使用TransitionDrawable实现渐变效果

private void setImageBitmap(ImageView imageView, Bitmap bitmap) {
		final TransitionDrawable td = new TransitionDrawable(new Drawable[] {
				new ColorDrawable(android.R.color.transparent),
				new BitmapDrawable(getResources(), bitmap) });
		imageView.setBackgroundDrawable(imageView.getDrawable());
		imageView.setImageDrawable(td);
		td.startTransition(200);
	}