如何在颤动中缩放ListView内的图像

Question

我正在编写一个 Flutter 应用程序，我想知道如何在 ListView 中使用/实现可缩放图像。我在我的应用中使用了以下插件。

• flutter_advanced_networkimage GitHub - DartPackages
• flutter_zoomable_image GitHub - DartPackages

他们都没有参与我的项目并引发了不同的异常。重现错误的示例代码：

flutter_advanced_networkimage：

import 'package:flutter/material.dart';
import 'package:flutter_advanced_networkimage/flutter_advanced_networkimage.dart';
import 'package:flutter_advanced_networkimage/transition_to_image.dart';
import 'package:flutter_advanced_networkimage/zoomable_widget.dart';

void main() {
  runApp(new ZoomableImageInListView());
}

class ZoomableImageInListView extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new _ZoomableImageInListViewState();
  }
}

final List<String> _urlList = [
  'https://www.w3schools.com/htmL/pic_trulli.jpg',
  'https://www.w3schools.com/htmL/img_girl.jpg',
  'https://www.w3schools.com/htmL/img_chania.jpg',
];

class _ZoomableImageInListViewState extends State<ZoomableImageInListView> {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Zoomable Image In ListView',
      debugShowCheckedModeBanner: false,
      home: new Scaffold(
        body: new Column(
          children: <Widget>[
            new Expanded(
              child: new ListView.builder(
                scrollDirection: Axis.vertical,
                itemBuilder: _buildVerticalChild,
              ),
            ),
          ],
        ),
      ),
    );
  }

  _buildVerticalChild(BuildContext context, int index) {
    index++;
    if (index > _urlList.length) return null;
    TransitionToImage imageWidget = TransitionToImage(
      AdvancedNetworkImage(
        _urlList[index],
        useDiskCache: true,
      ),
      useReload: true,
      reloadWidget: Icon(Icons.replay),
    );
    return new ZoomableWidget(
      minScale: 1.0,
      maxScale: 5.0,
      child: imageWidget,
      tapCallback: imageWidget.reloadImage,
    );
  }
}

抛出此异常：

I/flutter (13594): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (13594): The following assertion was thrown building ZoomableImageInListView(dirty, state:
I/flutter (13594): _ZoomableImageInListViewState#39144):
I/flutter (13594): type '(BuildContext, int) => dynamic' is not a subtype of type '(BuildContext, int) => Widget'
I/flutter (13594): 
I/flutter (13594): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter (13594): more information in this error message to help you determine and fix the underlying cause.
I/flutter (13594): In either case, please report this assertion by filing a bug on GitHub:
I/flutter (13594):   https://github.com/flutter/flutter/issues/new
.
.
.
I/flutter (13594): ════════════════════════════════════════════════════════════════════════════════════════════════════

可缩放图像：

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

void main() {
  runApp(new ZoomableImageInListView());
}

class ZoomableImageInListView extends StatefulWidget {
  @override
  _ZoomableImageInListViewState createState() =>
      new _ZoomableImageInListViewState();
}

final List<String> _urlList = [
  'https://www.w3schools.com/htmL/pic_trulli.jpg',
  'https://www.w3schools.com/htmL/img_girl.jpg',
  'https://www.w3schools.com/htmL/img_chania.jpg',
];

class _ZoomableImageInListViewState extends State<ZoomableImageInListView> {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Zoomable Image In ListView',
      debugShowCheckedModeBanner: false,
      home: new Scaffold(
        body: new Column(
          children: <Widget>[
            new Expanded(
              child: new ListView.builder(
                scrollDirection: Axis.vertical,
                itemBuilder: (context, index) => new ZoomableImage(
                    new NetworkImage(_urlList[index], scale: 1.0)),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

抛出此异常：

I/flutter (13594): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (13594): The following assertion was thrown building ZoomableImage(dirty, state: _ZoomableImageState#d60f4):
I/flutter (13594): A build function returned null.
I/flutter (13594): The offending widget is: ZoomableImage
I/flutter (13594): Build functions must never return null. To return an empty space that causes the building widget to
I/flutter (13594): fill available room, return "new Container()". To return an empty space that takes as little room as
I/flutter (13594): possible, return "new Container(width: 0.0, height: 0.0)".
.
.
.
I/flutter (13594): ════════════════════════════════════════════════════════════════════════════════════════════════════

