action_sheet.dart

// 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:ui' show ImageFilter;

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

import 'colors.dart';
import 'interface_level.dart';
import 'scrollbar.dart';
import 'theme.dart';

const TextStyle _kActionSheetActionStyle = TextStyle(
  fontFamily: '.SF UI Text',
  inherit: false,
  fontSize: 20.0,
  fontWeight: FontWeight.w400,
  textBaseline: TextBaseline.alphabetic,
);

const TextStyle _kActionSheetContentStyle = TextStyle(
  fontFamily: '.SF UI Text',
  inherit: false,
  fontSize: 13.0,
  fontWeight: FontWeight.w400,
  color: _kContentTextColor,
  textBaseline: TextBaseline.alphabetic,
);

// Translucent, very light gray that is painted on top of the blurred backdrop
// as the action sheet's background color.
// TODO(LongCatIsLooong): https://github.com/flutter/flutter/issues/39272. Use
// System Materials once we have them.
// Extracted from https://developer.apple.com/design/resources/.
const Color _kBackgroundColor = CupertinoDynamicColor.withBrightness(
  color: Color(0xC7F9F9F9),
  darkColor: Color(0xC7252525),
);

// Translucent, light gray that is painted on top of the blurred backdrop as
// the background color of a pressed button.
// Eye-balled from iOS 13 beta simulator.
const Color _kPressedColor = CupertinoDynamicColor.withBrightness(
  color: Color(0xFFE1E1E1),
  darkColor: Color(0xFF2E2E2E),
);

const Color _kCancelPressedColor = CupertinoDynamicColor.withBrightness(
  color: Color(0xFFECECEC),
  darkColor: Color(0xFF49494B),
);

// The gray color used for text that appears in the title area.
// Extracted from https://developer.apple.com/design/resources/.
const Color _kContentTextColor = Color(0xFF8F8F8F);

// Translucent gray that is painted on top of the blurred backdrop in the gap
// areas between the content section and actions section, as well as between
// buttons.
// Eye-balled from iOS 13 beta simulator.
const Color _kButtonDividerColor = _kContentTextColor;

const double _kBlurAmount = 20.0;
const double _kEdgeHorizontalPadding = 8.0;
const double _kCancelButtonPadding = 8.0;
const double _kEdgeVerticalPadding = 10.0;
const double _kContentHorizontalPadding = 40.0;
const double _kContentVerticalPadding = 14.0;
const double _kButtonHeight = 56.0;
const double _kCornerRadius = 14.0;
const double _kDividerThickness = 1.0;

/// An iOS-style action sheet.
///
/// An action sheet is a specific style of alert that presents the user
/// with a set of two or more choices related to the current context.
/// An action sheet can have a title, an additional message, and a list
/// of actions. The title is displayed above the message and the actions
/// are displayed below this content.
///
/// This action sheet styles its title and message to match standard iOS action
/// sheet title and message text style.
///
/// To display action buttons that look like standard iOS action sheet buttons,
/// provide [CupertinoActionSheetAction]s for the [actions] given to this action sheet.
///
/// To include a iOS-style cancel button separate from the other buttons,
/// provide an [CupertinoActionSheetAction] for the [cancelButton] given to this
/// action sheet.
///
/// An action sheet is typically passed as the child widget to
/// [showCupertinoModalPopup], which displays the action sheet by sliding it up
/// from the bottom of the screen.
///
/// See also:
///
///  * [CupertinoActionSheetAction], which is an iOS-style action sheet button.
///  * <https://developer.apple.com/design/human-interface-guidelines/ios/views/action-sheets/>
class CupertinoActionSheet extends StatelessWidget {
  /// Creates an iOS-style action sheet.
  ///
  /// An action sheet must have a non-null value for at least one of the
  /// following arguments: [actions], [title], [message], or [cancelButton].
  ///
  /// Generally, action sheets are used to give the user a choice between
  /// two or more choices for the current context.
  const CupertinoActionSheet({
    Key key,
    this.title,
    this.message,
    this.actions,
    this.messageScrollController,
    this.actionScrollController,
    this.cancelButton,
  }) : assert(actions != null || title != null || message != null || cancelButton != null,
          'An action sheet must have a non-null value for at least one of the following arguments: '
          'actions, title, message, or cancelButton'),
       super(key: key);

  /// An optional title of the action sheet. When the [message] is non-null,
  /// the font of the [title] is bold.
  ///
  /// Typically a [Text] widget.
  final Widget title;

  /// An optional descriptive message that provides more details about the
  /// reason for the alert.
  ///
  /// Typically a [Text] widget.
  final Widget message;

  /// The set of actions that are displayed for the user to select.
  ///
  /// Typically this is a list of [CupertinoActionSheetAction] widgets.
  final List<Widget> actions;

  /// A scroll controller that can be used to control the scrolling of the
  /// [message] in the action sheet.
  ///
  /// This attribute is typically not needed, as alert messages should be
  /// short.
  final ScrollController messageScrollController;

  /// A scroll controller that can be used to control the scrolling of the
  /// [actions] in the action sheet.
  ///
  /// This attribute is typically not needed.
  final ScrollController actionScrollController;

