Android:Volley: 为什么 volley imageLoader 必须从主线程调用？

Question

我的代码是:

class MyService extends Service{
    public void onCreate(){
          new ImageLoader(mRequestQueue, new VolleyLruCache(cacheSize))
                                            .get(url, new ImageListener(){..});
    }
}

我希望运行良好，但它抛出 IllegalStateException 异常。所以，打开 volley 的完整源代码，找到这个。

[ImageLoader.java]

    public ImageContainer get(String requestUrl, ImageListener imageListener,
        int maxWidth, int maxHeight) {
    // only fulfill requests that were initiated from the main thread.
    throwIfNotOnMainThread();   //<- why???

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

    // 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<?> newRequest =
        new ImageRequest(requestUrl, new Listener<Bitmap>() {
            @Override
            public void onResponse(Bitmap response) {
                onGetImageSuccess(cacheKey, response);
            }
        }, maxWidth, maxHeight,
        Config.RGB_565, new ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                onGetImageError(cacheKey, error);
            }
        });

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

我不明白 throwIfNotOnMainThread()..为什么 volley imageLoader 必须从主线程调用？

谢谢。

Answer 1

这是为了保证响应回调发生在主 UI 线程上，假设回调想要更新 UI。

ImageListener 的默认实现由 Volley 库提供更新 UI，因此必须从主线程使用它。

   /**
     * The default implementation of ImageListener which handles basic functionality
     * of showing a default image until the network response is received, at which point
     * it will switch to either the actual image or the error image.
     * @param imageView The imageView that the listener is associated with.
     * @param defaultImageResId Default image resource ID to use, or 0 if it doesn't exist.
     * @param errorImageResId Error image resource ID to use, or 0 if it doesn't exist.
     */
    public static ImageListener getImageListener(final ImageView view,
            final int defaultImageResId, final int errorImageResId) {
        return new ImageListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                if (errorImageResId != 0) {
                    view.setImageResource(errorImageResId);
                }
            }
            @Override
            public void onResponse(ImageContainer response, boolean isImmediate) {
                if (response.getBitmap() != null) {
                    view.setImageBitmap(response.getBitmap());
                } else if (defaultImageResId != 0) {
                    view.setImageResource(defaultImageResId);
                }
            }
        };
    }