我检查了 ListView 之外的两个插件，它们运行良好。我的实现有什么问题吗？这些插件支持 ListView 吗？如果答案是肯定的，请告诉我怎么做？

Answer 1

在您的第一个示例中，您需要这样定义函数_buildVerticalChild：

Widget _buildVerticalChild(BuildContext context, int index) {

不指定Widget 会使编译器认为_buildVerticalChild 可以返回任何东西。

在这两种情况下，您都需要指定itemCount

new ListView.builder(
    itemCount: _urlList.length
)

Answer 2

如果我错了，请纠正我，但从堆栈跟踪来看，我认为您的问题是您试图在父级中添加一个大小未知的子级，其大小也未知，并且颤动无法计算布局。要解决此问题，您需要创建一个具有固定大小的小部件（可能从其子项的初始状态计算，例如，在您的情况下为 Image），如 ClipRect。
虽然这解决了错误；它会让您的行为出现故障，因为在您的情况下，我们面临着here 提到的手势消歧，这意味着您有多个手势检测器 试图同时识别特定手势。确切地说，一个处理scale，它是pan 的超集，用于缩放和平移您的图像，另一个处理drag，用于滚动您的ListView。 为了克服这个问题，我认为您需要实现一个小部件来控制输入手势并手动决定是在 gesture arena 中宣布胜利还是宣布失败。
我附上了几行我发现的代码 here 和 there 一起为了实现所需的行为，对于这个特定示例，您将需要 flutter_advanced_networkimage 库，但您可以将 AdvancedNetworkImage 替换为其他小部件：

ZoomableCachedNetworkImage：

class ZoomableCachedNetworkImage extends StatelessWidget {
  String url;
  ImageProvider imageProvider;

  ZoomableCachedNetworkImage(this.url) {
    imageProvider = _loadImageProvider();
  }

  @override
  Widget build(BuildContext context) {
    return new ZoomablePhotoViewer(
      url: url,
    );
  }
  
  ImageProvider _loadImageProvider() {
    return new AdvancedNetworkImage(this.url);
  }
}

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

  final String url;

  @override
  _ZoomablePhotoViewerState createState() => new _ZoomablePhotoViewerState();
}

class _ZoomablePhotoViewerState extends State<ZoomablePhotoViewer>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation<Offset> _flingAnimation;
  Offset _offset = Offset.zero;
  double _scale = 1.0;
  Offset _normalizedOffset;
  double _previousScale;
  HitTestBehavior behavior;

  @override
  void initState() {
    super.initState();
    _controller = new AnimationController(vsync: this)
      ..addListener(_handleFlingAnimation);
  }

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

  // The maximum offset value is 0,0. If the size of this renderer's box is w,h
  // then the minimum offset value is w - _scale * w, h - _scale * h.
  Offset _clampOffset(Offset offset) {
    final Size size = context.size;
    final Offset minOffset =
        new Offset(size.width, size.height) * (1.0 - _scale);
    return new Offset(
        offset.dx.clamp(minOffset.dx, 0.0), offset.dy.clamp(minOffset.dy, 0.0));
  }

  void _handleFlingAnimation() {
    setState(() {
      _offset = _flingAnimation.value;
    });
  }

  void _handleOnScaleStart(ScaleStartDetails details) {
    setState(() {
      _previousScale = _scale;
      _normalizedOffset = (details.focalPoint - _offset) / _scale;
      // The fling animation stops if an input gesture starts.
      _controller.stop();
    });
  }

  void _handleOnScaleUpdate(ScaleUpdateDetails details) {
    setState(() {
      _scale = (_previousScale * details.scale).clamp(1.0, 4.0);
      // Ensure that image location under the focal point stays in the same place despite scaling.
      _offset = _clampOffset(details.focalPoint - _normalizedOffset * _scale);
    });
  }

  void _handleOnScaleEnd(ScaleEndDetails details) {
    const double _kMinFlingVelocity = 800.0;
    final double magnitude = details.velocity.pixelsPerSecond.distance;
    print('magnitude: ' + magnitude.toString());
    if (magnitude < _kMinFlingVelocity) return;
    final Offset direction = details.velocity.pixelsPerSecond / magnitude;
    final double distance = (Offset.zero & context.size).shortestSide;
    _flingAnimation = new Tween<Offset>(
            begin: _offset, end: _clampOffset(_offset + direction * distance))
        .animate(_controller);
    _controller
      ..value = 0.0
      ..fling(velocity: magnitude / 1000.0);
  }