  /// The optional cancel button that is grouped separately from the other
  /// actions.
  ///
  /// Typically this is an [CupertinoActionSheetAction] widget.
  final Widget cancelButton;

  Widget _buildContent(BuildContext context) {
    final List<Widget> content = <Widget>[];
    if (title != null || message != null) {
      final Widget titleSection = _CupertinoAlertContentSection(
        title: title,
        message: message,
        scrollController: messageScrollController,
      );
      content.add(Flexible(child: titleSection));
    }

    return Container(
      color: CupertinoDynamicColor.resolve(_kBackgroundColor, context),
      child: Column(
        mainAxisSize: MainAxisSize.min,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: content,
      ),
    );
  }

  Widget _buildActions() {
    if (actions == null || actions.isEmpty) {
      return Container(
        height: 0.0,
      );
    }
    return Container(
      child: _CupertinoAlertActionSection(
        children: actions,
        scrollController: actionScrollController,
        hasCancelButton: cancelButton != null,
      ),
    );
  }

  Widget _buildCancelButton() {
    final double cancelPadding = (actions != null || message != null || title != null)
        ? _kCancelButtonPadding : 0.0;
    return Padding(
      padding: EdgeInsets.only(top: cancelPadding),
      child: _CupertinoActionSheetCancelButton(
        child: cancelButton,
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    final List<Widget> children = <Widget>[
      Flexible(child: ClipRRect(
          borderRadius: BorderRadius.circular(12.0),
          child: BackdropFilter(
            filter: ImageFilter.blur(sigmaX: _kBlurAmount, sigmaY: _kBlurAmount),
            child: _CupertinoAlertRenderWidget(
              contentSection: Builder(builder: _buildContent),
              actionsSection: _buildActions(),
            ),
          ),
        ),
      ),
      if (cancelButton != null) _buildCancelButton(),
    ];

    final Orientation orientation = MediaQuery.of(context).orientation;
    double actionSheetWidth;
    if (orientation == Orientation.portrait) {
      actionSheetWidth = MediaQuery.of(context).size.width - (_kEdgeHorizontalPadding * 2);
    } else {
      actionSheetWidth = MediaQuery.of(context).size.height - (_kEdgeHorizontalPadding * 2);
    }

    return SafeArea(
      child: Semantics(
        namesRoute: true,
        scopesRoute: true,
        explicitChildNodes: true,
        label: 'Alert',
        child: CupertinoUserInterfaceLevel(
          data: CupertinoUserInterfaceLevelData.elevated,
          child: Container(
            width: actionSheetWidth,
            margin: const EdgeInsets.symmetric(
              horizontal: _kEdgeHorizontalPadding,
              vertical: _kEdgeVerticalPadding,
            ),
            child: Column(
              children: children,
              mainAxisSize: MainAxisSize.min,
              crossAxisAlignment: CrossAxisAlignment.stretch,
            ),
          ),
        ),
      ),
    );
  }
}

/// A button typically used in a [CupertinoActionSheet].
///
/// See also:
///
///  * [CupertinoActionSheet], an alert that presents the user with a set of two or
///    more choices related to the current context.
class CupertinoActionSheetAction extends StatelessWidget {
  /// Creates an action for an iOS-style action sheet.
  ///
  /// The [child] and [onPressed] arguments must not be null.
  const CupertinoActionSheetAction({
    @required this.onPressed,
    this.isDefaultAction = false,
    this.isDestructiveAction = false,
    @required this.child,
  }) : assert(child != null),
       assert(onPressed != null);

  /// The callback that is called when the button is tapped.
  ///
  /// This attribute must not be null.
  final VoidCallback onPressed;

  /// Whether this action is the default choice in the action sheet.
  ///
  /// Default buttons have bold text.
  final bool isDefaultAction;

  /// Whether this action might change or delete data.
  ///
  /// Destructive buttons have red text.
  final bool isDestructiveAction;

  /// The widget below this widget in the tree.
  ///
  /// Typically a [Text] widget.
  final Widget child;

  @override
  Widget build(BuildContext context) {
    TextStyle style = _kActionSheetActionStyle.copyWith(
      color: isDestructiveAction
        ? CupertinoDynamicColor.resolve(CupertinoColors.systemRed, context)
        : CupertinoTheme.of(context).primaryColor,
    );

    if (isDefaultAction) {
      style = style.copyWith(fontWeight: FontWeight.w600);
    }

    return GestureDetector(
      onTap: onPressed,
      behavior: HitTestBehavior.opaque,
      child: ConstrainedBox(
        constraints: const BoxConstraints(
          minHeight: _kButtonHeight,
        ),
        child: Semantics(
          button: true,
          child: Container(
            alignment: Alignment.center,
            padding: const EdgeInsets.symmetric(
              vertical: 16.0,
              horizontal: 10.0,
            ),
            child: DefaultTextStyle(
              style: style,
              child: child,
              textAlign: TextAlign.center,
            ),
          ),
        ),
      ),
    );
  }
}

class _CupertinoActionSheetCancelButton extends StatefulWidget {
  const _CupertinoActionSheetCancelButton({
    Key key,
    this.child,
  }) : super(key: key);

  final Widget child;

