Flutter刷新组件RefreshIndicator自定义样式demo

AnimatedBuilder(
  animation: _positionController,
  builder: (BuildContext context, Widget? child) {
    return ClipOval(
      child: Container(
          padding: const EdgeInsets.all(10),
          decoration: BoxDecoration(
              color: widget.backgroundColor ?? Colors.white),
          child: CupertinoActivityIndicator(
              color: widget.color)),
    );
  },
)

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'dart:async';
import 'dart:math' as math;
import 'dart:ui';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:get/get.dart';
// The over-scroll distance that moves the indicator to its maximum
// displacement, as a percentage of the scrollable's container extent.
const double _kDragContainerExtentPercentage = 0.25;
// How much the scroll's drag gesture can overshoot the RefreshIndicator's
// displacement; max displacement = _kDragSizeFactorLimit * displacement.
const double _kDragSizeFactorLimit = 1.5;
// When the scroll ends, the duration of the refresh indicator's animation
// to the RefreshIndicator's displacement.
const Duration _kIndicatorSnapDuration = Duration(milliseconds: 150);
// The duration of the ScaleTransition that starts when the refresh action
// has completed.
const Duration _kIndicatorScaleDuration = Duration(milliseconds: 200);
/// The signature for a function that's called when the user has dragged a
/// [RefreshIndicator] far enough to demonstrate that they want the app to
/// refresh. The returned [Future] must complete when the refresh operation is
/// finished.
///
/// Used by [RefreshIndicator.onRefresh].
typedef RefreshCallback = Future<void> Function();
// The state machine moves through these modes only when the scrollable
// identified by scrollableKey has been scrolled to its min or max limit.
enum _RefreshIndicatorMode {
  drag, // Pointer is down.
  armed, // Dragged far enough that an up event will run the onRefresh callback.
  snap, // Animating to the indicator's final "displacement".
  refresh, // Running the refresh callback.
  done, // Animating the indicator's fade-out after refreshing.
  canceled, // Animating the indicator's fade-out after not arming.
}
/// Used to configure how [RefreshIndicator] can be triggered.
enum RefreshIndicatorTriggerMode {
  /// The indicator can be triggered regardless of the scroll position
  /// of the [Scrollable] when the drag starts.
  anywhere,
  /// The indicator can only be triggered if the [Scrollable] is at the edge
  /// when the drag starts.
  onEdge,
}
/// A widget that supports the Material "swipe to refresh" idiom.
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=ORApMlzwMdM}
///
/// When the child's [Scrollable] descendant overscrolls, an animated circular
/// progress indicator is faded into view. When the scroll ends, if the
/// indicator has been dragged far enough for it to become completely opaque,
/// the [onRefresh] callback is called. The callback is expected to update the
/// scrollable's contents and then complete the [Future] it returns. The refresh
/// indicator disappears after the callback's [Future] has completed.
///
/// The trigger mode is configured by [RefreshIndicator.triggerMode].
///
/// {@tool dartpad}
/// This example shows how [RefreshIndicator] can be triggered in different ways.
///
/// ** See code in examples/api/lib/material/refresh_indicator/refresh_indicator.0.dart **
/// {@end-tool}
///
/// ## Troubleshooting
///
/// ### Refresh indicator does not show up
///
/// The [RefreshIndicator] will appear if its scrollable descendant can be
/// overscrolled, i.e. if the scrollable's content is bigger than its viewport.
/// To ensure that the [RefreshIndicator] will always appear, even if the
/// scrollable's content fits within its viewport, set the scrollable's
/// [Scrollable.physics] property to [AlwaysScrollableScrollPhysics]:
///
/// ```dart
/// ListView(
///   physics: const AlwaysScrollableScrollPhysics(),
///   children: ...
/// )
/// ```
///
/// A [RefreshIndicator] can only be used with a vertical scroll view.
///
/// See also:
///
///  * <https://material.io/design/platform-guidance/android-swipe-to-refresh.html>
///  * [RefreshIndicatorState], can be used to programmatically show the refresh indicator.
