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

Question

我的代码如下：

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

我希望工作顺利，但它抛出了非法状态异常。所以，打开Volley的完整源代码，找到如下内容。

[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;
}

我不理解 throwIfNotOnMainThread() 是什么意思。 为什么 Volley ImageLoader 必须从主线程调用？ 谢谢。

- JuL

---

java.lang.IllegalStateException: ImageLoader 必须在主线程中调用。 位于 com.android.volley.toolbox.ImageLoader.throwIfNotOnMainThread(ImageLoader.java:467) at com.android.volley.toolbox.ImageLoader.get(ImageLoader.java:189) at com.android.volley.toolbox.ImageLoader.get(ImageLoader.java:171) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:136) at android.os.HandlerThread.run(HandlerThread.java:61) - JuL

Answer 1

这是为了保证回调响应发生在主UI线程上，假设回调想要更新UI。

Volley库提供的ImageListener的默认实现会更新UI，因此必须从主线程中使用。

   /**
     * 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);
                }
            }
        };
    }

- Manish Mulimani