【Flutter 专题】33 图解自定义 View 之 Canvas (一)
【摘要】 0 基础学习 Flutter，第三十三步：尝试 Canvas 内容绘制(一)！
小菜最近在学习自定义 View，刚了解了一下 Paint 画笔的神奇之处，现在学习一下 Canvas 画布的神秘之处。Flutter 提供了众多的绘制方法，小菜接触不深，尽量都尝试一下。
Canvas 画布
drawColor 绘制背景色
drawColor 需要传入两个参数，第一个为色值，第二个为混合模式，有众多混合模式供选择，但注意使用混合模式后会与绘制其上的其他 View 颜色混合像素。
canvas.drawColor(Colors.pinkAccent, BlendMode.srcIn);
drawPoints 绘制点/线
drawPoints 不仅可以绘制点，还可以绘制点与点的连线；PointMode 包括 points 点 / lines 线 / polygon 多边形；注意 lines 为每两点之间的连线，若为奇数个点，最后一个没有与之相连的点。
// 绘制点
canvas.drawPoints(
PointMode.points,
[
Offset(30.0, 30.0), Offset(60.0, 30.0),
Offset(90.0, 30.0), Offset(90.0, 60.0),
Offset(60.0, 60.0), Offset(30.0, 60.0)
],
Paint()..strokeWidth = 4.0);
canvas.drawPoints(
PointMode.points,
[
Offset(160.0, 30.0), Offset(190.0, 30.0),
Offset(220.0, 30.0), Offset(220.0, 60.0),
Offset(190.0, 60.0), Offset(160.0, 60.0)
],
Paint()..strokeWidth = 4.0..strokeCap = StrokeCap.round);
// 绘制线
canvas.drawPoints(
PointMode.lines,
[
Offset(30.0, 100.0), Offset(60.0, 100.0),
Offset(90.0, 100.0), Offset(90.0, 130.0),
Offset(60.0, 130.0), Offset(30.0, 130.0)
],
Paint()..strokeWidth = 4.0..strokeCap = StrokeCap.round);
// 绘制多边形
canvas.drawPoints(
PointMode.polygon,
[
Offset(160.0, 100.0), Offset(190.0, 100.0),
Offset(220.0, 100.0), Offset(220.0, 130.0),
Offset(190.0, 130.0), Offset(160.0, 130.0)
],
Paint()..strokeWidth = 4.0..strokeCap = StrokeCap.round);
drawLine 绘制线
canvas.drawLine(Offset(30.0, 90.0), Offset(Screen.width - 30.0, 90.0),
Paint()..strokeWidth = 4.0);
canvas.drawLine(Offset(30.0, 120.0), Offset(Screen.width - 30.0, 120.0),
Paint()..strokeWidth = 4.0..strokeCap = StrokeCap.round);
canvas.drawLine(Offset(30.0, 150.0), Offset(Screen.width - 30.0, 150.0),
Paint()..strokeWidth = 4.0..strokeCap = StrokeCap.square);
drawArc 绘制弧/饼
drawArc 可以用来绘制圆弧甚至配合 Paint 绘制饼状图；drawArc 的第一个参数为矩形范围，即圆弧所在的圆的范围，若非正方形则圆弧所在的圆会拉伸；第二个参数为起始角度，0.0 为坐标系 x 轴正向方形；第三个参数为终止角度，若超过 2*PI，则为一个圆；第四个参数为是否由中心出发，false 时只绘制圆弧，true 时绘制圆饼；第五个参数即 Paint 画笔，可通过 PaintingStyle 属性绘制是否填充等；
const PI = 3.1415926;
canvas.drawArc(Rect.fromCircle(center: Offset(60.0, 60.0), radius: 80.0),
0.0, PI / 2, false,
Paint()..color = Colors.white..strokeCap = StrokeCap.round..strokeWidth = 4.0..style = PaintingStyle.stroke);
canvas.drawArc(Rect.fromCircle(center: Offset(200.0, 60.0), radius: 80.0),
0.0, PI / 2, false,
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.fill);
canvas.drawArc(Rect.fromCircle(center: Offset(90.0, 160.0), radius: 80.0),
0.0, PI * 2 / 3, true,
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
canvas.drawArc(Rect.fromCircle(center: Offset(250.0, 160.0), radius: 80.0),
0.0, PI * 2 / 3, true,
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.fill);
canvas.drawArc(Rect.fromLTWH(30.0, 300.0, 200.0, 100.0),
0.0, 5.0, true,
Paint()..color = Colors.white..style = PaintingStyle.fill);
canvas.drawArc(Rect.fromPoints(Offset(260.0, 260.0), Offset(320.0, 420.0)),
0.0, 5.0, true,
Paint()..color = Colors.white..style = PaintingStyle.fill);
drawRect 绘制矩形
drawRect 用来绘制矩形，Flutter 提供了多种绘制矩形方法：
