flutter - 我怎样才能在我的 Dismissible 小部件上有圆角？

Question

我目前正在开发一个需要圆角的小部件，但是每当我从左向右拖动我的 Dismissible 小部件时，Dismissible 小部件都有一个不可编辑的剪辑器。

一些截图的更多解释:

我现在拥有的:

我想要的(拖动 Dismissible 时的圆角)

这是我现在使用的代码，带有用于测试的功能性 Dartpad。

https://dartpad.dev/?id=89b956ba997c4dc2f74e97b479b05249

Answer 1

所以基本上 flutter 提供的默认 Dismissible 类是不可能的，所以我复制它并根据您的需要更新它。

工作要点:https://dartpad.dev/?id=e7a904d25c9ecf54caa5cb0d1208801c

import 'package:flutter/material.dart';
import 'package:flutter/gestures.dart';


void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({
    Key? key,
    required this.title,
  }) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            ...List.generate(
              3,
              (index) => Padding(
                padding:
                    const EdgeInsets.symmetric(horizontal: 20, vertical: 10),
                child: Stack(
                  children: [
                    Container(
                      padding: const EdgeInsets.symmetric(
                          horizontal: 20, vertical: 10),
                      width: double.infinity,
                      color: Colors.amber,
                      height: 150,
                    ),
                    Positioned.fill(
                      child: ClipRRect(
                        borderRadius: BorderRadius.circular(30),
                        child: MyDismissible(
                          radius: 50,
                            onUpdate: (details) {},
                            confirmDismiss: (direction) async {},
                            crossAxisEndOffset:
                                MediaQuery.of(context).size.width,
                            dismissThresholds: const {
                              DismissDirection.endToStart: .5,
                              DismissDirection.startToEnd: .5,
                            },
                            key: UniqueKey(),
                            background: Container(
                              decoration: BoxDecoration(
                                borderRadius: BorderRadius.circular(50),
                                color: Colors.green.withOpacity(.35),
                              ),
                            ),
                            secondaryBackground: Container(
                                decoration: BoxDecoration(
                              borderRadius: BorderRadius.circular(50),
                              color: Colors.red.withOpacity(.35),
                            )),
                            child: Container(
                              width: double.infinity,
                            )),
                      ),
                    ),
                  ],
                ),
              ),
            )
          ],
        ),
      ),
    );
  }
}

Modified Dismissible Class(你需要将其作为别名导入，因为它可以与默认类相冲突)

const Curve _kResizeTimeCurve = Interval(0.4, 1.0, curve: Curves.ease);
const double _kMinFlingVelocity = 700.0;
const double _kMinFlingVelocityDelta = 400.0;
const double _kFlingVelocityScale = 1.0 / 300.0;
const double _kDismissThreshold = 0.4;

/// Signature used by [MyDismissible] to indicate that it has been dismissed in
/// the given `direction`.
///
/// Used by [MyDismissible.onDismissed].
typedef DismissDirectionCallback = void Function(DismissDirection direction);

/// Signature used by [MyDismissible] to give the application an opportunity to
/// confirm or veto a dismiss gesture.
///
/// Used by [MyDismissible.confirmDismiss].
typedef ConfirmDismissCallback = Future<bool?> Function(DismissDirection direction);

/// Signature used by [MyDismissible] to indicate that the dismissible has been dragged.
///
/// Used by [MyDismissible.onUpdate].
typedef DismissUpdateCallback = void Function(DismissUpdateDetails details);

/// The direction in which a [MyDismissible] can be dismissed.
enum DismissDirection {
  /// The [MyDismissible] can be dismissed by dragging either up or down.
  vertical,

  /// The [MyDismissible] can be dismissed by dragging either left or right.
  horizontal,

  /// The [MyDismissible] can be dismissed by dragging in the reverse of the
  /// reading direction (e.g., from right to left in left-to-right languages).
  endToStart,

  /// The [MyDismissible] can be dismissed by dragging in the reading direction
  /// (e.g., from left to right in left-to-right languages).
  startToEnd,

