如何平移和缩放图像？

Question

我正在尝试为图像获得一些基本的平移和缩放功能。无状态版本可以显示图像（通过 Transform 旋转 180 度）并且 Scale 事件显示在日志中，但仅此而已。

是GestureDetector获取平移/捏/展开事件的正确小部件吗？我应该查看转换、动画，还是应该只修改Image小部件内的字段？

无状态版本

// Wraps an Image widget to provide pan and zoom functionality.
class InteractiveImage extends StatelessWidget {
  InteractiveImage(this._image, {Key key}) : super(key: key);

  final Image _image;

  @override
  Widget build(BuildContext context) {
    return new Center(
      child: new GestureDetector(
        onScaleStart: (ScaleStartDetails details) => print(details),
        onScaleUpdate: (ScaleUpdateDetails details) => print(details),
        onScaleEnd: (ScaleEndDetails details) => print(details),
        child: new Transform(
          transform: new Matrix4.rotationZ(math.PI),
          alignment: FractionalOffset.center,
          child: _image,
        ),
      ),
    );
  }
}

有状态版本（不起作用）

// Wraps an Image widget to provide pan and zoom functionality.
class InteractiveImage extends StatefulWidget {
  InteractiveImage(this._image, {Key key}) : super(key: key);

  final Image _image;

  @override
  _InteractiveImageState createState() => new _InteractiveImageState(_image);
}

class _InteractiveImageState extends State<InteractiveImage> {
  _InteractiveImageState(this._image);

  final Image _image;

  @override
  Widget build(BuildContext context) {
    setState(() => print("STATE SET\n"));
    return new GestureDetector(
      onScaleStart: (ScaleStartDetails details) => print(details),
      onScaleUpdate: (ScaleUpdateDetails details) => print(details),
      onScaleEnd: (ScaleEndDetails details) => print(details),
      child: new Transform(
        transform: new Matrix4.rotationZ(math.PI),
        alignment: FractionalOffset.center,
        child: _image,
      ),
    );
  }
}

更新（库解决方案）

使用https://pub.dartlang.org/packages/zoomable_image

（也许可以帮我添加物理和弹性边缘？）

Answer 1

您可以使用随 Flutter 开箱即用的InteractiveViewer。

@override
Widget build(BuildContext context) {
  return Center(
    child: InteractiveViewer(
      panEnabled: false, // Set it to false
      boundaryMargin: EdgeInsets.all(100),
      minScale: 0.5,
      maxScale: 2,
      child: Image.asset(
        'your_image_asset',
        width: 200,
        height: 200,
        fit: BoxFit.cover,
      ),
    ),
  );
}

Answer 2

我不确定你所说的“不起作用”是什么意思，但你走在正确的轨道上。

查看https://github.com/flutter/flutter/blob/master/examples/layers/widgets/gestures.dart以获得可以平移、捏合和缩放的小部件的工作示例。

Answer 3

根据问题中的代码并受到科林答案的鼓励，这就是我想到的：

import 'package:flutter/material.dart';
import 'package:vector_math/vector_math_64.dart';

class InteractiveImage extends StatefulWidget {
  InteractiveImage(this.image, {Key key}) : super(key: key);

  final Image image;

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

class _InteractiveImageState extends State<InteractiveImage> {
  _InteractiveImageState();

  double _scale = 1.0;
  double _previousScale = null;

  @override
  Widget build(BuildContext context) {
    setState(() => print("STATE SET\n"));
    return new GestureDetector(
      onScaleStart: (ScaleStartDetails details) {
        print(details);
        // Does this need to go into setState, too?
        // We are only saving the scale from before the zooming started
        // for later - this does not affect the rendering...
        _previousScale = _scale;
      },
      onScaleUpdate: (ScaleUpdateDetails details) {
        print(details);
        setState(() => _scale = _previousScale * details.scale);
      },
      onScaleEnd: (ScaleEndDetails details) {
        print(details);
        // See comment above
        _previousScale = null;
      },
      child: new Transform(
        transform: new Matrix4.diagonal3(new Vector3(_scale, _scale, _scale)),
        alignment: FractionalOffset.center,
        child: widget.image,
      ),
    );
  }
}

虽然它似乎有效，但如果能得到 Flutter 团队内部人员的确认或纠正那就太好了。

（此外，这仅允许缩放。未实现平移。）

编辑：编辑以反映科林的评论和他更详细的解释。但请注意，关于私有setState()和私有字段仍然存在一个悬而未决的问题，这些问题不会立即影响渲染。