Flutter 36: 图解自定义 View 之 Canvas

      小菜继续学习 Canvas 的相关方法：

drawVertices 绘制顶点

      小菜上次没有整理 drawVertices 的绘制方法，这次补上；Vertice 即顶点，通过绘制多个顶点，在进行连线，多用于 3D 模型中；

      drawVertices 包括三个参数，第一个是顶点属性，根据不同属性线的连接方式也不同；第二个是混合模式，即线的颜色与背景色混合效果；第三个是画笔，小菜测试调整 Paint 线的粗细无法调整整体连线的粗细；

      小菜借用 A B C D E F G H I 顶点来简单介绍：

1. VertexMode.triangles：每三个分割顶点相连，即 [A B C] [D E F] [G H I] 共 3 组；
2. VertexMode.triangleStrip：每相邻的三个顶点相连，即 [A B C] [B C D] [C D E] [D E F] [E F G] [F G H] [G H I] 共 7 组；
3. VertexMode.triangleFan：每相邻的两个顶点与首点相连，即 [A B C] [A C D] [A D E] [A E F] [A F G] [A G H] [A H I] 共 7 组；

canvas.drawVertices(
    Vertices(VertexMode.triangles, [
      Offset(30, 30), Offset(30, 60),Offset(60, 60),
      Offset(90, 60), Offset(120, 60), Offset(120, 30),
      Offset(60, 90), Offset(60, 120), Offset(90, 120),
    ]),
    BlendMode.colorBurn, Paint()..color = Colors.red);

canvas.drawVertices(
    Vertices(VertexMode.triangleStrip, [
      Offset(210, 30), Offset(210, 60), Offset(240, 60),
      Offset(270, 60), Offset(300, 60), Offset(300, 30),
      Offset(240, 90), Offset(240, 120), Offset(270, 120),
    ]),
    BlendMode.colorBurn, Paint()..color = Colors.green);

canvas.drawVertices(
    Vertices(VertexMode.triangleFan, [
      Offset(120, 150), Offset(120, 180), Offset(150, 180),
      Offset(180, 180), Offset(210, 180), Offset(210, 150),
      Offset(150, 210), Offset(150, 240), Offset(180, 240),
    ]),
    BlendMode.colorBurn, Paint()..color = Colors.blue);

画布操作

      小菜接下来介绍一下画布的基本操作，与 Android 很相似；

scale 缩放

      scale 即缩放效果，缩放的是画布大小，可以设置缩放倍数，且缩放倍数会叠加；

canvas.drawRect(
    Rect.fromLTWH(60, 60, 90, 50),
    Paint()..color = Colors.red
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);
// 缩放
canvas.scale(2);

canvas.drawRect(
    Rect.fromLTWH(60, 60, 90, 50),
    Paint()..color = Colors.red
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);
// 缩放
canvas.scale(0.25);

canvas.drawRect(
    Rect.fromLTWH(60, 60, 90, 50),
    Paint()..color = Colors.red
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);

translate 平移

      translate 即平移，平移的也是画布，并非画布中子元素，两个参数分别为水平方向和竖直方向距离；

canvas.drawLine(
    Offset(0, 0), Offset(60, 60),
    Paint()..color = Colors.red..strokeWidth = 2);
canvas.drawRect(
    Rect.fromLTWH(60, 60, 90, 50),
    Paint()..color = Colors.red
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);
// 平移
canvas.translate(30, 90);

canvas.drawLine(
    Offset(0, 0), Offset(0, Screen.height),
    Paint()..color = Colors.blueGrey..strokeWidth = 2);
canvas.drawLine(
    Offset(0, 0), Offset(Screen.width, 0),
    Paint()..color = Colors.blueGrey..strokeWidth = 2);
canvas.drawLine(
    Offset(0, 0), Offset(60, 60),
    Paint()..color = Colors.red..strokeWidth = 2);
canvas.drawRect(
    Rect.fromLTWH(60, 60, 90, 50),
    Paint()..color = Colors.red
      ..strokeWidth = 2.0.style = PaintingStyle.stroke);

rotate 旋转

      rotate 即旋转，原点为屏幕左上角，小菜为了效果先将画布平移一部分到屏幕中间在进行旋转测试，注意参数并非角度而是对应的 PI 值；

canvas.drawLine(
    Offset(0, 0), Offset(60, 60),
    Paint()..color = Colors.red..strokeWidth = 2);
canvas.drawRect(
    Rect.fromLTWH(60, 60, 90, 50),
    Paint()..color = Colors.red
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);
// 以当前原点旋转 90 度
canvas.rotate(degToRad(90));

canvas.drawLine(
    Offset(0, 0), Offset(60, 60),
    Paint()..color = Colors.green..strokeWidth = 2);
canvas.drawRect(
    Rect.fromLTWH(60, 60, 90, 50),
    Paint()..color = Colors.green
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);

skew 斜切

      skew 即斜切，两个参数为水平方向和竖直方向切度值，值为三角函数中的 tan 值，即 45 度时 tan 值为 1；

canvas.drawRect(
    Rect.fromLTWH(60, 0, 90, 50),
    Paint()..color = Colors.red
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);
// 水平方向斜近 30 度，竖直方向不变
canvas.skew(0.6, 0);

canvas.drawRect(
    Rect.fromLTWH(60, 0, 90, 50),
    Paint()..color = Colors.green
      ..strokeWidth = 2.0..style = PaintingStyle.stroke);

save/restore 保存/恢复

      save/savelayer 即保存当前画布，restore 即恢复当前画布，也可以理解为清空重新绘制；save/restore 可以多次，以栈的方式存储，可以通过进栈/出栈到当具体某一层；但是小菜测试时发现与 save/restore 需要成对出现，否则回报不匹配异常；

canvas.clipRect(Rect.fromLTWH(40, 40, Screen.width - 80, Screen.width - 80));
canvas.drawColor(Colors.green, BlendMode.srcIn);
// 保存画布1
canvas.save();

canvas.clipRect(
    Rect.fromLTWH(60, 60, Screen.width - 120, Screen.width - 120));
canvas.drawColor(Colors.grey, BlendMode.srcIn);
// 保存画布2
canvas.save();
canvas.clipRect(
    Rect.fromLTWH(80, 80, Screen.width - 160, Screen.width - 160));
canvas.drawColor(Colors.orange, BlendMode.srcIn);
//  canvas.save();
// 恢复画布1
canvas.restore();
// 恢复画布2
canvas.restore();
//  canvas.restore();
canvas.drawColor(Colors.blue, BlendMode.srcIn);

---

      Canvas 非常强大，还有很多研究不透彻的地方，小菜仍在不断学习，有错误的地方烦请多多指点；如下是小菜公众号，欢迎闲来吐槽～

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