  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      gestures: {
        AllowMultipleScaleRecognizer:
            GestureRecognizerFactoryWithHandlers<AllowMultipleScaleRecognizer>(
          () => AllowMultipleScaleRecognizer(), //constructor
          (AllowMultipleScaleRecognizer instance) {
            //initializer
            instance.onStart = (details) => this._handleOnScaleStart(details);
            instance.onEnd = (details) => this._handleOnScaleEnd(details);
            instance.onUpdate = (details) => this._handleOnScaleUpdate(details);
          },
        ),
        AllowMultipleHorizontalDragRecognizer:
            GestureRecognizerFactoryWithHandlers<AllowMultipleHorizontalDragRecognizer>(
          () => AllowMultipleHorizontalDragRecognizer(),
          (AllowMultipleHorizontalDragRecognizer instance) {
            instance.onStart = (details) => this._handleHorizontalDragAcceptPolicy(instance);
            instance.onUpdate = (details) => this._handleHorizontalDragAcceptPolicy(instance);
          },
        ),
        AllowMultipleVerticalDragRecognizer:
            GestureRecognizerFactoryWithHandlers<AllowMultipleVerticalDragRecognizer>(
          () => AllowMultipleVerticalDragRecognizer(),
          (AllowMultipleVerticalDragRecognizer instance) {
            instance.onStart = (details) => this._handleVerticalDragAcceptPolicy(instance);
            instance.onUpdate = (details) => this._handleVerticalDragAcceptPolicy(instance);
          },
        ),
      },
      //Creates the nested container within the first.
      behavior: HitTestBehavior.opaque,
      child: new ClipRect(
        child: new Transform(
          transform: new Matrix4.identity()
            ..translate(_offset.dx, _offset.dy)
            ..scale(_scale),
          child: Image(
            image: new AdvancedNetworkImage(widget.url),
            fit: BoxFit.cover,
          ),
        ),
      ),
    );
  }

  void _handleHorizontalDragAcceptPolicy(AllowMultipleHorizontalDragRecognizer instance) {
    _scale > 1.0 ? instance.alwaysAccept = true : instance.alwaysAccept = false;
  }

 void _handleVerticalDragAcceptPolicy(AllowMultipleVerticalDragRecognizer instance) {
   _scale > 1.0 ? instance.alwaysAccept = true : instance.alwaysAccept = false;
 }
}

AllowMultipleVerticalDragRecognizer：

import 'package:flutter/gestures.dart';

class AllowMultipleVerticalDragRecognizer extends VerticalDragGestureRecognizer {
  bool alwaysAccept;

  @override
  void rejectGesture(int pointer) {
    acceptGesture(pointer);
  }

  @override
  void resolve(GestureDisposition disposition) {
    if(alwaysAccept) {
      super.resolve(GestureDisposition.accepted);
    } else {
      super.resolve(GestureDisposition.rejected);
    }
  }
}

AllowMultipleHorizo​​ntalDragRecognizer：

import 'package:flutter/gestures.dart';

class AllowMultipleHorizontalDragRecognizer extends HorizontalDragGestureRecognizer {
  bool alwaysAccept;

  @override
  void rejectGesture(int pointer) {
    acceptGesture(pointer);
  }

  @override
  void resolve(GestureDisposition disposition) {
    if(alwaysAccept) {
      super.resolve(GestureDisposition.accepted);
    } else {
      super.resolve(GestureDisposition.rejected);
    }
  }
}

AllowMultipleScaleRecognizer

import 'package:flutter/gestures.dart';

class AllowMultipleScaleRecognizer extends ScaleGestureRecognizer {
  @override
  void rejectGesture(int pointer) {
    acceptGesture(pointer);
  }
}

然后像这样使用它：

@override
Widget build(BuildContext context) {
  return new MaterialApp(
    title: 'Zoomable Image In ListView',
    debugShowCheckedModeBanner: false,
    home: new Scaffold(
      body: new Column(
        children: <Widget>[
          new Expanded(
            child: new ListView.builder(
              scrollDirection: Axis.vertical,
              itemBuilder: (context, index) => ZoomableCachedNetworkImage(_urlList[index]),
            ),
          ),
        ],
      ),
    ),
  );
}

