Use dispatch_barrier to handle NSMutableDictionary thread unsafety instead of ma…

…in thread dispatching

@@ -20,6 +20,7 @@ NSString *const kCompletedCallbackKey = @"completed";
 
 
 @property (strong, nonatomic) NSOperationQueue *downloadQueue;
 @property (strong, nonatomic) NSOperationQueue *downloadQueue;
 @property (strong, nonatomic) NSMutableDictionary *URLCallbacks;
 @property (strong, nonatomic) NSMutableDictionary *URLCallbacks;
+@property (assign, nonatomic) dispatch_queue_t barrierQueue;
 
 
 @end
 @end
 
 
@@ -65,6 +66,7 @@ NSString *const kCompletedCallbackKey = @"completed";
         _downloadQueue = NSOperationQueue.new;
         _downloadQueue = NSOperationQueue.new;
         _downloadQueue.maxConcurrentOperationCount = 10;
         _downloadQueue.maxConcurrentOperationCount = 10;
         _URLCallbacks = NSMutableDictionary.new;
         _URLCallbacks = NSMutableDictionary.new;
+        _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
     }
     }
     return self;
     return self;
 }
 }
@@ -82,28 +84,9 @@ NSString *const kCompletedCallbackKey = @"completed";
 - (NSOperation *)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(void (^)(NSUInteger, long long))progressBlock completed:(void (^)(UIImage *, NSError *, BOOL))completedBlock
 - (NSOperation *)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(void (^)(NSUInteger, long long))progressBlock completed:(void (^)(UIImage *, NSError *, BOOL))completedBlock
 {
 {
     __block SDWebImageDownloaderOperation *operation;
     __block SDWebImageDownloaderOperation *operation;
+    __weak SDWebImageDownloader *wself = self;
 
 
-    dispatch_async(dispatch_get_main_queue(), ^ // NSDictionary isn't thread safe
-    {
-        BOOL performDownload = NO;
-
-        if (!self.URLCallbacks[url])
-        {
-            self.URLCallbacks[url] = NSMutableArray.new;
-            performDownload = YES;
-        }
-
-        // Handle single download of simultaneous download request for the same URL
-        {
-            NSMutableArray *callbacksForURL = self.URLCallbacks[url];
-            NSMutableDictionary *callbacks = NSMutableDictionary.new;
-            if (progressBlock) callbacks[kProgressCallbackKey] = progressBlock;
-            if (completedBlock) callbacks[kCompletedCallbackKey] = completedBlock;
-            [callbacksForURL addObject:callbacks];
-            self.URLCallbacks[url] = callbacksForURL;
-        }
-
-        if (performDownload)
+    [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
         // In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests
         NSMutableURLRequest *request = [NSMutableURLRequest.alloc initWithURL:url cachePolicy:NSURLCacheStorageNotAllowed timeoutInterval:15];
         NSMutableURLRequest *request = [NSMutableURLRequest.alloc initWithURL:url cachePolicy:NSURLCacheStorageNotAllowed timeoutInterval:15];
@@ -112,41 +95,74 @@ NSString *const kCompletedCallbackKey = @"completed";
         [request addValue:@"image/*" forHTTPHeaderField:@"Accept"];
         [request addValue:@"image/*" forHTTPHeaderField:@"Accept"];
         operation = [SDWebImageDownloaderOperation.alloc initWithRequest:request options:options progress:^(NSUInteger receivedSize, long long expectedSize)
         operation = [SDWebImageDownloaderOperation.alloc initWithRequest:request options:options progress:^(NSUInteger receivedSize, long long expectedSize)
         {
         {
-                dispatch_async(dispatch_get_main_queue(), ^
-                {
-                    NSMutableArray *callbacksForURL = self.URLCallbacks[url];
+            if (!wself) return;
+            SDWebImageDownloader *sself = wself;
+            NSArray *callbacksForURL = [sself removeAndReturnCallbacksForURL:url];
             for (NSDictionary *callbacks in callbacksForURL)
             for (NSDictionary *callbacks in callbacksForURL)
             {
             {
                 SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
                 SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
                 if (callback) callback(receivedSize, expectedSize);
                 if (callback) callback(receivedSize, expectedSize);
             }
             }
-                });
         }
         }
         completed:^(UIImage *image, NSError *error, BOOL finished)
         completed:^(UIImage *image, NSError *error, BOOL finished)
         {
         {
-                dispatch_async(dispatch_get_main_queue(), ^
-                {
-                    NSMutableArray *callbacksForURL = self.URLCallbacks[url];
-                    [self.URLCallbacks removeObjectForKey:url];
+            if (!wself) return;
+            SDWebImageDownloader *sself = wself;
+            NSArray *callbacksForURL = [sself removeAndReturnCallbacksForURL:url];
             for (NSDictionary *callbacks in callbacksForURL)
             for (NSDictionary *callbacks in callbacksForURL)
             {
             {
                 SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
                 SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
                 if (callback) callback(image, error, finished);
                 if (callback) callback(image, error, finished);
             }
             }
-                });
         }
         }
         cancelled:^
         cancelled:^
         {
         {
-                dispatch_async(dispatch_get_main_queue(), ^
-                {
-                    [self.URLCallbacks removeObjectForKey:url];
-                });
+            if (!wself) return;
+            SDWebImageDownloader *sself = wself;
+            [sself removeAndReturnCallbacksForURL:url];
         }];
         }];
         [self.downloadQueue addOperation:operation];
         [self.downloadQueue addOperation:operation];
+    }];
+
+
+    return operation;
+}
+
+- (void)addProgressCallback:(void (^)(NSUInteger, long long))progressBlock andCompletedBlock:(void (^)(UIImage *, NSError *, BOOL))completedBlock forURL:(NSURL *)url createCallback:(void (^)())createCallback
+{
+    dispatch_barrier_async(self.barrierQueue, ^
+    {
+        BOOL first = NO;
+        if (!self.URLCallbacks[url])
+        {
+            self.URLCallbacks[url] = NSMutableArray.new;
+            first = YES;
+        }
+
+        // Handle single download of simultaneous download request for the same URL
+        NSMutableArray *callbacksForURL = self.URLCallbacks[url];
+        NSMutableDictionary *callbacks = NSMutableDictionary.new;
+        if (progressBlock) callbacks[kProgressCallbackKey] = progressBlock;
+        if (completedBlock) callbacks[kCompletedCallbackKey] = completedBlock;
+        [callbacksForURL addObject:callbacks];
+        self.URLCallbacks[url] = callbacksForURL;
+
+        if (first)
+        {
+            createCallback();
         }
         }
     });
     });
+}
 
 
-    return operation;
+- (NSArray *)removeAndReturnCallbacksForURL:(NSURL *)url
+{
+    __block NSArray *callbacksForURL;
+    dispatch_barrier_sync(self.barrierQueue, ^
+    {
+        callbacksForURL = self.URLCallbacks[url];
+        [self.URLCallbacks removeObjectForKey:url];
+    });
+    return callbacksForURL;
 }
 }
 
 
 @end
 @end