Flutter 34: 图解自定义 View 之 Canvas (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：

1. Rect.fromPoints 根据两个点(左上角点/右下角点)来绘制；
2. 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；
3. Rect.fromLTWH 根据设置左上角的点与矩形宽高来绘制；
4. 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：

1. 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；
2. 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；
3. 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；
4. 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；
5. 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；
6. 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：

1. moveTo() 即从当前坐标点开始，不设置时默认为屏幕左上角位置；
2. lineTo() 即从起点绘制到设置的新的点位；
3. 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；
4. reset() 即清空连线；
5. addRect() 添加矩形连线；
6. addOval() 添加弧线，即贝塞尔(二阶)曲线；
7. cubicTo() 添加弧线，即贝塞尔(三阶)曲线；
8. 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；
9. 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);

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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！