Merge pull request #534 from pixable/ios-background

iOS Background: Provide background option through the manager

@@ -24,7 +24,10 @@ typedef enum
      * (to be combined with `SDWebImageDownloaderUseNSURLCache`).
      */
     SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
-
+    /**
+     * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
+     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
+     */
     SDWebImageDownloaderContinueInBackground = 1 << 4
 
 } SDWebImageDownloaderOptions;

@@ -40,7 +40,13 @@ typedef enum
      *
      * Use this flag only if you can't make your URLs static with embeded cache busting parameter.
      */
-    SDWebImageRefreshCached = 1 << 4
+    SDWebImageRefreshCached = 1 << 4,
+
+    /**
+     * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
+     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
+     */
+    SDWebImageContinueInBackground = 1 << 5
 } SDWebImageOptions;
 
 typedef void(^SDWebImageCompletedBlock)(UIImage *image, NSError *error, SDImageCacheType cacheType);

@@ -144,6 +144,7 @@
             if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
             if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
             if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
+            if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
             if (image && options & SDWebImageRefreshCached)
             {
                 // force progressive off if image already cached but forced refreshing