///  * [RefreshProgressIndicator], widget used by [RefreshIndicator] to show
///    the inner circular progress spinner during refreshes.
///  * [CupertinoSliverRefreshControl], an iOS equivalent of the pull-to-refresh pattern.
///    Must be used as a sliver inside a [CustomScrollView] instead of wrapping
///    around a [ScrollView] because it's a part of the scrollable instead of
///    being overlaid on top of it.
class RefreshIndicatorNeo extends StatefulWidget {
  /// Creates a refresh indicator.
  ///
  /// The [onRefresh], [child], and [notificationPredicate] arguments must be
  /// non-null. The default
  /// [displacement] is 40.0 logical pixels.
  ///
  /// The [semanticsLabel] is used to specify an accessibility label for this widget.
  /// If it is null, it will be defaulted to [MaterialLocalizations.refreshIndicatorSemanticLabel].
  /// An empty string may be passed to avoid having anything read by screen reading software.
  /// The [semanticsValue] may be used to specify progress on the widget.
  const RefreshIndicatorNeo({
    Key? key,
    required this.child,
    this.displacement = 40.0,
    this.edgeOffset = 0.0,
    required this.onRefresh,
    this.color,
    this.backgroundColor,
    this.notificationPredicate = defaultScrollNotificationPredicate,
    this.semanticsLabel,
    this.semanticsValue,
    this.strokeWidth = RefreshProgressIndicator.defaultStrokeWidth,
    this.triggerMode = RefreshIndicatorTriggerMode.onEdge,
  })  : assert(child != null),
        assert(onRefresh != null),
        assert(notificationPredicate != null),
        assert(strokeWidth != null),
        assert(triggerMode != null),
        super(key: key);
  /// The widget below this widget in the tree.
  ///
  /// The refresh indicator will be stacked on top of this child. The indicator
  /// will appear when child's Scrollable descendant is over-scrolled.
  ///
  /// Typically a [ListView] or [CustomScrollView].
  final Widget child;
  /// The distance from the child's top or bottom [edgeOffset] where
  /// the refresh indicator will settle. During the drag that exposes the refresh
  /// indicator, its actual displacement may significantly exceed this value.
  ///
  /// In most cases, [displacement] distance starts counting from the parent's
  /// edges. However, if [edgeOffset] is larger than zero then the [displacement]
  /// value is calculated from that offset instead of the parent's edge.
  final double displacement;
  /// The offset where [RefreshProgressIndicator] starts to appear on drag start.
  ///
  /// Depending whether the indicator is showing on the top or bottom, the value
  /// of this variable controls how far from the parent's edge the progress
  /// indicator starts to appear. This may come in handy when, for example, the
  /// UI contains a top [Widget] which covers the parent's edge where the progress
  /// indicator would otherwise appear.
  ///
  /// By default, the edge offset is set to 0.
  ///
  /// See also:
  ///
  ///  * [displacement], can be used to change the distance from the edge that
  ///    the indicator settles.
  final double edgeOffset;
  /// A function that's called when the user has dragged the refresh indicator
  /// far enough to demonstrate that they want the app to refresh. The returned
  /// [Future] must complete when the refresh operation is finished.
  final RefreshCallback onRefresh;
  /// The progress indicator's foreground color. The current theme's
  /// [ColorScheme.primary] by default.
  final Color? color;
  /// The progress indicator's background color. The current theme's
  /// [ThemeData.canvasColor] by default.
  final Color? backgroundColor;
  /// A check that specifies whether a [ScrollNotification] should be
  /// handled by this widget.
  ///
  /// By default, checks whether `notification.depth == 0`. Set it to something
  /// else for more complicated layouts.
  final ScrollNotificationPredicate notificationPredicate;
  /// {@macro flutter.progress_indicator.ProgressIndicator.semanticsLabel}
  ///
  /// This will be defaulted to [MaterialLocalizations.refreshIndicatorSemanticLabel]
  /// if it is null.
  final String? semanticsLabel;
  /// {@macro flutter.progress_indicator.ProgressIndicator.semanticsValue}
  final String? semanticsValue;
  /// Defines `strokeWidth` for `RefreshIndicator`.