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

class _CupertinoActionSheetCancelButtonState extends State<_CupertinoActionSheetCancelButton> {
  bool isBeingPressed = false;

  void _onTapDown(TapDownDetails event) {
    setState(() { isBeingPressed = true; });
  }

  void _onTapUp(TapUpDetails event) {
    setState(() { isBeingPressed = false; });
  }

  void _onTapCancel() {
    setState(() { isBeingPressed = false; });
  }

  @override
  Widget build(BuildContext context) {
    final Color backgroundColor = isBeingPressed
      ? _kCancelPressedColor
      : CupertinoColors.secondarySystemGroupedBackground;
    return GestureDetector(
      excludeFromSemantics: true,
      onTapDown: _onTapDown,
      onTapUp: _onTapUp,
      onTapCancel: _onTapCancel,
      child: Container(
        decoration: BoxDecoration(
          color: CupertinoDynamicColor.resolve(backgroundColor, context),
          borderRadius: BorderRadius.circular(_kCornerRadius),
        ),
        child: widget.child,
      ),
    );
  }
}

class _CupertinoAlertRenderWidget extends RenderObjectWidget {
  const _CupertinoAlertRenderWidget({
    Key key,
    @required this.contentSection,
    @required this.actionsSection,
  }) : super(key: key);

  final Widget contentSection;
  final Widget actionsSection;

  @override
  RenderObject createRenderObject(BuildContext context) {
    return _RenderCupertinoAlert(
      dividerThickness: _kDividerThickness / MediaQuery.of(context).devicePixelRatio,
      dividerColor: CupertinoDynamicColor.resolve(_kButtonDividerColor, context),
    );
  }

  @override
  void updateRenderObject(BuildContext context, _RenderCupertinoAlert renderObject) {
    super.updateRenderObject(context, renderObject);
    renderObject.dividerColor = CupertinoDynamicColor.resolve(_kButtonDividerColor, context);
  }

  @override
  RenderObjectElement createElement() {
    return _CupertinoAlertRenderElement(this);
  }
}

class _CupertinoAlertRenderElement extends RenderObjectElement {
  _CupertinoAlertRenderElement(_CupertinoAlertRenderWidget widget) : super(widget);

  Element _contentElement;
  Element _actionsElement;

  @override
  _CupertinoAlertRenderWidget get widget => super.widget;

  @override
  _RenderCupertinoAlert get renderObject => super.renderObject;

  @override
  void visitChildren(ElementVisitor visitor) {
    if (_contentElement != null) {
      visitor(_contentElement);
    }
    if (_actionsElement != null) {
      visitor(_actionsElement);
    }
  }

  @override
  void mount(Element parent, dynamic newSlot) {
    super.mount(parent, newSlot);
    _contentElement = updateChild(_contentElement,
        widget.contentSection, _AlertSections.contentSection);
    _actionsElement = updateChild(_actionsElement,
        widget.actionsSection, _AlertSections.actionsSection);
  }

  @override
  void insertChildRenderObject(RenderObject child, _AlertSections slot) {
    _placeChildInSlot(child, slot);
  }

  @override
  void moveChildRenderObject(RenderObject child, _AlertSections slot) {
    _placeChildInSlot(child, slot);
  }

  @override
  void update(RenderObjectWidget newWidget) {
    super.update(newWidget);
    _contentElement = updateChild(_contentElement,
        widget.contentSection, _AlertSections.contentSection);
    _actionsElement = updateChild(_actionsElement,
        widget.actionsSection, _AlertSections.actionsSection);
  }

  @override
  void forgetChild(Element child) {
    assert(child == _contentElement || child == _actionsElement);
    if (_contentElement == child) {
      _contentElement = null;
    } else if (_actionsElement == child) {
      _actionsElement = null;
    }
  }

  @override
  void removeChildRenderObject(RenderObject child) {
    assert(child == renderObject.contentSection || child == renderObject.actionsSection);
    if (renderObject.contentSection == child) {
      renderObject.contentSection = null;
    } else if (renderObject.actionsSection == child) {
      renderObject.actionsSection = null;
    }
  }

  void _placeChildInSlot(RenderObject child, _AlertSections slot) {
    assert(slot != null);
    switch (slot) {
      case _AlertSections.contentSection:
        renderObject.contentSection = child;
        break;
      case _AlertSections.actionsSection:
        renderObject.actionsSection = child;
        break;
    }
  }
}

