Flutter 手势缩放图片并且截取指定区域显示

效果图如上, 我是看的maka编辑器,照着来的,结果光这个动画居然写了两三天,不过还好弄好了,带有边界检测,手势放大,放大的区域可以在阿里云生成指定大小的图片,效果很完美, 贴代码

import 'dart:ui';

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

class ClipperImage2 extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _ClipperState();
  }
}

class _ClipperState extends State {
  String backgroundUrl = "https://img1.maka.im/materialStore/beijingshejia/tupianbeijinga/9/M_69F3SAO8/M_69F3SAO8_v1.jpg";

  GlobalKey _contentGlobalKey = GlobalKey();

  ///图片的唯一KEY
  GlobalKey _pictureGlobalKey = GlobalKey();

  double viewWidth = 125;
  double viewHeight = 196;

  double imageOriginWidth = 750;
  double imageOriginHeight = 1181;

  double _currentScale = 1.0;
  Offset _pictureDefOffset = Offset(100, 100);
  Offset _dragViewOffset = Offset(100, 100);

  double _lastViewScale = 1.0;
  Offset _lastViewPoint;

  @override
  void didUpdateWidget(covariant StatefulWidget oldWidget) {
    super.didUpdateWidget(oldWidget);
    // _currentScale = 1.82;
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      key: _contentGlobalKey,
      onScaleStart: _handleScaleStart,
      onScaleUpdate: (d) => _handleScaleUpdate(context.size, d),
      onScaleEnd: _handleScaleEnd,
      child: Stack(
        children: [
          Positioned(
            left: _dragViewOffset.dx,
            top: _dragViewOffset.dy,
            child: Transform.scale(
              scale: _currentScale,
              child: Image.network(
                backgroundUrl,
                key: _pictureGlobalKey,
                width: viewWidth,
                height: viewHeight,
                fit: BoxFit.cover,
              ),
            ),
          ),
          //
          CustomPaint(
            painter: DrawRectLight(Size(viewWidth, viewHeight), _pictureDefOffset),
            size: Size(double.infinity, double.infinity),
          ),
          //
          Positioned(
            bottom: 20,
            left: 0,
            right: 0,
            child: InkWell(
              onTap: saveClipperInfo,
              child: Text(
                "双指缩放调整图片大小",
                style: TextStyle(fontSize: 14, color: Colors.white),
                textAlign: TextAlign.center,
              ),
            ),
          ),
        ],
      ),
    );
  }

  /* Offset _getRelativePosition() {
    RenderBox imageRenderBox = _pictureGlobalKey.currentContext.findRenderObject();
    RenderBox contentRenderBox = _contentGlobalKey.currentContext.findRenderObject();
    Offset _relativePosition = imageRenderBox.globalToLocal(
      _pictureDefOffset,
      ancestor: contentRenderBox,
    );
    return _relativePosition;
  }*/

  void _handleScaleStart(ScaleStartDetails details) {
    print('_handleScaleStart');
    _lastViewScale = _currentScale;
    _lastViewPoint = details.focalPoint;
  }

  void _handleScaleUpdate(Size size, ScaleUpdateDetails details) {
    if (details.scale != 1) {
      ///缩放
      double tempScale = _lastViewScale * details.scale;

      //缩放可以保证不小于1, 放大到多大并不管
      if (tempScale < 1) return;
      //缩放生效
      _currentScale = tempScale;
    } else {
      Offset tempPositionPoint = _dragViewOffset + (details.focalPoint - _lastViewPoint);
      _dragViewOffset = tempPositionPoint;
      _lastViewPoint = details.focalPoint;
    }


    ///检测是否临界边界
    double _correctDx = _dragViewOffset.dx;
    double _correctDy = _dragViewOffset.dy;

    double leftVerticalLine = (viewWidth * _currentScale - viewWidth) / 2 + _pictureDefOffset.dx;
    double topVerticalLine = (viewHeight * _currentScale - viewHeight) / 2 + _pictureDefOffset.dy;
    double rightVerticalLine = _pictureDefOffset.dx - (leftVerticalLine - _pictureDefOffset.dx);
    double bottomHorizontalLine = _pictureDefOffset.dy - (topVerticalLine - _pictureDefOffset.dy);

    if (_correctDx > leftVerticalLine) {
      _correctDx = leftVerticalLine;
    }
    if (_correctDy > topVerticalLine) {
      _correctDy = topVerticalLine;
    }
    if (_correctDx < rightVerticalLine) {
      _correctDx = rightVerticalLine;
    }
    if(_correctDy < bottomHorizontalLine){
      _correctDy = bottomHorizontalLine;
    }

    _dragViewOffset = Offset(_correctDx, _correctDy);


    setState(() {});
  }

  void _handleScaleEnd(ScaleEndDetails details) {
    setState(() {});
  }

  void saveClipperInfo() {
    int clipWidth = (imageOriginWidth / _currentScale).round();
    int clipHeight = (imageOriginHeight / _currentScale).round();

    int offsetX = ((((viewWidth * _currentScale - viewWidth) / 2 + _pictureDefOffset.dx) - _dragViewOffset.dx) *
            (imageOriginWidth / (viewWidth * _currentScale)))
        .abs()
        .round();
    int offsetY = ((((viewHeight * _currentScale - viewHeight) / 2 + _pictureDefOffset.dy) - _dragViewOffset.dy) *
            (imageOriginHeight / (viewHeight * _currentScale)))
        .abs()
        .round();

    print(backgroundUrl +
        "?x-oss-process=image/crop,"
            "x_$offsetX,"
            "y_$offsetY,"
            "w_$clipWidth,"
            "h_$clipHeight");
  }
}

class DrawRectLight extends CustomPainter {
  Size _pictureSize;
  Offset _pictureOffset;

  DrawRectLight(this._pictureSize, this._pictureOffset);

  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint();

    paint
      ..style = PaintingStyle.fill
      ..color = Color(0xaa000000);

    ///假设是中心点
    canvas.clipRect(
      Rect.fromLTRB(
        _pictureOffset.dx,
        _pictureOffset.dy,
        _pictureOffset.dx + _pictureSize.width,
        _pictureOffset.dy + _pictureSize.height,
      ),
      clipOp: ClipOp.difference,
    );

    canvas.drawRect(Rect.fromLTRB(0, 0, size.width, size.height), paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}