Android - Volley的ImageLoader下载图片

// Update the caller to let them know that they should use the default bitmap.
imageListener.onResponse(imageContainer, true);

/**
* 从服务器下载图片
* onResponse会调用两次, 第一次Bitmap是null, 设置默认图片; 第二次是网络图片.
*
* @param listener 接收监听
*/
protected void parseImage(final ImageListener<Bitmap> listener) {
WebScheduler.getInstance().getImageLoader().get(NetworkHelper.getRealUrl(mImgUrl),
new ImageLoader.ImageListener() {
@Override
public void onResponse(ImageLoader.ImageContainer imageContainer, boolean b) {
mImgBitmap = imageContainer.getBitmap();
if (mImgBitmap != null)
listener.onResponse(mImgBitmap);
}

@Override
public void onErrorResponse(VolleyError volleyError) {
ToastHelper.getInstance().showToast(R.string.network_error);
}
}, 0, 0
);
}

/**
* Issues a bitmap request with the given URL if that image is not available
* in the cache, and returns a bitmap container that contains all of the data
* relating to the request (as well as the default image if the requested
* image is not available).
* @param requestUrl The url of the remote image
* @param imageListener The listener to call when the remote image is loaded
* @param maxWidth The maximum width of the returned image.
* @param maxHeight The maximum height of the returned image.
* @param scaleType The ImageViews ScaleType used to calculate the needed image size.
* @return A container object that contains all of the properties of the request, as well as
*     the currently available image (default if remote is not loaded).
*/
public ImageContainer get(String requestUrl, ImageListener imageListener,
int maxWidth, int maxHeight, ScaleType scaleType) {

// only fulfill requests that were initiated from the main thread.
throwIfNotOnMainThread();

final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight, scaleType);

// Try to look up the request in the cache of remote images.
Bitmap cachedBitmap = mCache.getBitmap(cacheKey);
if (cachedBitmap != null) {
// Return the cached bitmap.
ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null);
imageListener.onResponse(container, true);
return container;
}

// The bitmap did not exist in the cache, fetch it!
ImageContainer imageContainer =
new ImageContainer(null, requestUrl, cacheKey, imageListener);

// Update the caller to let them know that they should use the default bitmap.
imageListener.onResponse(imageContainer, true);

// Check to see if a request is already in-flight.
BatchedImageRequest request = mInFlightRequests.get(cacheKey);
if (request != null) {
// If it is, add this request to the list of listeners.
request.addContainer(imageContainer);
return imageContainer;
}

// The request is not already in flight. Send the new request to the network and
// track it.
Request<Bitmap> newRequest = makeImageRequest(requestUrl, maxWidth, maxHeight, scaleType,
cacheKey);

mRequestQueue.add(newRequest);
mInFlightRequests.put(cacheKey,
new BatchedImageRequest(newRequest, imageContainer));
return imageContainer;
}