我希望这会有所帮助。

更新：

根据 cmets 的要求，为了支持 双击，您应该进行以下更改：

AllowMultipleDoubleTapRecognizer：

import 'package:flutter/gestures.dart';

class AllowMultipleDoubleTapRecognizer extends DoubleTapGestureRecognizer {
  @override
  void rejectGesture(int pointer) {
    acceptGesture(pointer);
  }
}

AllowMultipleTapRecognizer

import 'package:flutter/gestures.dart';

class AllowMultipleTapRecognizer extends TapGestureRecognizer {
  @override
  void rejectGesture(int pointer) {
    acceptGesture(pointer);
  }
}

ZoomableCachedNetworkImage

class ZoomableCachedNetworkImage extends StatelessWidget {
  final String url;
  final bool closeOnZoomOut;
  final Offset focalPoint;
  final double initialScale;
  final bool animateToInitScale;

  ZoomableCachedNetworkImage({
    this.url,
    this.closeOnZoomOut = false,
    this.focalPoint,
    this.initialScale,
    this.animateToInitScale,
  });

  Widget loadImage() {
    return ZoomablePhotoViewer(
      url: url,
      closeOnZoomOut: closeOnZoomOut,
      focalPoint: focalPoint,
      initialScale: initialScale,
      animateToInitScale: animateToInitScale,
    );
  }
}

class ZoomablePhotoViewer extends StatefulWidget {
  const ZoomablePhotoViewer({
    Key key,
    this.url,
    this.closeOnZoomOut,
    this.focalPoint,
    this.initialScale,
    this.animateToInitScale,
  }) : super(key: key);

  final String url;
  final bool closeOnZoomOut;
  final Offset focalPoint;
  final double initialScale;
  final bool animateToInitScale;

  @override
  _ZoomablePhotoViewerState createState() => _ZoomablePhotoViewerState(url,
      closeOnZoomOut: closeOnZoomOut,
      focalPoint: focalPoint,
      animateToInitScale: animateToInitScale,
      initialScale: initialScale);
}

class _ZoomablePhotoViewerState extends State<ZoomablePhotoViewer>
    with TickerProviderStateMixin {
  static const double _minScale = 0.99;
  static const double _maxScale = 4.0;
  AnimationController _flingAnimationController;
  Animation<Offset> _flingAnimation;
  AnimationController _zoomAnimationController;
  Animation<double> _zoomAnimation;
  Offset _offset;
  double _scale;
  Offset _normalizedOffset;
  double _previousScale;
  AllowMultipleHorizontalDragRecognizer _allowMultipleHorizontalDragRecognizer;
  AllowMultipleVerticalDragRecognizer _allowMultipleVerticalDragRecognizer;
  Offset _tapDownGlobalPosition;
  String _url;
  bool _closeOnZoomOut;
  Offset _focalPoint;
  bool _animateToInitScale;
  double _initialScale;

  _ZoomablePhotoViewerState(
    String url, {
    bool closeOnZoomOut = false,
    Offset focalPoint = Offset.zero,
    double initialScale = 1.0,
    bool animateToInitScale = false,
  }) {
    this._url = url;
    this._closeOnZoomOut = closeOnZoomOut;
    this._offset = Offset.zero;
    this._scale = 1.0;
    this._initialScale = initialScale;
    this._focalPoint = focalPoint;
    this._animateToInitScale = animateToInitScale;
  }

  @override
  void initState() {
    super.initState();
    if (_animateToInitScale) {
      WidgetsBinding.instance.addPostFrameCallback(
          (_) => _zoom(_focalPoint, _initialScale, context));
    }
    _flingAnimationController = AnimationController(vsync: this)
      ..addListener(_handleFlingAnimation);
    _zoomAnimationController = AnimationController(
        duration: const Duration(milliseconds: 200), vsync: this);
  }

  @override
  void dispose() {
    _flingAnimationController.dispose();
    _zoomAnimationController.dispose();
    super.dispose();
  }