  /// The [MyDismissible] can be dismissed by dragging up only.
  up,

  /// The [MyDismissible] can be dismissed by dragging down only.
  down,

  /// The [MyDismissible] cannot be dismissed by dragging.
  none
}

/// A widget that can be dismissed by dragging in the indicated [direction].
///
/// Dragging or flinging this widget in the [DismissDirection] causes the child
/// to slide out of view. Following the slide animation, if [resizeDuration] is
/// non-null, the Dismissible widget animates its height (or width, whichever is
/// perpendicular to the dismiss direction) to zero over the [resizeDuration].
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=iEMgjrfuc58}
///
/// {@tool dartpad}
/// This sample shows how you can use the [MyDismissible] widget to
/// remove list items using swipe gestures. Swipe any of the list
/// tiles to the left or right to dismiss them from the [ListView].
///
/// ** See code in examples/api/lib/widgets/dismissible/dismissible.0.dart **
/// {@end-tool}
///
/// Backgrounds can be used to implement the "leave-behind" idiom. If a background
/// is specified it is stacked behind the Dismissible's child and is exposed when
/// the child moves.
///
/// The widget calls the [onDismissed] callback either after its size has
/// collapsed to zero (if [resizeDuration] is non-null) or immediately after
/// the slide animation (if [resizeDuration] is null). If the Dismissible is a
/// list item, it must have a key that distinguishes it from the other items and
/// its [onDismissed] callback must remove the item from the list.
class MyDismissible extends StatefulWidget {
  /// Creates a widget that can be dismissed.
  ///
  /// The [key] argument must not be null because [MyDismissible]s are commonly
  /// used in lists and removed from the list when dismissed. Without keys, the
  /// default behavior is to sync widgets based on their index in the list,
  /// which means the item after the dismissed item would be synced with the
  /// state of the dismissed item. Using keys causes the widgets to sync
  /// according to their keys and avoids this pitfall.
  const MyDismissible({
    required Key key,
    required this.child,
    this.radius=20,
    this.background,
    this.secondaryBackground,
    this.confirmDismiss,
    this.onResize,
    this.onUpdate,
    this.onDismissed,
    this.direction = DismissDirection.horizontal,
    this.resizeDuration = const Duration(milliseconds: 300),
    this.dismissThresholds = const <DismissDirection, double>{},
    this.movementDuration = const Duration(milliseconds: 200),
    this.crossAxisEndOffset = 0.0,
    this.dragStartBehavior = DragStartBehavior.start,
    this.behavior = HitTestBehavior.opaque,
  }) : assert(key != null),
        assert(secondaryBackground == null || background != null),
        assert(dragStartBehavior != null),
        super(key: key);

  /// The widget below this widget in the tree.
  ///
  /// {@macro flutter.widgets.ProxyWidget.child}
  final Widget child;

  /// A widget that is stacked behind the child. If secondaryBackground is also
  /// specified then this widget only appears when the child has been dragged
  /// down or to the right.
  final Widget? background;

  /// A widget that is stacked behind the child and is exposed when the child
  /// has been dragged up or to the left. It may only be specified when background
  /// has also been specified.
  final Widget? secondaryBackground;

  /// Gives the app an opportunity to confirm or veto a pending dismissal.
  ///
  /// The widget cannot be dragged again until the returned future resolves.
  ///
  /// If the returned Future<bool> completes true, then this widget will be
  /// dismissed, otherwise it will be moved back to its original location.
  ///
  /// If the returned Future<bool?> completes to false or null the [onResize]
  /// and [onDismissed] callbacks will not run.
  final ConfirmDismissCallback? confirmDismiss;

  /// Called when the widget changes size (i.e., when contracting before being dismissed).
  final VoidCallback? onResize;

  /// Called when the widget has been dismissed, after finishing resizing.
  final DismissDirectionCallback? onDismissed;

  /// The direction in which the widget can be dismissed.
  final DismissDirection direction;

  /// The amount of time the widget will spend contracting before [onDismissed] is called.
  ///
  /// If null, the widget will not contract and [onDismissed] will be called
  /// immediately after the widget is dismissed.
  final Duration? resizeDuration;

  /// The offset threshold the item has to be dragged in order to be considered
  /// dismissed.
  ///
  /// Represented as a fraction, e.g. if it is 0.4 (the default), then the item
  /// has to be dragged at least 40% towards one direction to be considered
  /// dismissed. Clients can define different thresholds for each dismiss
  /// direction.
  ///
  /// Flinging is treated as being equivalent to dragging almost to 1.0, so
  /// flinging can dismiss an item past any threshold less than 1.0.
  ///
  /// Setting a threshold of 1.0 (or greater) prevents a drag in the given
  /// [DismissDirection] even if it would be allowed by the [direction]
  /// property.
  ///
  /// See also:
  ///
  ///  * [direction], which controls the directions in which the items can
  ///    be dismissed.
  final Map<DismissDirection, double> dismissThresholds;