- Rect.fromPoints 根据两个点(左上角点/右下角点)来绘制；
- Rect.fromLTRB 根据以屏幕左上角为坐标系圆点，分别设置上下左右四个方向距离；
- Rect.fromLTWH 根据设置左上角的点与矩形宽高来绘制；
- Rect.fromCircle 最特殊，根据圆形绘制正方形；
canvas.drawRect(Rect.fromPoints(Offset(30.0, 30.0), Offset(150.0, 100.0)),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
canvas.drawRect(Rect.fromPoints(Offset(210.0, 30.0), Offset(330.0, 100.0)),
Paint()..color = Colors.white..style = PaintingStyle.fill);
canvas.drawRect(Rect.fromLTRB(30.0, 140.0, 150.0, 210.0),
Paint()..color = Colors.white);
canvas.drawRect(Rect.fromLTWH(210.0, 140.0, 120.0, 70.0),
Paint()..color = Colors.white);
canvas.drawRect(Rect.fromCircle(center: Offset(90.0, 300.0), radius: 60.0),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
drawRRect 绘制圆角矩形
drawRRect 绘制圆角矩形，Flutter 提供了多种绘制方法：
- RRect.fromLTRBXY 前四个参数用来绘制矩形位置，剩余两个参数绘制固定 x/y 弧度；
- RRect.fromLTRBR 前四个参数用来绘制矩形位置，最后一个参数绘制 Radius 弧度；
- RRect.fromLTRBAndCorners 前四个参数用来绘制矩形位置，剩余四个可选择参数，根据需求设置四个角 Radius 弧度，可不同；
- RRect.fromRectXY 第一个参数绘制矩形，可以用上面介绍的多种矩形绘制方式，剩余两个参数绘制固定 x/y 弧度；
- RRect.fromRectAndRadius 第一个参数绘制矩形，可以用上面介绍的多种矩形绘制方式，最后一个参数绘制 Radius 弧度；
- RRect.fromRectAndCorners第一个参数绘制矩形，可以用上面介绍的多种矩形绘制方式，剩余四个可选择参数，根据需求设置四个角 Radius 弧度，最为灵活。
// RRect.fromLTRBXY 方式
canvas.drawRRect(
RRect.fromLTRBXY(30.0, 30.0, 150.0, 100.0, 8.0, 8.0),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
canvas.drawRRect(
RRect.fromLTRBXY(210.0, 30.0, 330.0, 100.0, 8.0, 18.0),
Paint()..color = Colors.white..style = PaintingStyle.fill);
// RRect.fromLTRBR 方式
canvas.drawRRect(
RRect.fromLTRBR(30.0, 140.0, 150.0, 210.0, Radius.circular(8.0)),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
// RRect.fromLTRBAndCorners 方式
canvas.drawRRect(
RRect.fromLTRBAndCorners(210.0, 140.0, 330.0, 210.0,
topLeft: Radius.circular(5.0),
topRight: Radius.circular(20.0),
bottomRight: Radius.circular(5.0),
bottomLeft: Radius.circular(20.0)),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
// RRect.fromRectAndCorners 方式
canvas.drawRRect(
RRect.fromRectAndCorners(Rect.fromLTWH(30.0, 260.0, 120.0, 70.0),
topLeft: Radius.circular(5.0),
topRight: Radius.circular(20.0),
bottomRight: Radius.circular(5.0),
bottomLeft: Radius.circular(20.0)),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
// RRect.fromRectAndRadius 方式
canvas.drawRRect(
RRect.fromRectAndRadius(Rect.fromLTWH(210.0, 260.0, 120.0, 70.0),
Radius.elliptical(8.0, 18.0)),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
// RRect.fromRectXY 方式
canvas.drawRRect(
RRect.fromRectXY(
Rect.fromCircle(center: Offset(90.0, 420.0), radius: 60.0),
8.0, 8.0),
Paint()..color = Colors.white..strokeWidth = 4.0..style = PaintingStyle.stroke);
drawDRRect 绘制嵌套矩形
drawDRRect 绘制嵌套矩形，第一个参数为外部矩形，第二个参数为内部矩形，可用上述多种设置圆角矩形方式；最后一个参数为 Paint 画笔，且 PaintingStyle 为 fill 时填充的是两个矩形之间的范围。
canvas.drawDRRect(
RRect.fromRectXY(
Rect.fromCircle(center: Offset(90.0, 420.0), radius: 60.0), 8.0, 8.0),
RRect.fromRectXY(
Rect.fromCircle(center: Offset(90.0, 420.0), radius: 54.0), 8.0, 8.0),
Paint()..color = Colors.whit..strokeWidth = 3.0
..style = PaintingStyle.stroke);