// An iOS-style layout policy for sizing an alert's content section and action
// button section.
//
// The policy is as follows:
//
// If all content and buttons fit on the screen:
// The content section and action button section are sized intrinsically.
//
// If all content and buttons do not fit on the screen:
// A minimum height for the action button section is calculated. The action
// button section will not be rendered shorter than this minimum.  See
// _RenderCupertinoAlertActions for the minimum height calculation.
//
// With the minimum action button section calculated, the content section can
// take up as much of the remaining space as it needs.
//
// After the content section is laid out, the action button section is allowed
// to take up any remaining space that was not consumed by the content section.
class _RenderCupertinoAlert extends RenderBox {
  _RenderCupertinoAlert({
    RenderBox contentSection,
    RenderBox actionsSection,
    double dividerThickness = 0.0,
    @required Color dividerColor,
  }) : assert(dividerColor != null),
       _contentSection = contentSection,
       _actionsSection = actionsSection,
       _dividerThickness = dividerThickness,
       _dividerPaint = Paint()
        ..color = dividerColor
        ..style = PaintingStyle.fill;

  RenderBox get contentSection => _contentSection;
  RenderBox _contentSection;
  set contentSection(RenderBox newContentSection) {
    if (newContentSection != _contentSection) {
      if (null != _contentSection) {
        dropChild(_contentSection);
      }
      _contentSection = newContentSection;
      if (null != _contentSection) {
        adoptChild(_contentSection);
      }
    }
  }

  RenderBox get actionsSection => _actionsSection;
  RenderBox _actionsSection;
  set actionsSection(RenderBox newActionsSection) {
    if (newActionsSection != _actionsSection) {
      if (null != _actionsSection) {
        dropChild(_actionsSection);
      }
      _actionsSection = newActionsSection;
      if (null != _actionsSection) {
        adoptChild(_actionsSection);
      }
    }
  }

  Color get dividerColor => _dividerPaint.color;
  set dividerColor(Color value) {
    if (value == _dividerPaint.color)
      return;
    _dividerPaint.color = value;
    markNeedsPaint();
  }

  final double _dividerThickness;

  final Paint _dividerPaint;

  @override
  void attach(PipelineOwner owner) {
    super.attach(owner);
    if (null != contentSection) {
      contentSection.attach(owner);
    }
    if (null != actionsSection) {
      actionsSection.attach(owner);
    }
  }

  @override
  void detach() {
    super.detach();
    if (null != contentSection) {
      contentSection.detach();
    }
    if (null != actionsSection) {
      actionsSection.detach();
    }
  }

  @override
  void redepthChildren() {
    if (null != contentSection) {
      redepthChild(contentSection);
    }
    if (null != actionsSection) {
      redepthChild(actionsSection);
    }
  }

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

  @override
  void visitChildren(RenderObjectVisitor visitor) {
    if (contentSection != null) {
      visitor(contentSection);
    }
    if (actionsSection != null) {
      visitor(actionsSection);
    }
  }

  @override
  List<DiagnosticsNode> debugDescribeChildren() {
    final List<DiagnosticsNode> value = <DiagnosticsNode>[];
    if (contentSection != null) {
      value.add(contentSection.toDiagnosticsNode(name: 'content'));
    }
    if (actionsSection != null) {
      value.add(actionsSection.toDiagnosticsNode(name: 'actions'));
    }
    return value;
  }

  @override
  double computeMinIntrinsicWidth(double height) {
    return constraints.minWidth;
  }

  @override
  double computeMaxIntrinsicWidth(double height) {
    return constraints.maxWidth;
  }

  @override
  double computeMinIntrinsicHeight(double width) {
    final double contentHeight = contentSection.getMinIntrinsicHeight(width);
    final double actionsHeight = actionsSection.getMinIntrinsicHeight(width);
    final bool hasDivider = contentHeight > 0.0 && actionsHeight > 0.0;
    double height = contentHeight + (hasDivider ? _dividerThickness : 0.0) + actionsHeight;

    if (actionsHeight > 0 || contentHeight > 0)
      height -= 2 * _kEdgeVerticalPadding;
    if (height.isFinite)
      return height;
    return 0.0;
  }

  @override
  double computeMaxIntrinsicHeight(double width) {
    final double contentHeight = contentSection.getMaxIntrinsicHeight(width);
    final double actionsHeight = actionsSection.getMaxIntrinsicHeight(width);
    final bool hasDivider = contentHeight > 0.0 && actionsHeight > 0.0;
    double height = contentHeight + (hasDivider ? _dividerThickness : 0.0) + actionsHeight;

    if (actionsHeight > 0 || contentHeight > 0)
      height -= 2 * _kEdgeVerticalPadding;
    if (height.isFinite)
      return height;
    return 0.0;
  }

  @override
  void performLayout() {
    final bool hasDivider = contentSection.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0
        && actionsSection.getMaxIntrinsicHeight(constraints.maxWidth) > 0.0;
    final double dividerThickness = hasDivider ? _dividerThickness : 0.0;

    final double minActionsHeight = actionsSection.getMinIntrinsicHeight(constraints.maxWidth);

    // Size alert content.
    contentSection.layout(
      constraints.deflate(EdgeInsets.only(bottom: minActionsHeight + dividerThickness)),
      parentUsesSize: true,
    );
    final Size contentSize = contentSection.size;

    // Size alert actions.
    actionsSection.layout(
      constraints.deflate(EdgeInsets.only(top: contentSize.height + dividerThickness)),
      parentUsesSize: true,
    );
    final Size actionsSize = actionsSection.size;