  /// Defines the duration for card to dismiss or to come back to original position if not dismissed.
  final Duration movementDuration;

  /// Defines the end offset across the main axis after the card is dismissed.
  ///
  /// If non-zero value is given then widget moves in cross direction depending on whether
  /// it is positive or negative.
  final double crossAxisEndOffset;

  /// Determines the way that drag start behavior is handled.
  ///
  /// If set to [DragStartBehavior.start], the drag gesture used to dismiss a
  /// dismissible will begin at the position where the drag gesture won the arena.
  /// If set to [DragStartBehavior.down] it will begin at the position where
  /// a down event is first detected.
  ///
  /// In general, setting this to [DragStartBehavior.start] will make drag
  /// animation smoother and setting it to [DragStartBehavior.down] will make
  /// drag behavior feel slightly more reactive.
  ///
  /// By default, the drag start behavior is [DragStartBehavior.start].
  ///
  /// See also:
  ///
  ///  * [DragGestureRecognizer.dragStartBehavior], which gives an example for the different behaviors.
  final DragStartBehavior dragStartBehavior;

  /// How to behave during hit tests.
  ///
  /// This defaults to [HitTestBehavior.opaque].
  final HitTestBehavior behavior;

  /// Called when the dismissible widget has been dragged.
  ///
  /// If [onUpdate] is not null, then it will be invoked for every pointer event
  /// to dispatch the latest state of the drag. For example, this callback
  /// can be used to for example change the color of the background widget
  /// depending on whether the dismiss threshold is currently reached.
  final DismissUpdateCallback? onUpdate;

  final double radius;

  @override
  State<MyDismissible> createState() => _MyDismissibleState();
}

/// Details for [DismissUpdateCallback].
///
/// See also:
///
///   * [MyDismissible.onUpdate], which receives this information.
class DismissUpdateDetails {
  /// Create a new instance of [DismissUpdateDetails].
  DismissUpdateDetails({
    this.direction = DismissDirection.horizontal,
    this.reached = false,
    this.previousReached = false
  });

  /// The direction that the dismissible is being dragged.
  final DismissDirection direction;

  /// Whether the dismiss threshold is currently reached.
  final bool reached;

  /// Whether the dismiss threshold was reached the last time this callback was invoked.
  ///
  /// This can be used in conjunction with [DismissUpdateDetails.reached] to catch the moment
  /// that the [MyDismissible] is dragged across the threshold.
  final bool previousReached;
}

/// i changed here in this class it was Rect changed it to Rect
class _DismissibleClipper extends CustomClipper<RRect> {
  _DismissibleClipper({
    required this.axis,
    required this.moveAnimation,
    required this.radius,
  }) : assert(axis != null),
        assert(moveAnimation != null),
        super(reclip: moveAnimation);

  final Axis axis;
  final Animation<Offset> moveAnimation;
  final double radius;

  @override
  RRect getClip(Size size) {
    assert(axis != null);
    switch (axis) {
      case Axis.horizontal:
        final double offset = moveAnimation.value.dx * size.width;
        if (offset < 0) {
          return RRect.fromLTRBR(size.width + offset, 0.0, size.width, size.height,Radius.circular(radius));
        }
        return RRect.fromLTRBR(0.0, 0.0, offset, size.height,Radius.circular(radius));
      case Axis.vertical:
        final double offset = moveAnimation.value.dy * size.height;
        if (offset < 0) {
          return RRect.fromLTRBR(0.0, size.height + offset, size.width, size.height,Radius.circular(radius));
        }
        return RRect.fromLTRBR(0.0, 0.0, size.width, offset,Radius.circular(radius));
    }
  }

  @override
  RRect getApproximateClipRRect(Size size) => getClip(size);