  ///
  /// By default, the value of `strokeWidth` is 2.0 pixels.
  final double strokeWidth;
  /// Defines how this [RefreshIndicator] can be triggered when users overscroll.
  ///
  /// The [RefreshIndicator] can be pulled out in two cases,
  /// 1, Keep dragging if the scrollable widget at the edge with zero scroll position
  ///    when the drag starts.
  /// 2, Keep dragging after overscroll occurs if the scrollable widget has
  ///    a non-zero scroll position when the drag starts.
  ///
  /// If this is [RefreshIndicatorTriggerMode.anywhere], both of the cases above can be triggered.
  ///
  /// If this is [RefreshIndicatorTriggerMode.onEdge], only case 1 can be triggered.
  ///
  /// Defaults to [RefreshIndicatorTriggerMode.onEdge].
  final RefreshIndicatorTriggerMode triggerMode;
  @override
  RefreshIndicatorNeoState createState() => RefreshIndicatorNeoState();
}
/// Contains the state for a [RefreshIndicator]. This class can be used to
/// programmatically show the refresh indicator, see the [show] method.
class RefreshIndicatorNeoState extends State<RefreshIndicatorNeo>
    with TickerProviderStateMixin<RefreshIndicatorNeo> {
  late AnimationController _positionController;
  late AnimationController _scaleController;
  late Animation<double> _positionFactor;
  late Animation<double> _scaleFactor;
  late Animation<double> _value;
  late Animation<Color?> _valueColor;
  _RefreshIndicatorMode? _mode;
  late Future<void> _pendingRefreshFuture;
  bool? _isIndicatorAtTop;
  double? _dragOffset;
  static final Animatable<double> _threeQuarterTween =
      Tween<double>(begin: 0.0, end: 0.75);
  static final Animatable<double> _kDragSizeFactorLimitTween =
      Tween<double>(begin: 0.0, end: _kDragSizeFactorLimit);
  static final Animatable<double> _oneToZeroTween =
      Tween<double>(begin: 1.0, end: 0.0);
  @override
  void initState() {
    super.initState();
    _positionController = AnimationController(vsync: this);
    _positionFactor = _positionController.drive(_kDragSizeFactorLimitTween);
    _value = _positionController.drive(
        _threeQuarterTween); // The "value" of the circular progress indicator during a drag.
    _scaleController = AnimationController(vsync: this);
    _scaleFactor = _scaleController.drive(_oneToZeroTween);
  }
  @override
  void didChangeDependencies() {
    final ThemeData theme = Theme.of(context);
    _valueColor = _positionController.drive(
      ColorTween(
        begin: (widget.color ?? theme.colorScheme.primary).withOpacity(0.0),
        end: (widget.color ?? theme.colorScheme.primary).withOpacity(1.0),
      ).chain(CurveTween(
        curve: const Interval(0.0, 1.0 / _kDragSizeFactorLimit),
      )),
    );
    super.didChangeDependencies();
  }
  @override
  void didUpdateWidget(covariant RefreshIndicatorNeo oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.color != widget.color) {
      final ThemeData theme = Theme.of(context);
      _valueColor = _positionController.drive(
        ColorTween(
          begin: (widget.color ?? theme.colorScheme.primary).withOpacity(0.0),
          end: (widget.color ?? theme.colorScheme.primary).withOpacity(1.0),
        ).chain(CurveTween(
          curve: const Interval(0.0, 1.0 / _kDragSizeFactorLimit),
        )),
      );
    }
  }
  @override
  void dispose() {
    _positionController.dispose();
    _scaleController.dispose();
    super.dispose();
  }
  bool _shouldStart(ScrollNotification notification) {
    // If the notification.dragDetails is null, this scroll is not triggered by
    // user dragging. It may be a result of ScrollController.jumpTo or ballistic scroll.
