实现一个圆角图片，其实两段代码就是画图的先后顺序换了一下，然后模式换了一下，但是第二段代码没有效果，第一段可以实现，请问老师们怎么回事，谢谢！

第一段：

Bitmap newBitmap = Bitmap.createBitmap(bitmap.getWidth(),

bitmap.getHeight(), Config.ARGB_8888);

Canvas canvas = new Canvas(newBitmap);

Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

canvas.drawBitmap(bitmap, 0, 0, paint);

RectF rect = new RectF(0, 0, getWidth(), getHeight());

paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));

canvas.drawRoundRect(rect, 50, 50, paint);

paint.setXfermode(null);

第二段：

Bitmap newBitmap = Bitmap.createBitmap(bitmap.getWidth(),

bitmap.getHeight(), Config.ARGB_8888);

Canvas canvas = new Canvas(newBitmap);

Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

RectF rect = new RectF(0, 0, getWidth(), getHeight());

canvas.drawRoundRect(rect, 50, 50, paint);

paint.setXfermode(new PorterDuffXfermode(Mode.DST_IN));

canvas.drawBitmap(bitmap, 0, 0, paint);

paint.setXfermode(null);