创建几种优惠券的花边样式，我画了3种：
- 一种是圆形花边
- 一种是直角花边
- 还有一种是倒三角花边
需要覆写以下两个方法：
onSizeChanged(int, int, int, int) //Called when the size of this view has changed.
//View的尺寸发生改变时调用
onDraw(android.graphics.Canvas) //Called when the view should render its content.
//View需要绘制内容时调用
以绘制三角形为例： 在构造函数里初始化画笔，
Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
//mPaint.setAntiAlias(true);//该方法作用是抗锯齿
mPaint.setDither(true);//该方法是设置防抖动
mPaint.setColor(Color.WHITE);
mPaint.setStyle(Paint.Style.FILL);
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关于**setAntiAlias()**方法官方的解释为：
void setAntiAlias(boolean aa)
//Helper for setFlags(), setting or clearing the ANTI_ALIAS_FLAG bit AntiAliasing smooths out the edges of what is being drawn, but is has no impact on the interior of the shape. //我翻译一下：setFlags()的辅助方法，用于设置或清除ANTI_ALIAS_FLAG的标志位，绘制形状的抗锯齿光滑边缘效果，但是在形状内部不起作用。
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关于**setDither()**方法官方的解释为：
void setDither(boolean dither)
//Helper for setFlags(), setting or clearing the DITHER_FLAG bit Dithering affects how colors that are higher precision than the device are down-sampled. //setFlags()的辅助方法，用于设置或清除DITHER_FLAG的标志位，比一般设备更高颜色精度的抖动效果。
在**onDraw()**方法里绘制边缘图形，
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
for (int i = 0; i < circleNum; i++) {
float x = radius + remain / 2 + ((gap + radius * 2) * i);
// canvas.drawCircle(x, 0, radius, mPaint);//顶部画圆
// canvas.drawCircle(x, getHeight(), radius, mPaint);//底部画圆
Path pathTop = new Path();
pathTop.moveTo(x, 0);// 此点为多边形的起点
pathTop.lineTo(x + radius * 2, 0);
pathTop.lineTo(x + radius, height);
pathTop.close(); // 使这些点构成封闭的多边形
canvas.drawPath(pathTop, mPaint);
Path pathBottom = new Path();
pathBottom.moveTo(x, getHeight());// 此点为多边形的起点
pathBottom.lineTo(x + radius * 2, getHeight());
pathBottom.lineTo(x + radius, getHeight() - height);
pathBottom.close(); // 使这些点构成封闭的多边形
canvas.drawPath(pathBottom, mPaint);
}
}
Path类用于构建多边形图形，drawPath()方法有两个参数
void drawPath(Path path, Paint paint)
// Draw the specified path using the specified paint.
// 用指定的画笔对象绘制指定图形