    // Calculate overall alert height.
    final double actionSheetHeight = contentSize.height + dividerThickness + actionsSize.height;

    // Set our size now that layout calculations are complete.
    size = Size(constraints.maxWidth, actionSheetHeight);

    // Set the position of the actions box to sit at the bottom of the alert.
    // The content box defaults to the top left, which is where we want it.
    assert(actionsSection.parentData is MultiChildLayoutParentData);
    final MultiChildLayoutParentData actionParentData = actionsSection.parentData;
    actionParentData.offset = Offset(0.0, contentSize.height + dividerThickness);
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    final MultiChildLayoutParentData contentParentData = contentSection.parentData;
    contentSection.paint(context, offset + contentParentData.offset);

    final bool hasDivider = contentSection.size.height > 0.0 && actionsSection.size.height > 0.0;
    if (hasDivider) {
      _paintDividerBetweenContentAndActions(context.canvas, offset);
    }

    final MultiChildLayoutParentData actionsParentData = actionsSection.parentData;
    actionsSection.paint(context, offset + actionsParentData.offset);
  }

  void _paintDividerBetweenContentAndActions(Canvas canvas, Offset offset) {
    canvas.drawRect(
      Rect.fromLTWH(
        offset.dx,
        offset.dy + contentSection.size.height,
        size.width,
        _dividerThickness,
      ),
      _dividerPaint,
    );
  }

  @override
  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
    final MultiChildLayoutParentData contentSectionParentData = contentSection.parentData;
    final MultiChildLayoutParentData actionsSectionParentData = actionsSection.parentData;
    return result.addWithPaintOffset(
             offset: contentSectionParentData.offset,
             position: position,
             hitTest: (BoxHitTestResult result, Offset transformed) {
               assert(transformed == position - contentSectionParentData.offset);
               return contentSection.hitTest(result, position: transformed);
             },
           )
        || result.addWithPaintOffset(
             offset: actionsSectionParentData.offset,
             position: position,
             hitTest: (BoxHitTestResult result, Offset transformed) {
               assert(transformed == position - actionsSectionParentData.offset);
               return actionsSection.hitTest(result, position: transformed);
             },
           );
  }
}

// Visual components of an alert that need to be explicitly sized and
// laid out at runtime.
enum _AlertSections {
  contentSection,
  actionsSection,
}

// The "content section" of a CupertinoActionSheet.
//
// If title is missing, then only content is added.  If content is
// missing, then only a title is added. If both are missing, then it returns
// a SingleChildScrollView with a zero-sized Container.
class _CupertinoAlertContentSection extends StatelessWidget {
  const _CupertinoAlertContentSection({
    Key key,
    this.title,
    this.message,
    this.scrollController,
  }) : super(key: key);

  // An optional title of the action sheet. When the message is non-null,
  // the font of the title is bold.
  //
  // Typically a Text widget.
  final Widget title;

  // An optional descriptive message that provides more details about the
  // reason for the alert.
  //
  // Typically a Text widget.
  final Widget message;

  // A scroll controller that can be used to control the scrolling of the
  // content in the action sheet.
  //
  // Defaults to null, and is typically not needed, since most alert contents
  // are short.
  final ScrollController scrollController;

  @override
  Widget build(BuildContext context) {
    final List<Widget> titleContentGroup = <Widget>[];

    if (title != null) {
      titleContentGroup.add(Padding(
        padding: const EdgeInsets.only(
          left: _kContentHorizontalPadding,
          right: _kContentHorizontalPadding,
          bottom: _kContentVerticalPadding,
          top: _kContentVerticalPadding,
        ),
        child: DefaultTextStyle(
          style: message == null ? _kActionSheetContentStyle
              : _kActionSheetContentStyle.copyWith(fontWeight: FontWeight.w600),
          textAlign: TextAlign.center,
          child: title,
        ),
      ));
    }

    if (message != null) {
      titleContentGroup.add(
        Padding(
          padding: EdgeInsets.only(
            left: _kContentHorizontalPadding,
            right: _kContentHorizontalPadding,
            bottom: title == null ? _kContentVerticalPadding : 22.0,
            top: title == null ? _kContentVerticalPadding : 0.0,
          ),
          child: DefaultTextStyle(
            style: title == null ? _kActionSheetContentStyle.copyWith(fontWeight: FontWeight.w600)
                : _kActionSheetContentStyle,
            textAlign: TextAlign.center,
            child: message,
          ),
        ),
      );
    }

    if (titleContentGroup.isEmpty) {
      return SingleChildScrollView(
        controller: scrollController,
        child: Container(
          width: 0.0,
          height: 0.0,
        ),
      );
    }

    // Add padding between the widgets if necessary.
    if (titleContentGroup.length > 1) {
      titleContentGroup.insert(1, const Padding(padding: EdgeInsets.only(top: 8.0)));
    }

    return CupertinoScrollbar(
      child: SingleChildScrollView(
        controller: scrollController,
        child: Column(
          mainAxisSize: MainAxisSize.max,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: titleContentGroup,
        ),
      ),
    );
  }
}

