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,但它会抛出IllegalStateException Exception。 所以,打开凌空全部源,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().. 为什么必须从主线程调用volley imageLoader?
感谢。
答案 0 :(得分:2)
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。
Volley库提供的ImageListener的默认实现更新了UI,因此必须在主线程中使用它。
/**
* 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);
}
}
};
}