各种ViewGroup的圆角实现，包括listView等

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
mPath.reset();
mRect.set(0, 0, w, h);
mPath.addRoundRect(mRect ,4 , 4 , Path.Direction.CW);
}

@Override
protected void dispatchDraw(Canvas canvas) {
canvas.save();
canvas.clipPath(mPath);
super.dispatchDraw(canvas);
canvas.restore();
}