Flutter CustomPaint自定义绘画示例详解

正文

CustomPaint是Flutter中用于自由绘制的一个widget,它与android原生的绘制规则基本一致,以当前Canves(画布)的左上角为原点进行绘制。在有些场景中,我们会需要绘制一些高度定制化的组件,比如 UI 设计师给我们出了个难题 —— 弄一个奇形怪状的边框。这个时候我们就不能直接使用 Flutter 自带的那些组件了,而是需要手动绘制组件,那就会需要用到 CuntomPaint 组件。CustomPaint 组件和前端的 Canvas差不多,允许我们在一个画布上绘制各种元素,包括点、线、矩形、圆弧、文字、图片等等。

CustomPaint 介绍

CustomPaint是一个 Widget,其中有三个重要的参数:

CustomPaint(
 child: childWidget(),
 foregroundPainter: foregroundPainter(),
 painter: backgroundPainter(),
)

childCustomPaint的子组件;

painterforegroundPainter:都是 CustomPainter 类,用于定义 canvas 绘制的内容。区别在于,首先是执行 painter 的绘制指令。然后是在背景上渲染 child 子组件。最后,foregroundPainter 的内容会绘制在 child 上一层。

案例展示:

import 'package:demo202112/utils/common_appbar.dart';
import "package:flutter/material.dart";
class MyPaint extends StatelessWidget {
 const MyPaint({Key? key}) : super(key: key);
 @override
 Widget build(BuildContext context) {
 return Scaffold(
 appBar: getAppBar('CustomPaint'),
 body: CustomPaint(
 painter: MyPainer(),
 child: Container(height: 80,width: 80,child: Text('child测试'),color: Colors.red,),
 foregroundPainter: MyForeGroundPainer(),
 ),
 );
 }
}
class MyPainer extends CustomPainter{
 late Paint _paint;
 @override
 void paint(Canvas canvas, Size size) {
 _paint = Paint();
 _paint.color = Colors.blue;
 canvas.drawCircle(Offset(100, 100), 100, _paint);
 canvas.drawLine(Offset(300, 300), Offset(400, 400), _paint);
 // TODO: implement paint
 }
 @override
 bool shouldRepaint(covariant CustomPainter oldDelegate) {
 // TODO: implement shouldRepaint
 throw UnimplementedError();
 }
}
class MyForeGroundPainer extends CustomPainter{
 late Paint _paint;
 @override
 void paint(Canvas canvas, Size size) {
 _paint = Paint();
 _paint.color = Colors.green;
 canvas.drawCircle(Offset(100, 100), 70, _paint);
 // canvas.drawLine(Offset(300, 300), Offset(400, 400), _paint);
 // TODO: implement paint
 }
 @override
 bool shouldRepaint(covariant CustomPainter oldDelegate) {
 // TODO: implement shouldRepaint
 throw UnimplementedError();
 }
}

运行效果:

child: 红色区域,传入一个子widget,这个widget图层会在painter在上,在foregroundPainter之下。

painter:蓝色区域。

foregroundPainter:绿色区域,它与painter都是CustomPainter类型的。通过名字大概也就知道了,它会在painter的上层,也就是说在同样的位置去绘制,foregroundPainter 会覆盖painter。

CustomPainter提供了一个paint绘图方法供我们绘制图形,该方法携带canvassize两个参数,其中 canvas 是画布,size 是画布大小。canvas 提供了很多绘制图形的方法,比如绘制路径、矩形、圆形和线条等等。

//画圆
drawCircle(Offset c, double radius, Paint paint) → void
//画图片
drawImage(Image image, Offset p, Paint paint) → void
//画九宫图
drawImageNine(Image image, Rect center, Rect dst, Paint paint) → void
//画线
drawLine(Offset p1, Offset p2, Paint paint) → void
//画椭圆
drawOval(Rect rect, Paint paint) → void
//画文字
drawParagraph(Paragraph paragraph, Offset offset) → void
//画Rect区域
drawRect(Rect rect, Paint paint) → void
//画阴影
drawShadow(Path path, Color color, double elevation, bool transparentOccluder) → void

绘制点

class MyPoints extends CustomPainter{
 Paint _paint = Paint()
 ..color = Colors.red
 ..strokeWidth = 15;
 @override
 void paint(Canvas canvas, Size size) {
 // TODO: implement paint
 var points =[
 Offset(0, 0),
 Offset(size.width/2, size.height/2),
 Offset(size.width, size.height),
 ];
 canvas.drawPoints(PointMode.points, points, _paint);
 }
 @override
 bool shouldRepaint(covariant CustomPainter oldDelegate) {
 // TODO: implement shouldRepaint
 throw UnimplementedError();
 }
}

运行效果:

