private void setupTargetBitmap(int alpha) {

mBitmap = Bitmap.createBitmap(getMeasuredWidth(), getMeasuredHeight(),

Config.ARGB_8888);

mCanvas = new Canvas(mBitmap);

mPaint = new Paint();

mPaint.setColor(mColor);

mPaint.setAntiAlias(true);

mPaint.setDither(true);

mPaint.setAlpha(alpha); // 这个是可以设置alpha 的

mCanvas.drawRect(mIconRect, mPaint); // 我们只是在我们的icon 区域绘制纯色的背景

mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));

mPaint.setAlpha(255); 

mCanvas.drawBitmap(mIconBitmap, null, mIconRect, mPaint);

}