  // The maximum offset value is 0,0. If the size of this renderer's box is w,h
  // then the minimum offset value is w - _scale * w, h - _scale * h.
  Offset _clampOffset(Offset offset) {
    final Size size = context.size;
    final Offset minOffset = Offset(size.width, size.height) * (1.0 - _scale);
    return Offset(
        offset.dx.clamp(minOffset.dx, 0.0), offset.dy.clamp(minOffset.dy, 0.0));
  }

  void _handleFlingAnimation() {
    setState(() {
      _offset = _flingAnimation.value;
    });
  }

  void _handleOnScaleStart(ScaleStartDetails details) {
    setState(() {
      _previousScale = _scale;
      _normalizedOffset = (details.focalPoint - _offset) / _scale;
      // The fling animation stops if an input gesture starts.
      _flingAnimationController.stop();
    });
  }

  void _handleOnScaleUpdate(ScaleUpdateDetails details) {
    if (_scale < 1.0 && _closeOnZoomOut) {
      _zoom(Offset.zero, 1.0, context);
      Navigator.pop(context);
      return;
    }
    setState(() {
      _scale = (_previousScale * details.scale).clamp(_minScale, _maxScale);
      // Ensure that image location under the focal point stays in the same place despite scaling.
      _offset = _clampOffset(details.focalPoint - _normalizedOffset * _scale);
    });
  }

  void _handleOnScaleEnd(ScaleEndDetails details) {
    const double _kMinFlingVelocity = 2000.0;
    final double magnitude = details.velocity.pixelsPerSecond.distance;
//    print('magnitude: ' + magnitude.toString());
    if (magnitude < _kMinFlingVelocity) return;
    final Offset direction = details.velocity.pixelsPerSecond / magnitude;
    final double distance = (Offset.zero & context.size).shortestSide;
    _flingAnimation = Tween<Offset>(
            begin: _offset, end: _clampOffset(_offset + direction * distance))
        .animate(_flingAnimationController);
    _flingAnimationController
      ..value = 0.0
      ..fling(velocity: magnitude / 2000.0);
  }

  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      gestures: {
        AllowMultipleScaleRecognizer:
            GestureRecognizerFactoryWithHandlers<AllowMultipleScaleRecognizer>(
          () => AllowMultipleScaleRecognizer(), //constructor
          (AllowMultipleScaleRecognizer instance) {
            //initializer
            instance.onStart = (details) => this._handleOnScaleStart(details);
            instance.onEnd = (details) => this._handleOnScaleEnd(details);
            instance.onUpdate = (details) => this._handleOnScaleUpdate(details);
          },
        ),
        AllowMultipleHorizontalDragRecognizer:
            GestureRecognizerFactoryWithHandlers<
                AllowMultipleHorizontalDragRecognizer>(
          () => AllowMultipleHorizontalDragRecognizer(),
          (AllowMultipleHorizontalDragRecognizer instance) {
            _allowMultipleHorizontalDragRecognizer = instance;
            instance.onStart =
                (details) => this._handleHorizontalDragAcceptPolicy(instance);
            instance.onUpdate =
                (details) => this._handleHorizontalDragAcceptPolicy(instance);
          },
        ),
        AllowMultipleVerticalDragRecognizer:
            GestureRecognizerFactoryWithHandlers<
                AllowMultipleVerticalDragRecognizer>(
          () => AllowMultipleVerticalDragRecognizer(),
          (AllowMultipleVerticalDragRecognizer instance) {
            _allowMultipleVerticalDragRecognizer = instance;
            instance.onStart =
                (details) => this._handleVerticalDragAcceptPolicy(instance);
            instance.onUpdate =
                (details) => this._handleVerticalDragAcceptPolicy(instance);
          },
        ),
        AllowMultipleDoubleTapRecognizer: GestureRecognizerFactoryWithHandlers<
            AllowMultipleDoubleTapRecognizer>(
          () => AllowMultipleDoubleTapRecognizer(),
          (AllowMultipleDoubleTapRecognizer instance) {
            instance.onDoubleTap = () => this._handleDoubleTap();
          },
        ),
        AllowMultipleTapRecognizer:
            GestureRecognizerFactoryWithHandlers<AllowMultipleTapRecognizer>(
          () => AllowMultipleTapRecognizer(),
          (AllowMultipleTapRecognizer instance) {
            instance.onTapDown =
                (details) => this._handleTapDown(details.globalPosition);
          },
        ),
      },
      //Creates the nested container within the first.
      behavior: HitTestBehavior.opaque,
      child: Transform(
        transform: Matrix4.identity()
          ..translate(_offset.dx, _offset.dy)
          ..scale(_scale),
        child: _buildTransitionToImage(),
      ),
    );
  }

