Flutter实现PS钢笔工具，实现高精度抠图的效果。

演示：

 代码：

import 'dart:ui';
 
import 'package:flutter/material.dart' hide Image;
import 'package:flutter/services.dart';
import 'package:flutter_screenutil/flutter_screenutil.dart';
import 'package:kq_flutter_widgets/widgets/animate/stack.dart';
import 'package:kq_flutter_widgets/widgets/button/kq_small_button.dart';
import 'package:kq_flutter_widgets/widgets/update/update_view.dart';
 
///抠图软件原型
class DrawPathTest extends StatefulWidget {
  const DrawPathTest({super.key});
 
  @override
  State createState() => DrawPathTestState();
}
 
class DrawPathTestState extends State<DrawPathTest> {
  ///是否绑定左右操作点，即操作一个点，另一个点自动计算
  static bool isBind = true;
 
  ///击中范围半径
  static double hitRadius = 5;
 
  ///绘制区域state持有
  UpdateViewState? state;
 
  ///背景图
  Image? _image;
 
  ///历史步骤存储
  KqStack stackHistory = KqStack();
 
  ///回收站步骤存储
  KqStack stackRecycleBin = KqStack();
 
  ///绘制步骤集合
  List drawList = [];
 
  ///手指按下时点击的控制点的位置缓存
  Step? hitControlStep;
 
  ///手指按下时点击的画线点的位置缓存
  Step? hitDrawStep;
 
  ///闭合绘制完成状态，不再添加点
  bool drawFinish = false;
 
  @override
  void initState() {
    super.initState();
    _load("https://c-ssl.duitang.com/uploads/item/201903/19/20190319001325_bjvzi.jpg")
        .then((value) {
      _image = value;
      update();
    });
  }
 
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Expanded(
          child: LayoutBuilder(builder: (c, lc) {
            return Container(
              color: Colors.white60,
              child: Listener(
                onPointerDown: (v) {
                  Offset src = v.localPosition;
 
                  ///判断是否hit
                  hitDrawStep = _isHitDrawPoint(src);
                  if (!drawFinish) {
                    if (hitDrawStep != null && hitDrawStep!.isFirst) {
                      _add(src, isLast: true);
                      drawFinish = true;
                    } else {
                      hitControlStep = _isHitControlPoint(src);
                      hitControlStep ??= _add(src);
                    }
                    update();
                  } else {
                    hitControlStep = _isHitControlPoint(src);
                  }
                },
                onPointerMove: (v) {
                  if (hitDrawStep != null) {
                    _update(hitDrawStep!, v.localPosition);
                    update();
                  } else if (hitControlStep != null) {
                    _update(hitControlStep!, v.localPosition);
                    update();
                  }
                },
                child: UpdateView(
                  build: (UpdateViewState state) {
                    this.state = state;
                    return CustomPaint(
                      size: Size(lc.maxWidth, lc.maxHeight),
                      painter: TestDraw(_image, drawList),
                    );
                  },
                ),
              ),
            );
          }),
        ),
        Row(
          children: [
            SizedBox(width: 20.r),
            Expanded(
              child: KqSmallButton(
                title: "撤销",
                onTap: (disabled) {
                  _undo();
                  update();
                },
              ),
            ),
            SizedBox(width: 20.r),
            Expanded(
              child: KqSmallButton(
                title: "重做",
                onTap: (disabled) {
                  _redo();
                  update();
                },
              ),
            ),
            SizedBox(width: 20.r),
            Expanded(
              child: KqSmallButton(
                title: "选择",
                onTap: (disabled) {
                  _select();
                  update();
                },
              ),
            ),
            SizedBox(width: 20.r),
            Expanded(
              child: KqSmallButton(
                title: "反选",
                onTap: (disabled) {
                  _invert();
                  update();
                },
              ),
            ),
            SizedBox(width: 20.r),
            Expanded(
              child: KqSmallButton(
                title: "删除",
                onTap: (disabled) {
                  _delete();
                  update();
                },
              ),
            ),
            SizedBox(width: 20.r),
          ],
        ),
        SizedBox(height: 20.r),
      ],
    );
  }
 
  ///更新绘制区域
  update() {
    state?.update();
  }
 
  ///添加点
  Step _add(Offset offset, {bool isLast = false}) {
    Step step = Step(offset, offset, offset);
    step.isLast = isLast;
    if (drawList.isEmpty) {
      step.isFirst = true;
    }
    //添加到历史
    stackHistory.push(step);
    //添加到绘制列表
    drawList.add(step);
    //清除垃圾箱
    stackRecycleBin.clear();
    return step;
  }
 
  ///判断是否点击在控制点上
  Step? _isHitControlPoint(Offset src) {
    for (Step step in drawList) {
      if (_distance(step.pointRight, src) < hitRadius) {
        step.hitPointType = PointType.pointRight;
        return step;
      } else if (_distance(step.pointLeft, src) < hitRadius) {
        step.hitPointType = PointType.pointLeft;
        return step;
      }
    }
    return null;
  }
 
  ///判断是否点击在连接点上
  Step? _isHitDrawPoint(Offset src) {
    for (Step step in drawList) {
      if (_distance(step.point, src) < hitRadius) {
        step.hitPointType = PointType.point;
        return step;
      }
    }
    return null;
  }
 
