解決方案

        不應該將getDrawable的值拿來當?shù)谝粚覦rawable, 而是先判斷一下這個值的類型,
        如果是TransitionDrawable, 應該獲取它第二層Drawable作為我們的第一層, 
        這樣原來的第一層Drawable就會失去到GC Roots的引用鏈, 從而可以被回收

        Drawable oldDrawable  = mShutterButton.getDrawable();
        Drawable oldBitmapDrawable;
        if (oldDrawable == null) {
            oldBitmapDrawable = new ColorDrawable(Color.TRANSPARENT);
        } else if (oldDrawable instanceof TransitionDrawable) {
            oldBitmapDrawable = ((TransitionDrawable) oldDrawable).getDrawable(1);
        } else {
            oldBitmapDrawable = oldDrawable;
        }
        TransitionDrawable transitionDrawable = crossfadeDrawable(
                oldBitmapDrawable,
                getResources().getDrawable(resId));
        mShutterButton.setImageDrawable(transitionDrawable);