使用TransitionDrawable实现渐变效果

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