flutter绘制半圆和绘制其他图形

在Flutter中，绘制半圆可以使用CustomPainter和Canvas类。以下是绘制半圆的示例代码：

class SemiCirclePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..color = Colors.blue
      ..style = PaintingStyle.fill;
 
    Rect rect = Rect.fromLTWH(0, 0, size.width, size.height / 2);
    canvas.drawArc(rect, -math.pi, math.pi, true, paint);
  }
 
  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return false;
  }
}
 
class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return SizedBox(
      width: 100,
      height: 100,
      child: CustomPaint(
        painter: SemiCirclePainter(),
      ),
    );
  }
}

在这个示例中，SemiCirclePainter类继承自CustomPainter，并在paint方法中使用Canvas的drawArc方法绘制一个半圆。MyWidget是一个使用了CustomPaint控件的无状态小部件，以便显示绘制的半圆。

绘制其他形状（如三角形、多边形等）时，可以使用Path类来创建自定义形状，并使用Canvas的drawPath方法来绘制。以下是一个绘制三角形的示例：

class TrianglePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..color = Colors.red
      ..style = PaintingStyle.fill;
 
    Path path = Path();
    path.moveTo(size.width / 2, 0);
    path.lineTo(0, size.height);
    path.lineTo(size.width, size.height);
    path.close();
 
    canvas.drawPath(path, paint);
  }
 
  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return false;
  }
}
 
class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return SizedBox(
      width: 100,
      height: 100,
      child: CustomPaint(
        painter: TrianglePainter(),
      ),
    );
  }
}

在这个示例中，TrianglePainter类创建了一个三角形，并使用drawPath方法将其绘制出来。MyWidget小部件同样使用CustomPaint控件来展示绘制的三角形。