    // In this case, we don't want to trigger the refresh indicator.
    return ((notification is ScrollStartNotification &&
                notification.dragDetails != null) ||
            (notification is ScrollUpdateNotification &&
                notification.dragDetails != null &&
                widget.triggerMode == RefreshIndicatorTriggerMode.anywhere)) &&
        ((notification.metrics.axisDirection == AxisDirection.up &&
                notification.metrics.extentAfter == 0.0) ||
            (notification.metrics.axisDirection == AxisDirection.down &&
                notification.metrics.extentBefore == 0.0)) &&
        _mode == null &&
        _start(notification.metrics.axisDirection);
  }
  bool _handleScrollNotification(ScrollNotification notification) {
    if (!widget.notificationPredicate(notification)) return false;
    if (_shouldStart(notification)) {
      setState(() {
        _mode = _RefreshIndicatorMode.drag;
      });
      return false;
    }
    bool? indicatorAtTopNow;
    switch (notification.metrics.axisDirection) {
      case AxisDirection.down:
      case AxisDirection.up:
        indicatorAtTopNow = true;
        break;
      case AxisDirection.left:
      case AxisDirection.right:
        indicatorAtTopNow = true;
        break;
    }
    if (indicatorAtTopNow != _isIndicatorAtTop) {
      if (_mode == _RefreshIndicatorMode.drag ||
          _mode == _RefreshIndicatorMode.armed)
        _dismiss(_RefreshIndicatorMode.canceled);
    } else if (notification is ScrollUpdateNotification) {
      if (_mode == _RefreshIndicatorMode.drag ||
          _mode == _RefreshIndicatorMode.armed) {
        if ((notification.metrics.axisDirection == AxisDirection.down &&
                notification.metrics.extentBefore > 0.0) ||
            (notification.metrics.axisDirection == AxisDirection.up &&
                notification.metrics.extentAfter > 0.0)) {
          _dismiss(_RefreshIndicatorMode.canceled);
        } else {
          if (notification.metrics.axisDirection == AxisDirection.down) {
            _dragOffset = _dragOffset! - notification.scrollDelta!;
          } else if (notification.metrics.axisDirection == AxisDirection.up) {
            _dragOffset = _dragOffset! + notification.scrollDelta!;
          }
          _checkDragOffset(notification.metrics.viewportDimension);
        }
      }
      if (_mode == _RefreshIndicatorMode.armed &&
          notification.dragDetails == null) {
        // On iOS start the refresh when the Scrollable bounces back from the
        // overscroll (ScrollNotification indicating this don't have dragDetails
        // because the scroll activity is not directly triggered by a drag).
        _show();
      }
    } else if (notification is OverscrollNotification) {
      if (_mode == _RefreshIndicatorMode.drag ||
          _mode == _RefreshIndicatorMode.armed) {
        if (notification.metrics.axisDirection == AxisDirection.down) {
          _dragOffset = _dragOffset! - notification.overscroll;
        } else if (notification.metrics.axisDirection == AxisDirection.up) {
          _dragOffset = _dragOffset! + notification.overscroll;
        }
        _checkDragOffset(notification.metrics.viewportDimension,
            needIntercept: true);
      }
    } else if (notification is ScrollEndNotification) {
      switch (_mode) {
        case _RefreshIndicatorMode.armed:
          _show();
          break;
        case _RefreshIndicatorMode.drag:
          _dismiss(_RefreshIndicatorMode.canceled);
          break;
        case _RefreshIndicatorMode.canceled:
        case _RefreshIndicatorMode.done:
        case _RefreshIndicatorMode.refresh:
        case _RefreshIndicatorMode.snap:
        case null:
          // do nothing
          break;
      }
    }
    return false;
  }
  bool _handleGlowNotification(OverscrollIndicatorNotification notification) {
    if (notification.depth != 0 || !notification.leading) return false;
    if (_mode == _RefreshIndicatorMode.drag) {
      notification.disallowGlow();
      return true;
    }
    return false;
  }
  bool _start(AxisDirection direction) {
    assert(_mode == null);
    assert(_isIndicatorAtTop == null);
    assert(_dragOffset == null);
    switch (direction) {
      case AxisDirection.down:
      case AxisDirection.up:
        _isIndicatorAtTop = true;
        break;
      case AxisDirection.left:
      case AxisDirection.right:
        _isIndicatorAtTop = null;
        // we do not support horizontal scroll views.
