Ensure image isn't decoded twice if not necessary when SDWebImageRefreshCached flag is used #326

@@ -14,7 +14,16 @@ typedef enum
 {
     SDWebImageDownloaderLowPriority = 1 << 0,
     SDWebImageDownloaderProgressiveDownload = 1 << 1,
-    SDWebImageDownloaderEnableNSURLCache = 1 << 2
+    /**
+     * By default, request prevent the of NSURLCache. With this flag, NSURLCache
+     * is used with default policies.
+     */
+    SDWebImageDownloaderUseNSURLCache = 1 << 2,
+    /**
+     * Call completion block with nil image/imageData if the image was read from NSURLCache
+     * (to be combined with `SDWebImageDownloaderUseNSURLCache`).
+     */
+    SDWebImageDownloaderIgnoreCachedResponse = 1 << 3
 } SDWebImageDownloaderOptions;
 
 typedef enum

@@ -120,7 +120,7 @@ static NSString *const kCompletedCallbackKey = @"completed";
     [self addProgressCallback:progressBlock andCompletedBlock:completedBlock forURL:url createCallback:^
     {
         // In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
-        NSMutableURLRequest *request = [NSMutableURLRequest.alloc initWithURL:url cachePolicy:(options & SDWebImageDownloaderEnableNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:15];
+        NSMutableURLRequest *request = [NSMutableURLRequest.alloc initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:15];
         request.HTTPShouldHandleCookies = NO;
         request.HTTPShouldUsePipelining = YES;
         request.allHTTPHeaderFields = wself.HTTPHeaders;

@@ -28,6 +28,7 @@
 @implementation SDWebImageDownloaderOperation
 {
     size_t width, height;
+    BOOL responseFromCached;
 }
 
 - (id)initWithRequest:(NSURLRequest *)request queue:(dispatch_queue_t)queue options:(SDWebImageDownloaderOptions)options progress:(void (^)(NSUInteger, long long))progressBlock completed:(void (^)(UIImage *, NSData *, NSError *, BOOL))completedBlock cancelled:(void (^)())cancelBlock
@@ -43,6 +44,7 @@
         _executing = NO;
         _finished = NO;
         _expectedSize = 0;
+        responseFromCached = YES; // Initially wrong until `connection:willCacheResponse:` is called or not called
     }
     return self;
 }
@@ -271,8 +273,17 @@
     [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:nil];
 
     SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
+
     if (completionBlock)
     {
+        if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached)
+        {
+            completionBlock(nil, nil, nil, YES);
+            self.completionBlock = nil;
+            [self done];
+        }
+        else
+        {
             dispatch_async(self.queue, ^
             {
                 UIImage *image = [UIImage decodedImageWithImage:SDScaledImageForPath(self.request.URL.absoluteString, self.imageData)];
@@ -291,6 +302,7 @@
                 });
             });
         }
+    }
     else
     {
         [self done];
@@ -311,6 +323,7 @@
 
 - (NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse
 {
+    responseFromCached = NO; // If this method is called, it means the response wasn't read from cache
     if (self.request.cachePolicy == NSURLRequestReloadIgnoringLocalCacheData)
     {
         // Prevents caching of responses

@@ -33,10 +33,12 @@ typedef enum
      */
     SDWebImageProgressiveDownload = 1 << 3,
     /**
-     * Even if the image is cached, fetch the URL again anyway. When set, NSURLCache is enabled in the downloader.
-     * NSURLCache will handle the protocol caching logic while SDWebImage remains useful for offline images.
+     * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
+     * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
      * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
-     * If a cached image exists, the completion block is called once with the cached image and again with the final image.
+     * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
+     *
+     * Use this flag only if you can't make your URLs static with embeded cache busting parameter.
      */
     SDWebImageRefreshCached = 1 << 4
 } SDWebImageOptions;

@@ -107,8 +107,14 @@
             SDWebImageDownloaderOptions downloaderOptions = 0;
             if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
             if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
-            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderEnableNSURLCache;
-            if (image && options & SDWebImageRefreshCached) downloaderOptions ^= SDWebImageDownloaderProgressiveDownload; // force progressive off if image is refreshing
+            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
+            if (image && options & SDWebImageRefreshCached)
+            {
+                // force progressive off if image already cached but forced refreshing
+                downloaderOptions ^= SDWebImageDownloaderProgressiveDownload;
+                // ignore image read from NSURLCache if image if cached but force refreshing
+                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
+            }
             __block id<SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished)
             {
                 if (weakOperation.cancelled)
@@ -138,7 +144,11 @@
                         cacheOnDisk = NO;
                     }
 
-                    if (downloadedImage && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)])
+                    if (options & SDWebImageRefreshCached && image && !downloadedImage)
+                    {
+                        // Image refresh hit the NSURLCache cache, do not call the completion block
+                    }
+                    else if (downloadedImage && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)])
                     {
                         dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^
                         {