  Widget _buildTransitionToImage() {
    return CachedNetworkImage(
      imageUrl: this._url,
      fit: BoxFit.contain,
      fadeOutDuration: Duration(milliseconds: 0),
      fadeInDuration: Duration(milliseconds: 0),
    );
  }

  void _handleHorizontalDragAcceptPolicy(
      AllowMultipleHorizontalDragRecognizer instance) {
    _scale != 1.0
        ? instance.alwaysAccept = true
        : instance.alwaysAccept = false;
  }

  void _handleVerticalDragAcceptPolicy(
      AllowMultipleVerticalDragRecognizer instance) {
    _scale != 1.0
        ? instance.alwaysAccept = true
        : instance.alwaysAccept = false;
  }

  void _handleDoubleTap() {
    setState(() {
      if (_scale >= 1.0 && _scale <= 1.2) {
        _previousScale = _scale;
        _normalizedOffset = (_tapDownGlobalPosition - _offset) / _scale;
        _scale = 2.75;
        _offset = _clampOffset(
            context.size.center(Offset.zero) - _normalizedOffset * _scale);
        _allowMultipleVerticalDragRecognizer.alwaysAccept = true;
        _allowMultipleHorizontalDragRecognizer.alwaysAccept = true;
      } else {
        if (_closeOnZoomOut) {
          _zoom(Offset.zero, 1.0, context);
          _zoomAnimation.addListener(() {
            if (_zoomAnimation.isCompleted) {
              Navigator.pop(context);
            }
          });
          return;
        }
        _scale = 1.0;
        _offset = _clampOffset(Offset.zero - _normalizedOffset * _scale);
        _allowMultipleVerticalDragRecognizer.alwaysAccept = false;
        _allowMultipleHorizontalDragRecognizer.alwaysAccept = false;
      }
    });
  }

  _handleTapDown(Offset globalPosition) {
    final RenderBox referenceBox = context.findRenderObject();
    _tapDownGlobalPosition = referenceBox.globalToLocal(globalPosition);
  }

  _zoom(Offset focalPoint, double scale, BuildContext context) {
    final RenderBox referenceBox = context.findRenderObject();
    focalPoint = referenceBox.globalToLocal(focalPoint);
    _previousScale = _scale;
    _normalizedOffset = (focalPoint - _offset) / _scale;
    _allowMultipleVerticalDragRecognizer.alwaysAccept = true;
    _allowMultipleHorizontalDragRecognizer.alwaysAccept = true;
    _zoomAnimation = Tween<double>(begin: _scale, end: scale)
        .animate(_zoomAnimationController);
    _zoomAnimation.addListener(() {
      setState(() {
        _scale = _zoomAnimation.value;
        _offset = scale < _scale
            ? _clampOffset(Offset.zero - _normalizedOffset * _scale)
            : _clampOffset(
                context.size.center(Offset.zero) - _normalizedOffset * _scale);
      });
    });
    _zoomAnimationController.forward(from: 0.0);
  }
}

abstract class ScaleDownHandler {
  void handleScaleDown();
}

Answer 3

我遇到了这个问题，但是一旦您将 ZoomableWidget 包装在容器中，它就会得到解决。所以，基本上高度没有界限。我是 Flutter 新手，请检查一次。

    children: <Widget>[

                   Container(
                    height: 450.0,
                    child: ZoomableWidget(
                      minScale: 0.3,
                      maxScale: 2.0,
                      // default factor is 1.0, use 0.0 to disable boundary
                      panLimit: 0.8,


                        child: TransitionToImage(

                          AdvancedNetworkImage(imageUrl, timeoutDuration: Duration(minutes: 2), useDiskCache: true),
                          // This is the default placeholder widget at loading status,
                          // you can write your own widget with CustomPainter.
                          placeholder: CircularProgressIndicator(),
                          // This is default duration
                          duration: Duration(milliseconds: 300),
                          height: 350.0,
                          width: 400.0,
                        ),

                    ),
                  ),
//                ),
                new Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: new Center(
                    child: new Text(
                      desc,
                      style: new TextStyle(fontSize: 16.0),
                      textAlign: TextAlign.start,
                    ),
                  ),
                ),

              ],