  @override
  bool shouldReclip(_DismissibleClipper oldClipper) {
    return oldClipper.axis != axis
        || oldClipper.moveAnimation.value != moveAnimation.value;
  }
}

enum _FlingGestureKind { none, forward, reverse }

class _MyDismissibleState extends State<MyDismissible> with TickerProviderStateMixin, AutomaticKeepAliveClientMixin {
  @override
  void initState() {
    super.initState();
    _moveController = AnimationController(duration: widget.movementDuration, vsync: this)
      ..addStatusListener(_handleDismissStatusChanged)
      ..addListener(_handleDismissUpdateValueChanged);
    _updateMoveAnimation();
  }

  AnimationController? _moveController;
  late Animation<Offset> _moveAnimation;

  AnimationController? _resizeController;
  Animation<double>? _resizeAnimation;

  double _dragExtent = 0.0;
  bool _confirming = false;
  bool _dragUnderway = false;
  Size? _sizePriorToCollapse;
  bool _dismissThresholdReached = false;

  @override
  bool get wantKeepAlive => _moveController?.isAnimating == true || _resizeController?.isAnimating == true;

  @override
  void dispose() {
    _moveController!.dispose();
    _resizeController?.dispose();
    super.dispose();
  }

  bool get _directionIsXAxis {
    return widget.direction == DismissDirection.horizontal
        || widget.direction == DismissDirection.endToStart
        || widget.direction == DismissDirection.startToEnd;
  }

  DismissDirection _extentToDirection(double extent) {
    if (extent == 0.0)
      return DismissDirection.none;
    if (_directionIsXAxis) {
      switch (Directionality.of(context)) {
        case TextDirection.rtl:
          return extent < 0 ? DismissDirection.startToEnd : DismissDirection.endToStart;
        case TextDirection.ltr:
          return extent > 0 ? DismissDirection.startToEnd : DismissDirection.endToStart;
      }
    }
    return extent > 0 ? DismissDirection.down : DismissDirection.up;
  }

  DismissDirection get _dismissDirection => _extentToDirection(_dragExtent);

  bool get _isActive {
    return _dragUnderway || _moveController!.isAnimating;
  }

  double get _overallDragAxisExtent {
    final Size size = context.size!;
    return _directionIsXAxis ? size.width : size.height;
  }

  void _handleDragStart(DragStartDetails details) {
    if (_confirming)
      return;
    _dragUnderway = true;
    if (_moveController!.isAnimating) {
      _dragExtent = _moveController!.value * _overallDragAxisExtent * _dragExtent.sign;
      _moveController!.stop();
    } else {
      _dragExtent = 0.0;
      _moveController!.value = 0.0;
    }
    setState(() {
      _updateMoveAnimation();
    });
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    if (!_isActive || _moveController!.isAnimating)
      return;

    final double delta = details.primaryDelta!;
    final double oldDragExtent = _dragExtent;
    switch (widget.direction) {
      case DismissDirection.horizontal:
      case DismissDirection.vertical:
        _dragExtent += delta;
        break;

      case DismissDirection.up:
        if (_dragExtent + delta < 0)
          _dragExtent += delta;
        break;

      case DismissDirection.down:
        if (_dragExtent + delta > 0)
          _dragExtent += delta;
        break;

      case DismissDirection.endToStart:
        switch (Directionality.of(context)) {
          case TextDirection.rtl:
            if (_dragExtent + delta > 0)
              _dragExtent += delta;
            break;
          case TextDirection.ltr:
            if (_dragExtent + delta < 0)
              _dragExtent += delta;
            break;
        }
        break;

      case DismissDirection.startToEnd:
        switch (Directionality.of(context)) {
          case TextDirection.rtl:
            if (_dragExtent + delta < 0)
              _dragExtent += delta;
            break;
          case TextDirection.ltr:
            if (_dragExtent + delta > 0)
              _dragExtent += delta;
            break;
        }
        break;

      case DismissDirection.none:
        _dragExtent = 0;
        break;
    }
    if (oldDragExtent.sign != _dragExtent.sign) {
      setState(() {
        _updateMoveAnimation();
      });
    }
    if (!_moveController!.isAnimating) {
      _moveController!.value = _dragExtent.abs() / _overallDragAxisExtent;
    }
  }

