add support for high priority download

Author: jason

SDWebImage/SDWebImageDownloader.h

@@ -37,7 +37,12 @@ typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
      * Enable to allow untrusted SSL ceriticates.
      * Useful for testing purposes. Use with caution in production.
      */
-            SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6
+            SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
+    /**
+     * 
+     */
+        SDWebImageDownloaderHighPriority = 1 << 7,
+    
 
 };
 

SDWebImage/SDWebImageDownloader.m

@@ -152,6 +152,10 @@ - (NSInteger)maxConcurrentDownloads {
                                                                      SDWebImageDownloader *sself = wself;
                                                                      [sself removeCallbacksForURL:url];
                                                                  }];
+        
+        if (options & SDWebImageDownloaderHighPriority) {
+            operation.queuePriority = NSOperationQueuePriorityHigh;
+        }
 
         [wself.downloadQueue addOperation:operation];
         if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {

SDWebImage/SDWebImageManager.h

@@ -55,7 +55,13 @@ typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
      * Enable to allow untrusted SSL ceriticates.
      * Useful for testing purposes. Use with caution in production.
      */
-            SDWebImageAllowInvalidSSLCertificates = 1 << 7
+            SDWebImageAllowInvalidSSLCertificates = 1 << 7,
+    /**
+     * By default, image are loaded in the order they were queued. This flag move them to
+     * the front of the queue and is loaded immediately instead of waiting for the current queue to be loaded (which 
+     * could take a while).
+     */
+            SDWebImageHighPriority = 1 << 8
 };
 
 typedef void(^SDWebImageCompletedBlock)(UIImage *image, NSError *error, SDImageCacheType cacheType);

SDWebImage/SDWebImageManager.m

@@ -127,6 +127,7 @@ - (BOOL)diskImageExistsForURL:(NSURL *)url {
             if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
             if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
             if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
+            if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
             if (image && options & SDWebImageRefreshCached) {
                 // force progressive off if image already cached but forced refreshing
                 downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;