        return false;
    }
    _dragOffset = 0.0;
    _scaleController.value = 0.0;
    _positionController.value = 0.0;
    return true;
  }
  void _checkDragOffset(double containerExtent, {bool needIntercept = true}) {
    if (needIntercept) {
      assert(_mode == _RefreshIndicatorMode.drag ||
          _mode == _RefreshIndicatorMode.armed);
    }
    double newValue =
        _dragOffset! / (containerExtent * _kDragContainerExtentPercentage);
    if (_mode == _RefreshIndicatorMode.armed) {
      newValue = math.max(newValue, 1.0 / _kDragSizeFactorLimit);
    }
    _positionController.value =
        newValue.clamp(0.0, 1.0); // this triggers various rebuilds
    if (_mode == _RefreshIndicatorMode.drag &&
        _valueColor.value!.alpha == 0xFF) {
      _mode = _RefreshIndicatorMode.armed;
    }
  }
  // Stop showing the refresh indicator.
  Future<void> _dismiss(_RefreshIndicatorMode newMode, {Duration? time}) async {
    await Future<void>.value();
    // This can only be called from _show() when refreshing and
    // _handleScrollNotification in response to a ScrollEndNotification or
    // direction change.
    assert(newMode == _RefreshIndicatorMode.canceled ||
        newMode == _RefreshIndicatorMode.done);
    setState(() {
      _mode = newMode;
    });
    switch (_mode!) {
      // 注释：刷新结束，关闭动画
      case _RefreshIndicatorMode.done:
        _scaleController
            .animateTo(1.0, duration: time ?? _kIndicatorScaleDuration)
            .whenComplete(() {});
        _doneAnimation = Tween<double>(begin: getPos(pos.value), end: 0)
            .animate(_scaleController);
        if (_doneAnimation != null) {
          _doneAnimation?.addListener(() {
            //赋值高度
            pos(_doneAnimation?.value ?? 0);
            if ((_doneAnimation?.value ?? 0) == 0) {
              _doneAnimation = null;
            }
          });
        }
        break;
      case _RefreshIndicatorMode.canceled:
        await _positionController.animateTo(0.0,
            duration: time ?? _kIndicatorScaleDuration);
        break;
      case _RefreshIndicatorMode.armed:
      case _RefreshIndicatorMode.drag:
      case _RefreshIndicatorMode.refresh:
      case _RefreshIndicatorMode.snap:
        assert(false);
    }
    if (mounted && _mode == newMode) {
      _dragOffset = null;
      _isIndicatorAtTop = null;
      setState(() {
        _mode = null;
      });
    }
  }
  void _show() {
    assert(_mode != _RefreshIndicatorMode.refresh);
    assert(_mode != _RefreshIndicatorMode.snap);
    // final Completer<void> completer = Completer<void>();
    // _pendingRefreshFuture = completer.future;
    _mode = _RefreshIndicatorMode.snap;
    _positionController
        .animateTo(1.0 / _kDragSizeFactorLimit,
            duration: _kIndicatorSnapDuration)
        .then<void>((void value) {
      if (mounted && _mode == _RefreshIndicatorMode.snap) {
        assert(widget.onRefresh != null);
        setState(() {
          // Show the indeterminate progress indicator.