  void _handleDismissUpdateValueChanged() {
    if(widget.onUpdate != null) {
      final bool oldDismissThresholdReached = _dismissThresholdReached;
      _dismissThresholdReached = _moveController!.value > (widget.dismissThresholds[_dismissDirection] ?? _kDismissThreshold);
      final DismissUpdateDetails details = DismissUpdateDetails(
        direction: _dismissDirection,
        reached: _dismissThresholdReached,
        previousReached: oldDismissThresholdReached,
      );
      widget.onUpdate!(details);
    }
  }

  void _updateMoveAnimation() {
    final double end = _dragExtent.sign;
    _moveAnimation = _moveController!.drive(
      Tween<Offset>(
        begin: Offset.zero,
        end: _directionIsXAxis
            ? Offset(end, widget.crossAxisEndOffset)
            : Offset(widget.crossAxisEndOffset, end),
      ),
    );
  }

  _FlingGestureKind _describeFlingGesture(Velocity velocity) {
    assert(widget.direction != null);
    if (_dragExtent == 0.0) {
      // If it was a fling, then it was a fling that was let loose at the exact
      // middle of the range (i.e. when there's no displacement). In that case,
      // we assume that the user meant to fling it back to the center, as
      // opposed to having wanted to drag it out one way, then fling it past the
      // center and into and out the other side.
      return _FlingGestureKind.none;
    }
    final double vx = velocity.pixelsPerSecond.dx;
    final double vy = velocity.pixelsPerSecond.dy;
    DismissDirection flingDirection;
    // Verify that the fling is in the generally right direction and fast enough.
    if (_directionIsXAxis) {
      if (vx.abs() - vy.abs() < _kMinFlingVelocityDelta || vx.abs() < _kMinFlingVelocity)
        return _FlingGestureKind.none;
      assert(vx != 0.0);
      flingDirection = _extentToDirection(vx);
    } else {
      if (vy.abs() - vx.abs() < _kMinFlingVelocityDelta || vy.abs() < _kMinFlingVelocity)
        return _FlingGestureKind.none;
      assert(vy != 0.0);
      flingDirection = _extentToDirection(vy);
    }
    assert(_dismissDirection != null);
    if (flingDirection == _dismissDirection)
      return _FlingGestureKind.forward;
    return _FlingGestureKind.reverse;
  }

  void _handleDragEnd(DragEndDetails details) {
    if (!_isActive || _moveController!.isAnimating)
      return;
    _dragUnderway = false;
    if (_moveController!.isCompleted) {
      _handleMoveCompleted();
      return;
    }
    final double flingVelocity = _directionIsXAxis ? details.velocity.pixelsPerSecond.dx : details.velocity.pixelsPerSecond.dy;
    switch (_describeFlingGesture(details.velocity)) {
      case _FlingGestureKind.forward:
        assert(_dragExtent != 0.0);
        assert(!_moveController!.isDismissed);
        if ((widget.dismissThresholds[_dismissDirection] ?? _kDismissThreshold) >= 1.0) {
          _moveController!.reverse();
          break;
        }
        _dragExtent = flingVelocity.sign;
        _moveController!.fling(velocity: flingVelocity.abs() * _kFlingVelocityScale);
        break;
      case _FlingGestureKind.reverse:
        assert(_dragExtent != 0.0);
        assert(!_moveController!.isDismissed);
        _dragExtent = flingVelocity.sign;
        _moveController!.fling(velocity: -flingVelocity.abs() * _kFlingVelocityScale);
        break;
      case _FlingGestureKind.none:
        if (!_moveController!.isDismissed) { // we already know it's not completed, we check that above
          if (_moveController!.value > (widget.dismissThresholds[_dismissDirection] ?? _kDismissThreshold)) {
            _moveController!.forward();
          } else {
            _moveController!.reverse();
          }
        }
        break;
    }
  }

  Future<void> _handleDismissStatusChanged(AnimationStatus status) async {
    if (status == AnimationStatus.completed && !_dragUnderway) {
      await _handleMoveCompleted();
    }
    if (mounted) {
      updateKeepAlive();
    }
  }

  Future<void> _handleMoveCompleted() async {
    if ((widget.dismissThresholds[_dismissDirection] ?? _kDismissThreshold) >= 1.0) {
      _moveController!.reverse();
      return;
    }
    final bool result = await _confirmStartResizeAnimation();
    if (mounted) {
      if (result)
        _startResizeAnimation();
      else
        _moveController!.reverse();
    }
  }