  ///更新点信息
  _update(Step hitStep, Offset target) {
    if (hitStep.hitPointType == PointType.pointRight) {
      hitStep.pointRight = target;
      if (isBind) {
        hitStep.pointLeft = hitStep.point.scale(2, 2) - target;
      }
    } else if (hitStep.hitPointType == PointType.pointLeft) {
      hitStep.pointLeft = target;
      if (isBind) {
        hitStep.pointRight = hitStep.point.scale(2, 2) - target;
      }
    } else if (hitStep.hitPointType == PointType.point) {
      hitStep.pointLeft = hitStep.pointLeft - hitStep.point + target;
      hitStep.pointRight = hitStep.pointRight - hitStep.point + target;
      hitStep.point = target;
    }
  }
 
  ///两点距离
  double _distance(Offset one, Offset two) {
    return (one - two).distance;
  }
 
  ///撤销、后退
  _undo() {
    Step? step = stackHistory.pop();
    if (step != null) {
      drawList.remove(step);
      stackRecycleBin.push(step);
    }
  }
 
  ///重做、前进
  _redo() {
    Step? step = stackRecycleBin.pop();
    if (step != null) {
      drawList.add(step);
      stackHistory.push(step);
    }
  }
 
  ///选择、完成
  _select() {}
 
  ///反选、完成
  _invert() {}
 
  ///删除
  _delete() {}
 
  ///加载图片
  Future _load(String url) async {
    ByteData data = await NetworkAssetBundle(Uri.parse(url)).load(url);
    Codec codec = await instantiateImageCodec(data.buffer.asUint8List());
    FrameInfo fi = await codec.getNextFrame();
    return fi.image;
  }
}
 
class TestDraw extends CustomPainter {
  static double width = 260;
  static double width1 = 50;
  static double height1 = 100;
 
  ///绘制集合
  final List draw;
 
  ///背景图片
  final Image? image;
 
  Step? tempStep;
  Step? tempFirstStep;
 
  TestDraw(this.image, this.draw);
 
  @override
  void paint(Canvas canvas, Size size) {
    ///绘制背景
    if (image != null) {
      canvas.drawImageRect(
        image!,
        Rect.fromLTRB(
          0,
          0,
          image!.width.toDouble(),
          image!.height.toDouble(),
        ),
        Rect.fromLTRB(
          width1,
          height1,
          width + width1,
          width * image!.height / image!.width + height1,
        ),
        Paint(),
      );
    }
 
    if (draw.isNotEmpty) {
      ///构建画点与点之间的连线的path
      Path path = Path();
 
      ///绘制点和线
      for (int i = 0; i < draw.length; i++) {
        Step step = draw[i];
        if (!step.isLast) {
          canvas.drawCircle(step.point, 4.r, Paint()..color = Colors.red);
          canvas.drawCircle(
              step.pointLeft, 4.r, Paint()..color = Colors.purple);
          canvas.drawCircle(
              step.pointRight, 4.r, Paint()..color = Colors.purple);
 
          ///画控制点和连线点之间的线段
          canvas.drawLine(
              step.point,
              step.pointLeft,
              Paint()
                ..color = Colors.green
                ..style = PaintingStyle.stroke);
          canvas.drawLine(
              step.point,
              step.pointRight,
              Paint()
                ..color = Colors.green
                ..style = PaintingStyle.stroke);
        }
 
        ///构建画点与点之间的连线的path
        if (step.isLast) {
          if (tempFirstStep != null && tempStep != null) {
            path.cubicTo(
              tempStep!.pointRight.dx,
              tempStep!.pointRight.dy,
              tempFirstStep!.pointLeft.dx,
              tempFirstStep!.pointLeft.dy,
              tempFirstStep!.point.dx,
              tempFirstStep!.point.dy,
            );
          }
        } else {
          //处理初始点
          if (step.isFirst) {
            tempFirstStep = step;
            path.moveTo(step.point.dx, step.point.dy);
          }
          if (tempStep != null) {
            path.cubicTo(
              tempStep!.pointRight.dx,
              tempStep!.pointRight.dy,
              step.pointLeft.dx,
              step.pointLeft.dy,
              step.point.dx,
              step.point.dy,
            );
          }
        }
 
        tempStep = step;
      }
 
      if (draw.length >= 2) {
        canvas.drawPath(
          path,
          Paint()
            ..color = Colors.red
            ..style = PaintingStyle.stroke
            ..strokeWidth = 1.5,
        );
      }
    }
  }
 
  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return true;
  }
}
 
class Step {
  ///线条连接点
  Offset point;
 
  ///右控制点
  Offset pointRight;
 
  ///左控制点（起始点没有左控制点的）
  Offset pointLeft;
 
  ///是否选中了点的类型
  PointType hitPointType = PointType.pointRight;
 
  ///是否是第一个控制点
  bool isFirst = false;
 
  ///是否是最后一个控制点
  bool isLast = false;
 
  Step(
    this.point,
    this.pointRight,
    this.pointLeft,
  );
}
 
///点类型
enum PointType {
  ///线条连接点
  point,
 
  ///右控制点
  pointRight,
 
  ///左控制点
  pointLeft
}

stack代码：

///栈，先进后出
class KqStack<T> {
  final List _stack = [];
 
  ///入栈
  push(T obj) {
    _stack.add(obj);
  }
 
  ///出栈
  T? pop() {
    if (_stack.isEmpty) {
      return null;
    } else {
      return _stack.removeLast();
    }
  }
 
  ///栈长度
  length() {
    return _stack.length;
  }
 
  ///清除栈
  clear() {
    _stack.clear();
  }
}

主要思路：

更具手指点击屏幕的位置，记录点击的位置，并生成绘制点和两个控制点，手指拖动控制点时，动态刷新控制点位置，然后利用flutter绘制机制，在canvas上根据点的位置和控制点的位置绘制三阶贝塞尔曲线，实现钢笔工具效果。具体实现可以看代码，有注释，逻辑应该还算清晰。