// The "actions section" of a CupertinoActionSheet.
//
// See _RenderCupertinoAlertActions for details about action button sizing
// and layout.
class _CupertinoAlertActionSection extends StatefulWidget {
  const _CupertinoAlertActionSection({
    Key key,
    @required this.children,
    this.scrollController,
    this.hasCancelButton,
  }) : assert(children != null),
       super(key: key);

  final List<Widget> children;

  // A scroll controller that can be used to control the scrolling of the
  // actions in the action sheet.
  //
  // Defaults to null, and is typically not needed, since most alerts
  // don't have many actions.
  final ScrollController scrollController;

  final bool hasCancelButton;

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

class _CupertinoAlertActionSectionState extends State<_CupertinoAlertActionSection> {
  @override
  Widget build(BuildContext context) {
    final double devicePixelRatio = MediaQuery.of(context).devicePixelRatio;

    final List<Widget> interactiveButtons = <Widget>[];
    for (int i = 0; i < widget.children.length; i += 1) {
      interactiveButtons.add(
        _PressableActionButton(
          child: widget.children[i],
        ),
      );
    }

    return CupertinoScrollbar(
      child: SingleChildScrollView(
        controller: widget.scrollController,
        child: _CupertinoAlertActionsRenderWidget(
          actionButtons: interactiveButtons,
          dividerThickness: _kDividerThickness / devicePixelRatio,
          hasCancelButton: widget.hasCancelButton,
        ),
      ),
    );
  }
}

// A button that updates its render state when pressed.
//
// The pressed state is forwarded to an _ActionButtonParentDataWidget. The
// corresponding _ActionButtonParentData is then interpreted and rendered
// appropriately by _RenderCupertinoAlertActions.
class _PressableActionButton extends StatefulWidget {
  const _PressableActionButton({
    @required this.child,
  });

  final Widget child;

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

class _PressableActionButtonState extends State<_PressableActionButton> {
  bool _isPressed = false;

  @override
  Widget build(BuildContext context) {
    return _ActionButtonParentDataWidget(
      isPressed: _isPressed,
      // TODO(mattcarroll): Button press dynamics need overhaul for iOS: https://github.com/flutter/flutter/issues/19786
      child: GestureDetector(
        excludeFromSemantics: true,
        behavior: HitTestBehavior.opaque,
        onTapDown: (TapDownDetails details) => setState(() => _isPressed = true),
        onTapUp: (TapUpDetails details) => setState(() => _isPressed = false),
        // TODO(mattcarroll): Cancel is currently triggered when user moves past slop instead of off button: https://github.com/flutter/flutter/issues/19783
        onTapCancel: () => setState(() => _isPressed = false),
        child: widget.child,
      ),
    );
  }
}

// ParentDataWidget that updates _ActionButtonParentData for an action button.
//
// Each action button requires knowledge of whether or not it is pressed so that
// the alert can correctly render the button. The pressed state is held within
// _ActionButtonParentData. _ActionButtonParentDataWidget is responsible for
// updating the pressed state of an _ActionButtonParentData based on the
// incoming isPressed property.
class _ActionButtonParentDataWidget extends ParentDataWidget<_CupertinoAlertActionsRenderWidget> {
  const _ActionButtonParentDataWidget({
    Key key,
    this.isPressed,
    @required Widget child,
  }) : super(key: key, child: child);

  final bool isPressed;

  @override
  void applyParentData(RenderObject renderObject) {
    assert(renderObject.parentData is _ActionButtonParentData);
    final _ActionButtonParentData parentData = renderObject.parentData;
    if (parentData.isPressed != isPressed) {
      parentData.isPressed = isPressed;

      // Force a repaint.
      final AbstractNode targetParent = renderObject.parent;
      if (targetParent is RenderObject)
        targetParent.markNeedsPaint();
    }
  }
}

// ParentData applied to individual action buttons that report whether or not
// that button is currently pressed by the user.
class _ActionButtonParentData extends MultiChildLayoutParentData {
  _ActionButtonParentData({
    this.isPressed = false,
  });

  bool isPressed;
}

// An iOS-style alert action button layout.
//
// See _RenderCupertinoAlertActions for specific layout policy details.
class _CupertinoAlertActionsRenderWidget extends MultiChildRenderObjectWidget {
  _CupertinoAlertActionsRenderWidget({
    Key key,
    @required List<Widget> actionButtons,
    double dividerThickness = 0.0,
    bool hasCancelButton = false,
  }) : _dividerThickness = dividerThickness,
       _hasCancelButton = hasCancelButton,
       super(key: key, children: actionButtons);

  final double _dividerThickness;
  final bool _hasCancelButton;

  @override
  RenderObject createRenderObject(BuildContext context) {
    return _RenderCupertinoAlertActions(
      dividerThickness: _dividerThickness,
      dividerColor: CupertinoDynamicColor.resolve(_kButtonDividerColor, context),
      hasCancelButton: _hasCancelButton,
      backgroundColor: CupertinoDynamicColor.resolve(_kBackgroundColor, context),
      pressedColor: CupertinoDynamicColor.resolve(_kPressedColor, context),
    );
  }

