Displaying Bitmaps Efficiently之Processing Bitmaps Off the UI Thread

我们一般用AsyncTask来loader 图片，避免UI线程ANR。但是AsyncTask是一个抽象类，必须实现子类.
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    private final WeakReference<ImageView> imageViewReference;
    private int data = 0;


    public BitmapWorkerTask(ImageView imageView) {
        // Use a WeakReference to ensure the ImageView can be garbage collected
        imageViewReference = new WeakReference<ImageView>(imageView);
    }


    // Decode image in background.
    @Override
    protected Bitmap doInBackground(Integer... params) {
        data = params[0];
        return decodeSampledBitmapFromResource(getResources(), data, 100, 100));
    }


    // Once complete, see if ImageView is still around and set bitmap.
    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            if (imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}
由于imageViewReference;是WeakReference，因此要在onPostExtcute方法中判断imageViewReference是否还存在。
怎么使用BitmapWorkerTask 呢？参考如下:
public void loadBitmap(int resId, ImageView imageView) {
    BitmapWorkerTask task = new BitmapWorkerTask(imageView);
    task.execute(resId);
}
当使用ListView和GridView时，每一个子view 都会触发AsyncTask，没有办法保证这些AsyncTask完成的顺序.或者AsyncTask 完成时，他用于现实的view可能已经被回收了.可以采用下面的方法解决.
可以保存最近使用的AsyncTask的reference，当task完成是可以checkreference。具体code如下：
定义一个bitmapWorkerTaskReference 保存AsyncTask的实例，其采用的WeakReference。
static class AsyncDrawable extends BitmapDrawable {
    private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;


    public AsyncDrawable(Resources res, Bitmap bitmap,
            BitmapWorkerTask bitmapWorkerTask) {
        super(res, bitmap);
        bitmapWorkerTaskReference =
            new WeakReference<BitmapWorkerTask>(bitmapWorkerTask);
    }


    public BitmapWorkerTask getBitmapWorkerTask() {
        return bitmapWorkerTaskReference.get();
    }
}
在执行BitmapWorkerTask的时候，先new一个AsyncDrawable ，然后绑定到目标的ImageView。
public void loadBitmap(int resId, ImageView imageView) {
    if (cancelPotentialWork(resId, imageView)) {
        final BitmapWorkerTask task = new BitmapWorkerTask(imageView);
        final AsyncDrawable asyncDrawable =
                new AsyncDrawable(getResources(), mPlaceHolderBitmap, task);
        imageView.setImageDrawable(asyncDrawable);
        task.execute(resId);
    }
}


cancelPotentialWork 会检查bitmapWorkerTask  是否为NULL，如果不为NULL，则说明有另外一个task在使用这个Imageview。则调用cancel 取消这个先前的task.


public static boolean cancelPotentialWork(int data, ImageView imageView) {
    final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);


    if (bitmapWorkerTask != null) {
        final int bitmapData = bitmapWorkerTask.data;
        // If bitmapData is not yet set or it differs from the new data
        if (bitmapData == 0 || bitmapData != data) {
            // Cancel previous task
            bitmapWorkerTask.cancel(true);
        } else {
            // The same work is already in progress
            return false;
        }
    }
    // No task associated with the ImageView, or an existing task was cancelled
    return true;
}
下面这个函数可以取回和AsyncTask 绑定的Imageview.
private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) {
   if (imageView != null) {
       final Drawable drawable = imageView.getDrawable();
       if (drawable instanceof AsyncDrawable) {
           final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
           return asyncDrawable.getBitmapWorkerTask();
       }
    }
    return null;
}
最后一步是更新onPostExecute方法.
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    ...


    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (isCancelled()) {
            bitmap = null;
        }


        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            final BitmapWorkerTask bitmapWorkerTask =
                    getBitmapWorkerTask(imageView);
            if (this == bitmapWorkerTask && imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}