211_1668739627.gif
利用canvas画布给“刮奖区”上面盖一层蒙板，再利用Paint画笔的BlendMode.clear属性，把canvas上的蒙板擦掉
Listener(
onPointerDown: (PointerDownEvent event) {
},
onPointerUp: (PointerUpEvent event) {
},
onPointerCancel: (PointerCancelEvent event) {
},
onPointerMove: (PointerMoveEvent event) {
},
child: RepaintBoundary(
child: CustomPaint(
foregroundPainter: MyPainter(pointsNotifier),
child: Container(
width: width,
height: height,
color: Colors.redAccent,
alignment: Alignment.center,
child: Text(
'再来一瓶',
style: TextStyle(
fontSize: 28,
fontWeight: FontWeight.bold,
color: Colors.yellow,
),
),
),
),
),
)
foregroundPainter是前景画板，蒙层的绘制就在前景画板上
画布外面套了一层Listener，用来监听刮奖的手势，方便在蒙板上画“透明”的线，而RepaintBoundary可以避免因各种父组件造成的过多的重绘。
Canvas画板要根据手势进行画线和实时刷新，先准备一个ValueNotifier和Point类：
enum PointType {
tap,
move,
}
class Point {
Offset offset;
PointType type;
Point(this.offset, this.type);
}
class PointsNotifier extends ValueNotifier<List<Point>> {
PointsNotifier({List<Point>? points}) : super(points ?? <Point>[]);
void addPoint(Point point) {
value.add(point);
notifyListeners();
}
}
PointsNotifier继承自ValueNotifier，我们可以手动notifyListeners，完成刷新通知，如果直接用ValueNotifier监听List，List的add不会触发刷新，必须给List重新赋值才行
比如下面的方式不会触发刷新：
ValueNotifier<List<Point>> points = ValueNotifier([]);
points.add(Point(offset, type));
这样才会触发：
ValueNotifier<List<Point>> points = ValueNotifier([]);
points = otherPoints;
接下来在手势监听回调中修改Point列表数据：
onPointerDown: (PointerDownEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.tap,
));
},
onPointerUp: (PointerUpEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.tap,
));
},
onPointerCancel: (PointerCancelEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.tap,
));
},
onPointerMove: (PointerMoveEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.move,
));
},
Point里保存了一个PointType，如果是拖动类型（PointType.move），我们就把它和上一条线连起来
接下来准备Canvas画布：
class MyPainter extends CustomPainter {
PointsNotifier _pointsNotifier;
Paint _paint;
MyPainter(this._pointsNotifier)
: _paint = Paint(),
super(repaint: _pointsNotifier) {
_paint
..strokeWidth = 15
..blendMode = BlendMode.clear;
}
@override
void paint(Canvas canvas, Size size) {
Rect rect = Rect.fromCenter(
center: Offset(size.width / 2, size.height / 2),
width: size.width,
height: size.height);
canvas.saveLayer(rect, Paint());
// 绘制蒙板
canvas.drawRect(rect, Paint()..color = Colors.grey);
final List<Point> points = _pointsNotifier.value;
for (int i = 0; i < (points.length - 1); i++) {
if (points[i + 1].type == PointType.move) {
canvas.drawLine(
points[i].offset,
points[i + 1].offset,
_paint,
);
}
}
canvas.restore();
}
@override
bool shouldRepaint(CustomPainter other) => true;
}
CustomPainter类中可以传入一个repaint来触发刷新，把PointsNotifier传进去即可，Paint的strokeWidth设置画笔粗细，blendMode设置为BlendMode.clear就是“橡皮擦”的效果
在void paint(Canvas canvas, Size size)方法中进行绘制，canvas.drawRect用来绘制蒙板，蒙板完成后，遍历pointsNotifier中保存的Point数组，完成橡皮擦线条绘制
下面这两条很重要：
canvas.saveLayer(rect, Paint());
canvas.restore();
如果blendMode设为BlendMode.clear，必须在绘制前调用saveLayer保存画布状态，绘制完成后restore恢复，否则“橡皮擦”不生效，线条颜色会变黑色
完整代码：
import 'package:flutter/material.dart';
class HomePage extends StatefulWidget {
const HomePage({Key? key}) : super(key: key);
@override
_HomePageState createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
double width = 200;
double height = 100;
PointsNotifier pointsNotifier = PointsNotifier();
@override
Widget build(BuildContext context) {
return Center(
child: Listener(
onPointerDown: (PointerDownEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.tap,
));
},
onPointerUp: (PointerUpEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.tap,
));
},
onPointerCancel: (PointerCancelEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.tap,
));
},
onPointerMove: (PointerMoveEvent event) {
pointsNotifier.addPoint(Point(
event.localPosition,
PointType.move,
));
},
child: RepaintBoundary(
child: CustomPaint(
foregroundPainter: MyPainter(pointsNotifier),
child: Container(
width: width,
height: height,
color: Colors.redAccent,
alignment: Alignment.center,
child: Text(
'再来一瓶',
style: TextStyle(
fontSize: 28,
fontWeight: FontWeight.bold,
color: Colors.yellow,
),
),
),
),
),
),
);
}
}
enum PointType {
tap,
move,
}
class Point {
Offset offset;
PointType type;
Point(this.offset, this.type);
}
class MyPainter extends CustomPainter {
PointsNotifier _pointsNotifier;
Paint _paint;
MyPainter(this._pointsNotifier)
: _paint = Paint(),
super(repaint: _pointsNotifier) {
_paint
..strokeWidth = 15
..blendMode = BlendMode.clear;
}
@override
void paint(Canvas canvas, Size size) {
Rect rect = Rect.fromCenter(
center: Offset(size.width / 2, size.height / 2),
width: size.width,
height: size.height);
canvas.saveLayer(rect, Paint());
// 绘制蒙板
canvas.drawRect(rect, Paint()..color = Colors.grey);
final List<Point> points = _pointsNotifier.value;
for (int i = 0; i < (points.length - 1); i++) {
if (points[i + 1].type == PointType.move) {
canvas.drawLine(
points[i].offset,
points[i + 1].offset,
_paint,
);
}
}
canvas.restore();
}
@override
bool shouldRepaint(CustomPainter other) => true;
}
class PointsNotifier extends ValueNotifier<List<Point>> {
PointsNotifier({List<Point>? points}) : super(points ?? <Point>[]);
void addPoint(Point point) {
value.add(point);
notifyListeners();
}
}
PS：期间碰到一个问题，如果画笔Paint设为圆头strokeCap = StrokeCap.round，疯狂绘制线条会越来越卡顿，没能找到解决办法，只能不设置strokeCap属性了