  @override
  void updateRenderObject(BuildContext context, _RenderCupertinoAlertActions renderObject) {
    renderObject
      ..dividerThickness = _dividerThickness
      ..dividerColor = CupertinoDynamicColor.resolve(_kButtonDividerColor, context)
      ..hasCancelButton = _hasCancelButton
      ..backgroundColor = CupertinoDynamicColor.resolve(_kBackgroundColor, context)
      ..pressedColor = CupertinoDynamicColor.resolve(_kPressedColor, context);
  }
}

// An iOS-style layout policy for sizing and positioning an action sheet's
// buttons.
//
// The policy is as follows:
//
// Action sheet buttons are always stacked vertically. In the case where the
// content section and the action section combined can not fit on the screen
// without scrolling, the height of the action section is determined as
// follows.
//
// If the user has included a separate cancel button, the height of the action
// section can be up to the height of 3 action buttons (i.e., the user can
// include 1, 2, or 3 action buttons and they will appear without needing to
// be scrolled). If 4+ action buttons are provided, the height of the action
// section shrinks to 1.5 buttons tall, and is scrollable.
//
// If the user has not included a separate cancel button, the height of the
// action section is at most 1.5 buttons tall.
class _RenderCupertinoAlertActions extends RenderBox
    with ContainerRenderObjectMixin<RenderBox, MultiChildLayoutParentData>,
        RenderBoxContainerDefaultsMixin<RenderBox, MultiChildLayoutParentData> {
  _RenderCupertinoAlertActions({
    List<RenderBox> children,
    double dividerThickness = 0.0,
    @required Color dividerColor,
    bool hasCancelButton = false,
    Color backgroundColor,
    Color pressedColor,
  }) : _dividerThickness = dividerThickness,
       _hasCancelButton = hasCancelButton,
       _buttonBackgroundPaint = Paint()
          ..style = PaintingStyle.fill
          ..color = backgroundColor,
       _pressedButtonBackgroundPaint = Paint()
          ..style = PaintingStyle.fill
          ..color = pressedColor,
       _dividerPaint = Paint()
          ..color = dividerColor
          ..style = PaintingStyle.fill {
    addAll(children);
  }

  // The thickness of the divider between buttons.
  double get dividerThickness => _dividerThickness;
  double _dividerThickness;
  set dividerThickness(double newValue) {
    if (newValue == _dividerThickness) {
      return;
    }

    _dividerThickness = newValue;
    markNeedsLayout();
  }

  Color get backgroundColor => _buttonBackgroundPaint.color;
  set backgroundColor(Color newValue) {
    if (newValue == _buttonBackgroundPaint.color) {
      return;
    }

    _buttonBackgroundPaint.color = newValue;
    markNeedsPaint();
  }

  Color get pressedColor => _pressedButtonBackgroundPaint.color;
  set pressedColor(Color newValue) {
    if (newValue == _pressedButtonBackgroundPaint.color) {
      return;
    }

    _pressedButtonBackgroundPaint.color = newValue;
    markNeedsPaint();
  }

  Color get dividerColor => _dividerPaint.color;
  set dividerColor(Color value) {
    if (value == _dividerPaint.color) {
      return;
    }
    _dividerPaint.color = value;
    markNeedsPaint();
  }

  bool _hasCancelButton;
  bool get hasCancelButton => _hasCancelButton;
  set hasCancelButton(bool newValue) {
    if (newValue == _hasCancelButton) {
      return;
    }

    _hasCancelButton = newValue;
    markNeedsLayout();
  }

  final Paint _buttonBackgroundPaint;
  final Paint _pressedButtonBackgroundPaint;

  final Paint _dividerPaint;

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

  @override
  double computeMinIntrinsicWidth(double height) {
    return constraints.minWidth;
  }

  @override
  double computeMaxIntrinsicWidth(double height) {
    return constraints.maxWidth;
  }

  @override
  double computeMinIntrinsicHeight(double width) {
    if (childCount == 0)
      return 0.0;
    if (childCount == 1)
      return firstChild.computeMaxIntrinsicHeight(width) + dividerThickness;
    if (hasCancelButton && childCount < 4)
      return _computeMinIntrinsicHeightWithCancel(width);
    return _computeMinIntrinsicHeightWithoutCancel(width);
  }

  // The minimum height for more than 2-3 buttons when a cancel button is
  // included is the full height of button stack.
  double _computeMinIntrinsicHeightWithCancel(double width) {
    assert(childCount == 2 || childCount == 3);
    if (childCount == 2) {
      return firstChild.getMinIntrinsicHeight(width)
        + childAfter(firstChild).getMinIntrinsicHeight(width)
        + dividerThickness;
    }
    return firstChild.getMinIntrinsicHeight(width)
      + childAfter(firstChild).getMinIntrinsicHeight(width)
      + childAfter(childAfter(firstChild)).getMinIntrinsicHeight(width)
      + (dividerThickness * 2);
  }