PointMode3种模式

  • points:点
  • lines:将2个点绘制为线段,如果点的个数为奇数,最后一个点将会被忽略
  • polygon:将整个点绘制为一条线

绘制线 和路径

class MyGraph extends CustomPainter{
 final Paint _paint = Paint()
 ..color = Colors.red
 ..strokeWidth = 15;
 final Paint _paintPath = Paint()
 ..color = Colors.blue
 ..strokeWidth = 5
 ..style = PaintingStyle.fill;
 @override
 void paint(Canvas canvas, Size size) {
 // TODO: implement paint
 //绘制线
 canvas.drawLine(Offset(0, 30),Offset(size.width-30, size.height), _paint);
 //绘制路径
 var _path = Path()
 ..moveTo(0, 0)
 ..lineTo(size.width, 0)
 ..lineTo(size.width, size.height)
 ..close();
 canvas.drawPath(_path, _paintPath);
 //这里注意Paint.style,还可以设置为PaintingStyle.fill,
 //绘制圆形
 canvas.drawCircle(Offset(size.width/2+50, size.height/2+50), 20, _paint);
 //绘制椭圆
 canvas.drawOval(Rect.fromLTRB(0, 0, size.width, size.height/2), _paint);
 //绘制弧
 canvas.drawArc(Rect.fromLTRB(0, 0, size.width, size.height), 0, pi/2, true, _paint);
 //绘制圆角矩形
 canvas.drawRRect(RRect.fromLTRBR(0, 0, size.width, size.height, Radius.circular(10)), _paint);
 }
 @override
 bool shouldRepaint(covariant CustomPainter oldDelegate) {
 // TODO: implement shouldRepaint
 throw UnimplementedError();
 }
}

运行效果:

绘制五子棋

首先绘制背景,淡黄色,再绘制棋盘网格线,随后绘制黑白子,具体代码:

class CustomPaintRoute extends StatelessWidget {
 @override
 Widget build(BuildContext context) {
 return Center(
 child: CustomPaint(
 size: Size(300, 300), //指定画布大小
 painter: MyPainter(),
 ),
 );
 }
}
class MyPainter extends CustomPainter {
 @override
 void paint(Canvas canvas, Size size) {
 double eWidth = size.width / 15;
 double eHeight = size.height / 15;
 //画棋盘背景
 var paint = Paint()
 ..isAntiAlias = true
 ..style = PaintingStyle.fill //填充
 ..color = Color(0x77cdb175); //背景为纸黄色
 canvas.drawRect(Offset.zero & size, paint);
 //画棋盘网格
 paint
 ..style = PaintingStyle.stroke //线
 ..color = Colors.black87
 ..strokeWidth = 1.0;
 for (int i = 0; i <= 15; ++i) {
 double dy = eHeight * i;
 canvas.drawLine(Offset(0, dy), Offset(size.width, dy), paint);
 }
 for (int i = 0; i <= 15; ++i) {
 double dx = eWidth * i;
 canvas.drawLine(Offset(dx, 0), Offset(dx, size.height), paint);
 }
 //画一个黑子
 paint
 ..style = PaintingStyle.fill
 ..color = Colors.black;
 canvas.drawCircle(
 Offset(size.width / 2 - eWidth / 2, size.height / 2 - eHeight / 2),
 min(eWidth / 2, eHeight / 2) - 2,
 paint,
 );
 //画一个白子
 paint.color = Colors.white;
 canvas.drawCircle(
 Offset(size.width / 2 + eWidth / 2, size.height / 2 - eHeight / 2),
 min(eWidth / 2, eHeight / 2) - 2,
 paint,
 );
 }
 //在实际场景中正确利用此回调可以避免重绘开销,本示例我们简单的返回true
 @override
 bool shouldRepaint(CustomPainter oldDelegate) => true;
}

运行效果:

绘制是比较昂贵的操作,所以我们在实现自绘控件时应该考虑到性能开销,下面是两条关于性能优化的建议:

  • 尽可能的利用好shouldRepaint返回值;在UI树重新build时,控件在绘制前都会先调用该方法以确定是否有必要重绘;假如我们绘制的UI不依赖外部状态,那么就应该始终返回false,因为外部状态改变导致重新build时不会影响我们的UI外观;如果绘制依赖外部状态,那么我们就应该在shouldRepaint中判断依赖的状态是否改变,如果已改变则应返回true来重绘,反之则应返回false不需要重绘。
  • 绘制尽可能多的分层;在上面五子棋的示例中,我们将棋盘和棋子的绘制放在了一起,这样会有一个问题:由于棋盘始终是不变的,用户每次落子时变的只是棋子,但是如果按照上面的代码来实现,每次绘制棋子时都要重新绘制一次棋盘,这是没必要的。优化的方法就是将棋盘单独抽为一个Widget,并设置其shouldRepaint回调值为false,然后将棋盘Widget作为背景。然后将棋子的绘制放到另一个Widget中,这样落子时只需要绘制棋子。

总结

CustomPaint class提供了让用户自定义widget的能力,它暴露了一个canvas,可以通过这个canvas来绘制widget,CustomPaint会先调用painter绘制背景,然后再绘制child,最后调用foregroundPainter来绘制前景。

canvas--画布,真正的绘制是由canvas跟paint来完成的,画布提供了各种绘制的接口来绘制图形,除此以外画布还提供了平移、缩放、旋转等矩阵变换接口,画布都有固定大小跟形状,还可以使用画布提供的裁剪接口来裁剪画布的大小形状等等

Paint---笔画,是用来设置在画布上面绘制图形时的一些笔画属性,如:颜色、线宽、绘制模式、抗锯齿等等.

自绘控件非常强大,理论上可以实现任何2D图像外观,想更深入的了解,可以找到其对应的RenderObject对象,如Text Widget最终会通过RenderParagraph对象来通过Canvas实现文本绘制逻辑。了解了更底层的绘制逻辑,才能更好的在实际项目中灵活应用。

作者:风雨_83

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