  Future<bool> _confirmStartResizeAnimation() async {
    if (widget.confirmDismiss != null) {
      _confirming = true;
      final DismissDirection direction = _dismissDirection;
      try {
        return await widget.confirmDismiss!(direction) ?? false;
      } finally {
        _confirming = false;
      }
    }
    return true;
  }

  void _startResizeAnimation() {
    assert(_moveController!.isCompleted);
    assert(_resizeController == null);
    assert(_sizePriorToCollapse == null);
    if (widget.resizeDuration == null) {
      if (widget.onDismissed != null) {
        final DismissDirection direction = _dismissDirection;
        widget.onDismissed!(direction);
      }
    } else {
      _resizeController = AnimationController(duration: widget.resizeDuration, vsync: this)
        ..addListener(_handleResizeProgressChanged)
        ..addStatusListener((AnimationStatus status) => updateKeepAlive());
      _resizeController!.forward();
      setState(() {
        _sizePriorToCollapse = context.size;
        _resizeAnimation = _resizeController!.drive(
          CurveTween(
            curve: _kResizeTimeCurve,
          ),
        ).drive(
          Tween<double>(
            begin: 1.0,
            end: 0.0,
          ),
        );
      });
    }
  }

  void _handleResizeProgressChanged() {
    if (_resizeController!.isCompleted) {
      widget.onDismissed?.call(_dismissDirection);
    } else {
      widget.onResize?.call();
    }
  }

  @override
  Widget build(BuildContext context) {
    super.build(context); // See AutomaticKeepAliveClientMixin.

    assert(!_directionIsXAxis || debugCheckHasDirectionality(context));

    Widget? background = widget.background;
    if (widget.secondaryBackground != null) {
      final DismissDirection direction = _dismissDirection;
      if (direction == DismissDirection.endToStart || direction == DismissDirection.up)
        background = widget.secondaryBackground;
    }

    if (_resizeAnimation != null) {
      // we've been dragged aside, and are now resizing.
      assert(() {
        if (_resizeAnimation!.status != AnimationStatus.forward) {
          assert(_resizeAnimation!.status == AnimationStatus.completed);
          throw FlutterError.fromParts(<DiagnosticsNode>[
            ErrorSummary('A dismissed Dismissible widget is still part of the tree.'),
            ErrorHint(
              'Make sure to implement the onDismissed handler and to immediately remove the Dismissible '
                  'widget from the application once that handler has fired.',
            ),
          ]);
        }
        return true;
      }());

      return SizeTransition(
        sizeFactor: _resizeAnimation!,
        axis: _directionIsXAxis ? Axis.vertical : Axis.horizontal,
        child: SizedBox(
          width: _sizePriorToCollapse!.width,
          height: _sizePriorToCollapse!.height,
          child: background,
        ),
      );
    }

    Widget content = SlideTransition(
      position: _moveAnimation,
      child: widget.child,
    );

    if (background != null) {
      content = Stack(children: <Widget>[
        if (!_moveAnimation.isDismissed)
          Positioned.fill(
            child: ClipRRect(
              clipper: _DismissibleClipper(
                axis: _directionIsXAxis ? Axis.horizontal : Axis.vertical,
                moveAnimation: _moveAnimation,
                radius: widget.radius //todo specify radius here
              ),
              child: background,
            ),
          ),
        content,
      ]);
    }

    // If the DismissDirection is none, we do not add drag gestures because the content
    // cannot be dragged.
    if (widget.direction == DismissDirection.none)
      return content;

    // We are not resizing but we may be being dragging in widget.direction.
    return GestureDetector(
      onHorizontalDragStart: _directionIsXAxis ? _handleDragStart : null,
      onHorizontalDragUpdate: _directionIsXAxis ? _handleDragUpdate : null,
      onHorizontalDragEnd: _directionIsXAxis ? _handleDragEnd : null,
      onVerticalDragStart: _directionIsXAxis ? null : _handleDragStart,
      onVerticalDragUpdate: _directionIsXAxis ? null : _handleDragUpdate,
      onVerticalDragEnd: _directionIsXAxis ? null : _handleDragEnd,
      behavior: widget.behavior,
      dragStartBehavior: widget.dragStartBehavior,
      child: content,
    );
  }
}

这段代码很长，但它有效我添加了一些注释以及我修改的内容。