AsyncDrawable.java

/**
 * 该Drawable能够为与之绑定的ImageView保持一个BitmapWorkerTask（AsyncTask）的引用，使得
 * BitmapWorkerTask能够在需要的时候被取消，并且保证只有最后与ImageView绑定的BitmapWorkerTask
 * 才能够更新ImageView
 */
private static class AsyncDrawable extends BitmapDrawable {
    private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

    /**
     * bitmap参数是占位图片（placeholder），代表图片正在加载中
     */
    public AsyncDrawable(Resources res, Bitmap bitmap, BitmapWorkerTask bitmapWorkerTask) {
        super(res, bitmap);
        bitmapWorkerTaskReference =
            new WeakReference<BitmapWorkerTask>(bitmapWorkerTask);
    }

    public BitmapWorkerTask getBitmapWorkerTask() {
        return bitmapWorkerTaskReference.get();
    }
}