          _mode = _RefreshIndicatorMode.refresh;
        });
        // 注释:删掉这段代码，因为需要跟随手势，在手势释放的时候才执行，见下方手势控制onPointerUp
        // final Future<void> refreshResult = widget.onRefresh();
        // assert(() {
        //   if (refreshResult == null)
        //     FlutterError.reportError(FlutterErrorDetails(
        //       exception: FlutterError(
        //         'The onRefresh callback returned null.\n'
        //         'The RefreshIndicator onRefresh callback must return a Future.',
        //       ),
        //       context: ErrorDescription('when calling onRefresh'),
        //       library: 'material library',
        //     ));
        //   return true;
        // }());
        // if (refreshResult == null) return;
        // refreshResult.whenComplete(() {
        //   if (mounted && _mode == _RefreshIndicatorMode.refresh) {
        //     completer.complete();
        //     _dismiss(_RefreshIndicatorMode.done);
        //   }
        // });
      }
    });
  }
  /// Show the refresh indicator and run the refresh callback as if it had
  /// been started interactively. If this method is called while the refresh
  /// callback is running, it quietly does nothing.
  ///
  /// Creating the [RefreshIndicator] with a [GlobalKey<RefreshIndicatorState>]
  /// makes it possible to refer to the [RefreshIndicatorState].
  ///
  /// The future returned from this method completes when the
  /// [RefreshIndicator.onRefresh] callback's future completes.
  ///
  /// If you await the future returned by this function from a [State], you
  /// should check that the state is still [mounted] before calling [setState].
  ///
  /// When initiated in this manner, the refresh indicator is independent of any
  /// actual scroll view. It defaults to showing the indicator at the top. To
  /// show it at the bottom, set `atTop` to false.
  Future<void> show({bool atTop = true}) {
    if (_mode != _RefreshIndicatorMode.refresh &&
        _mode != _RefreshIndicatorMode.snap) {
      if (_mode == null) _start(atTop ? AxisDirection.down : AxisDirection.up);
      _show();
    }
    return _pendingRefreshFuture;
  }
  //点击时的Y
  double _downY = 0.0;
  //最后的移动Y
  double _lastMoveY = 0.0;
  //手势移动距离，对应下拉效果的位移
  //因为需要制造弹性效果，调用getPos()模拟弹性
  RxDouble pos = 0.0.obs;
  //手势状态
  MoveType moveType = MoveType.UP;
  final double bottomImg = 10;
  //手势下拉动画，主要对pos赋值
  late Animation<double>? _animation;
  //结束动画，主要对pos重新赋值至0
  late Animation<double>? _doneAnimation;
  late AnimationController _controller;
  ///模拟下拉的弹性
  double getPos(double pos) {
    if (pos <= 0) {
      return 0;
    } else if (pos < 100) {
      return pos * 0.7;
    } else if (pos < 200) {
      return 70 + ((pos - 100) * 0.5);
    } else if (pos < 300) {
      return 120 + ((pos - 200) * 0.3);
    } else {
      return 150 + ((pos - 300) * 0.1);
    }
  }
  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMaterialLocalizations(context));
    final Widget child = NotificationListener<ScrollNotification>(
      onNotification: _handleScrollNotification,
      child: widget.child,
      // NotificationListener<OverscrollIndicatorNotification>(
      //   // onNotification: _handleGlowNotification,
      //   child: widget.child,
      // ),
    );
    assert(() {
      if (_mode == null) {
        assert(_dragOffset == null);
        assert(_isIndicatorAtTop == null);
      } else {
        assert(_dragOffset != null);
        assert(_isIndicatorAtTop != null);
      }
      return true;
    }());
    final bool showIndeterminateIndicator =
        _mode == _RefreshIndicatorMode.refresh ||
            _mode == _RefreshIndicatorMode.done;
    double imgHeight = MediaQueryData.fromWindow(window).size.width / 7;
    double imgAllHeight = imgHeight + bottomImg;
    return Listener(
        onPointerDown: (PointerDownEvent event) {
          //手指按下的距离
          _downY = event.position.distance;
          moveType = MoveType.DOWN;
        },
        onPointerMove: (PointerMoveEvent event) {
          if (moveType != MoveType.MOVE || _mode == null) {
            setState(() {
              moveType = MoveType.MOVE;
            });
          }
          moveType = MoveType.MOVE;
          //手指移动的距离
          var position = event.position.distance;
          //判断距离差
          var detal = position - _lastMoveY;
          ///到达顶部才计算
          if (_isIndicatorAtTop != null &&
              _isIndicatorAtTop! &&
              _mode != null) {