  // The minimum height for more than 2 buttons when no cancel button or 4+
  // buttons when a cancel button is included is the height of the 1st button
  // + 50% the height of the 2nd button + 2 dividers.
  double _computeMinIntrinsicHeightWithoutCancel(double width) {
    assert(childCount >= 2);
    return firstChild.getMinIntrinsicHeight(width)
      + dividerThickness
      + (0.5 * childAfter(firstChild).getMinIntrinsicHeight(width));
  }

  @override
  double computeMaxIntrinsicHeight(double width) {
    if (childCount == 0)
      return 0.0;
    if (childCount == 1)
      return firstChild.computeMaxIntrinsicHeight(width) + dividerThickness;
    return _computeMaxIntrinsicHeightStacked(width);
  }

  // Max height of a stack of buttons is the sum of all button heights + a
  // divider for each button.
  double _computeMaxIntrinsicHeightStacked(double width) {
    assert(childCount >= 2);

    final double allDividersHeight = (childCount - 1) * dividerThickness;
    double heightAccumulation = allDividersHeight;
    RenderBox button = firstChild;
    while (button != null) {
      heightAccumulation += button.getMaxIntrinsicHeight(width);
      button = childAfter(button);
    }
    return heightAccumulation;
  }

  @override
  void performLayout() {
    final BoxConstraints perButtonConstraints = constraints.copyWith(
      minHeight: 0.0,
      maxHeight: double.infinity,
    );

    RenderBox child = firstChild;
    int index = 0;
    double verticalOffset = 0.0;
    while (child != null) {
      child.layout(
        perButtonConstraints,
        parentUsesSize: true,
      );

      assert(child.parentData is MultiChildLayoutParentData);
      final MultiChildLayoutParentData parentData = child.parentData;
      parentData.offset = Offset(0.0, verticalOffset);

      verticalOffset += child.size.height;
      if (index < childCount - 1) {
        // Add a gap for the next divider.
        verticalOffset += dividerThickness;
      }

      index += 1;
      child = childAfter(child);
    }

    size = constraints.constrain(
      Size(constraints.maxWidth, verticalOffset)
    );
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    final Canvas canvas = context.canvas;
    _drawButtonBackgroundsAndDividersStacked(canvas, offset);
    _drawButtons(context, offset);
  }

  void _drawButtonBackgroundsAndDividersStacked(Canvas canvas, Offset offset) {
    final Offset dividerOffset = Offset(0.0, dividerThickness);

    final Path backgroundFillPath = Path()
      ..fillType = PathFillType.evenOdd
      ..addRect(Rect.fromLTWH(0.0, 0.0, size.width, size.height));

    final Path pressedBackgroundFillPath = Path();

    final Path dividersPath = Path();

    Offset accumulatingOffset = offset;

    RenderBox child = firstChild;
    RenderBox prevChild;
    while (child != null) {
      assert(child.parentData is _ActionButtonParentData);
      final _ActionButtonParentData currentButtonParentData = child.parentData;
      final bool isButtonPressed = currentButtonParentData.isPressed;

      bool isPrevButtonPressed = false;
      if (prevChild != null) {
        assert(prevChild.parentData is _ActionButtonParentData);
        final _ActionButtonParentData previousButtonParentData = prevChild
            .parentData;
        isPrevButtonPressed = previousButtonParentData.isPressed;
      }

      final bool isDividerPresent = child != firstChild;
      final bool isDividerPainted = isDividerPresent && !(isButtonPressed || isPrevButtonPressed);
      final Rect dividerRect = Rect.fromLTWH(
        accumulatingOffset.dx,
        accumulatingOffset.dy,
        size.width,
        _dividerThickness,
      );

      final Rect buttonBackgroundRect = Rect.fromLTWH(
        accumulatingOffset.dx,
        accumulatingOffset.dy + (isDividerPresent ? dividerThickness : 0.0),
        size.width,
        child.size.height,
      );

      // If this button is pressed, then we don't want a white background to be
      // painted, so we erase this button from the background path.
      if (isButtonPressed) {
        backgroundFillPath.addRect(buttonBackgroundRect);
        pressedBackgroundFillPath.addRect(buttonBackgroundRect);
      }

      // If this divider is needed, then we erase the divider area from the
      // background path, and on top of that we paint a translucent gray to
      // darken the divider area.
      if (isDividerPainted) {
        backgroundFillPath.addRect(dividerRect);
        dividersPath.addRect(dividerRect);
      }

      accumulatingOffset += (isDividerPresent ? dividerOffset : Offset.zero)
          + Offset(0.0, child.size.height);

      prevChild = child;
      child = childAfter(child);
    }

    canvas.drawPath(backgroundFillPath, _buttonBackgroundPaint);
    canvas.drawPath(pressedBackgroundFillPath, _pressedButtonBackgroundPaint);
    canvas.drawPath(dividersPath, _dividerPaint);
  }

  void _drawButtons(PaintingContext context, Offset offset) {
    RenderBox child = firstChild;
    while (child != null) {
      final MultiChildLayoutParentData childParentData = child.parentData;
      context.paintChild(child, childParentData.offset + offset);
      child = childAfter(child);
    }
  }

  @override
  bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
    return defaultHitTestChildren(result, position: position);
  }
}