参考：

https://github.com/angcyo/RoundAngleFrameLayout

核心代码：

@Override    protected void dispatchDraw(Canvas canvas) {canvas.saveLayer(new RectF(0, 0, canvas.getWidth(), canvas.getHeight()), imagePaint,Canvas.ALL_SAVE_FLAG);        super.dispatchDraw(canvas);        drawTopLeft(canvas);//用PorterDuffXfermode        drawTopRight(canvas);//用PorterDuffXfermode        drawBottomLeft(canvas);//用PorterDuffXfermode        drawBottomRight(canvas);//用PorterDuffXfermode        canvas.restore();    }

　　

roundPaint = new Paint();        roundPaint.setColor(Color.WHITE);        roundPaint.setAntiAlias(true);        roundPaint.setStyle(Paint.Style.FILL);        roundPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));        imagePaint = new Paint();        imagePaint.setXfermode(null);