            pos(position - _downY);
            if (detal > 0) {
              //================向下移动================
            } else {
              //================向上移动================
              ///当刷新动画执行时，手指上滑就直接取消刷新动画
              if (_mode == _RefreshIndicatorMode.refresh && pos.value != 0) {
                _dismiss(_RefreshIndicatorMode.canceled,
                    time: Duration(microseconds: 500));
              }
            }
          }
          _lastMoveY = position;
        },
        onPointerUp: (PointerUpEvent event) {
          if (_isIndicatorAtTop != null && _isIndicatorAtTop!) {
            double heightPos = pos.value;
            double imgHeight = 0;
            ///计算图片高度，因为最终转成pos，因为pos被转换过getPos()
            //所以反转的时候需要再次计算
            if (imgAllHeight < 100) {
              imgHeight = imgAllHeight / 0.7;
            } else if (imgAllHeight < 200) {
              imgHeight = (imgAllHeight - 20) / 0.5;
            } else if (imgAllHeight < 300) {
              imgHeight = (imgAllHeight - 60) / 0.3;
            }
            //松手后的回弹效果
            _controller = AnimationController(
              vsync: this,
              duration: Duration(milliseconds: 250),
            )..forward().whenComplete(() {
                ///动画结束后触发onRefresh()方法
                if (_mode == _RefreshIndicatorMode.refresh) {
                  final Completer<void> completer = Completer<void>();
                  _pendingRefreshFuture = completer.future;
                  final Future<void> refreshResult = widget.onRefresh();
                  assert(() {
                    if (refreshResult == null) {
                      FlutterError.reportError(FlutterErrorDetails(
                        exception: FlutterError(
                          'The onRefresh callback returned null.\n'
                          'The RefreshIndicator onRefresh callback must return a Future.',
                        ),
                        context: ErrorDescription('when calling onRefresh'),
                        library: 'material library',
                      ));
                    }
                    return true;
                  }());
                  if (refreshResult == null) return;
                  refreshResult.whenComplete(() {
                    if (mounted && _mode == _RefreshIndicatorMode.refresh) {
                      completer.complete();
                      ///onRefresh()执行完后关闭动画
                      _dismiss(_RefreshIndicatorMode.done);
                    }
                  });
                }
              });
            _animation = Tween<double>(begin: heightPos, end: imgHeight)
                .animate(_controller);
            _animation?.addListener(() {
              //下拉动画变化，赋值高度
              if (_mode == _RefreshIndicatorMode.refresh) {
                pos(_animation?.value ?? 0);
                if (_animation?.value == imgHeight) {
                  _animation = null;
                }
              }
            });
          }
          moveType = MoveType.UP;
        },
        child: Obx(() => Column(
              children: [
                if (_isIndicatorAtTop != null &&
                        _isIndicatorAtTop! &&
                        _mode != null &&
                        moveType == MoveType.MOVE ||
                    pos.value != 0)
                  ScaleTransition(
                    scale: _scaleFactor,
                    child: AnimatedBuilder(
                      animation: _positionController,
                      builder: (BuildContext context, Widget? child) {
                        //使用gif动画
                        return Obx(() => Container(
                              height: getPos(pos.value),
                              alignment: Alignment.bottomCenter,
                              child: Container(
                                padding: EdgeInsets.only(bottom: bottomImg),
                                child: Image.asset(
                                  "assets/gif_load.gif",
                                  width: imgHeight * 2,
                                  height: imgHeight,
                                ),
                              ),
                            ));
                      },
                    ),
                  ),
                Expanded(child: child),
              ],
            )));
  }
}
enum MoveType {
  DOWN,
  MOVE,
  UP,
}