisPosition,
        crossAxisPosition: crossAxisPosition,
      );
  }

  @override
  bool hitTestChildren(SliverHitTestResult result, {double mainAxisPosition, double crossAxisPosition}) {
    return child != null
      && child.geometry.hitTestExtent > 0
      && child.hitTest(
        result,
        mainAxisPosition: mainAxisPosition,
        crossAxisPosition: crossAxisPosition,
      );
  }

  @override
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    assert(child != null);
    final SliverPhysicalParentData childParentData = child.parentData;
    childParentData.applyPaintTransform(transform);
  }

  @override
  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
    if (child != null && !_effectiveIgnoringSemantics)
      visitor(child);
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<bool>('ignoring', ignoring));
    properties.add(DiagnosticsProperty<bool>('ignoringSemantics', _effectiveIgnoringSemantics, description: ignoringSemantics == null ? 'implicitly $_effectiveIgnoringSemantics' : null));
  }
}

/// Lays the sliver child out as if it was in the tree, but without painting
/// anything, without making the sliver child available for hit testing, and
/// without taking any room in the parent.
class RenderSliverOffstage extends RenderSliver with RenderObjectWithChildMixin<RenderSliver> {
  /// Creates an offstage render object.
  RenderSliverOffstage({
    bool offstage = true,
    RenderSliver sliver,
  }) : assert(offstage != null),
       _offstage = offstage {
    child = sliver;
  }

  /// Whether the sliver child is hidden from the rest of the tree.
  ///
  /// If true, the sliver child is laid out as if it was in the tree, but
  /// without painting anything, without making the sliver child available for
  /// hit testing, and without taking any room in the parent.
  ///
  /// If false, the sliver child is included in the tree as normal.
  bool get offstage => _offstage;
  bool _offstage;

  set offstage(bool value) {
    assert(value != null);
    if (value == _offstage)
      return;
    _offstage = value;
    markNeedsLayoutForSizedByParentChange();
  }

  @override
  void setupParentData(RenderObject child) {
    if (child.parentData is! SliverPhysicalParentData)
      child.parentData = SliverPhysicalParentData();
  }

  @override
  void performLayout() {
    assert(child != null);
    child.layout(constraints, parentUsesSize: true);
    geometry = child.geometry;
  }

  @override
  bool hitTest(SliverHitTestResult result, {double mainAxisPosition, double crossAxisPosition}) {
    return !offstage && super.hitTest(
      result,
      mainAxisPosition: mainAxisPosition,
      crossAxisPosition: crossAxisPosition,
    );
  }

  @override
  bool hitTestChildren(SliverHitTestResult result, {double mainAxisPosition, double crossAxisPosition}) {
    return !offstage
      && child != null
      && child.geometry.hitTestExtent > 0
      && child.hitTest(
        result,
        mainAxisPosition: mainAxisPosition,
        crossAxisPosition: crossAxisPosition,
      );
  }

  @override
  double childMainAxisPosition(RenderSliver child) {
    assert(child != null);
    assert(child == this.child);
    return 0.0;
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    if (offstage)
      return;
    super.paint(context, offset);
  }

  @override
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    assert(child != null);
    final SliverPhysicalParentData childParentData = child.parentData;
    childParentData.applyPaintTransform(transform);
  }

  @override
  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
    if (offstage)
      return;
    super.visitChildrenForSemantics(visitor);
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<bool>('offstage', offstage));
  }

  @override
  List<DiagnosticsNode> debugDescribeChildren() {
    if (child == null)
      return <DiagnosticsNode>[];
    return <DiagnosticsNode>[
      child.toDiagnosticsNode(
        name: 'child',
        style: offstage ? DiagnosticsTreeStyle.offstage : DiagnosticsTreeStyle.sparse,
      ),
    ];
  }
}