canvas.drawDRRect(
RRect.fromRectXY(
Rect.fromCircle(center: Offset(270.0, 420.0), radius: 60.0), 8.0, 8.0),
RRect.fromRectXY(
Rect.fromCircle(center: Offset(270.0, 420.0), radius: 54.0), 8.0, 8.0),
Paint()..color = Colors.white..strokeWidth = 3.0
..style = PaintingStyle.fill);
drawCircle 绘制圆形
drawCircle 绘制圆形，仅需设置原点及半径即可；
canvas.drawCircle(Offset(90.0, 420.0), 60.0,
Paint()..color = Colors.white..strokeWidth = 3.0
..style = PaintingStyle.stroke);
canvas.drawCircle(Offset(270.0, 420.0), 60.0,
Paint()..color = Colors.white..strokeWidth = 3.0
..style = PaintingStyle.fill);
drawOval 绘制椭圆
drawOval 绘制椭圆方式很简单，主要绘制一个矩形即可；
canvas.drawOval(Rect.fromLTRB(30.0, 30.0, 150.0, 100.0),
Paint()..color = Colors.white..strokeWidth = 3.0
..style = PaintingStyle.stroke);
canvas.drawOval(Rect.fromLTRB(210.0, 30.0, 330.0, 100.0),
Paint()..color = Colors.white..strokeWidth = 3.0
..style = PaintingStyle.fill);
drawPath 绘制路径
drawPath 用来绘制路径，Flutter 提供了众多路径方法，小菜尝试几种常用的方法：
- moveTo() 即从当前坐标点开始，不设置时默认为屏幕左上角位置；
- lineTo() 即从起点绘制到设置的新的点位；
- close() 即最后的点到起始点连接，但对于中间绘制矩形/弧等时最后不会相连；
- reset() 即清空连线；
- addRect() 添加矩形连线；
- addOval() 添加弧线，即贝塞尔(二阶)曲线；
- cubicTo() 添加弧线，即贝塞尔(三阶)曲线；
- relativeMoveTo() 相对于移动到当前点位，小菜认为与 moveTo 相比整个坐标系移动；
- relativeLineTo() 相对连接到当前点位，并将坐标系移动到当前点位；
canvas.drawPath(
Path()
..moveTo(30.0, 100.0)..lineTo(120.0, 100.0)
..lineTo(90.0, 130.0)..lineTo(180.0, 130.0)
..close(),
Paint()
..color = Colors.white..strokeWidth = 3.0
..style = PaintingStyle.stroke);
canvas.drawPath(
Path()
..moveTo(200.0, 100.0)..lineTo(290.0, 100.0)
..lineTo(260.0, 130.0)..lineTo(350.0, 130.0)
..close(),
Paint()
..color = Colors.white..strokeWidth = 3.0
..style = PaintingStyle.fill);
canvas.drawPath(
Path()
..moveTo(30.0, 170.0)..lineTo(120.0, 170.0)
..lineTo(90.0, 210.0)..lineTo(180.0, 210.0)
..addRect(Rect.fromLTWH(180.0, 210.0, 120.0, 70.0))
..addOval(Rect.fromLTWH(180.0, 210.0, 120.0, 70.0))
..moveTo(230.0, 170.0)..lineTo(320.0, 170.0)
..close(),
Paint()
..color = Colors.white..strokeWidth = 3.0..style = PaintingStyle.stroke);
canvas.drawPath(
Path()
..arcTo(Rect.fromCircle(center: Offset(60, 300), radius: 80), -PI / 6,
PI * 2 / 3, false),
Paint()
..color = Colors.white..strokeWidth = 3.0..style = PaintingStyle.stroke);
canvas.drawPath(
Path()
..moveTo(210.0, 300.0)
..cubicTo(210.0, 390.0, 270.0, 330.0, 330.0, 300.0),
Paint()
..color = Colors.black..strokeWidth = 3.0..style = PaintingStyle.stroke);
小菜绘制了一个基本的坐标系来比较一下 moveTo()/lineTo() 与 relativeMoveTo()/relativeLineTo() 的区别：
canvas.drawPath(
Path()
..relativeMoveTo(30.0, 30.0)..relativeLineTo(120.0, 30.0)
..relativeLineTo(90.0, 60.0)..relativeLineTo(180.0, 60.0),
Paint()
..color = Colors.blue..strokeWidth = 6.0
..style = PaintingStyle.stroke);
canvas.drawPath(
Path()
..moveTo(30.0, 30.0)..lineTo(120.0, 30.0)
..lineTo(90.0, 60.0)..lineTo(180.0, 60.0),
Paint()
..color = Colors.orange..strokeWidth = 6.0
..style = PaintingStyle.stroke);
小菜对自定义 View 研究还不深入，有很多方法还没有尝试，有错误的地方希望多多指导！
来源：阿策小和尚
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