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) NSMutableDictionary *URLCallbacks;
+@property (assign, nonatomic) dispatch_queue_t barrierQueue;
 
 @end
 
@@ -65,6 +66,7 @@ NSString *const kCompletedCallbackKey = @"completed";
         _downloadQueue = NSOperationQueue.new;
         _downloadQueue.maxConcurrentOperationCount = 10;
         _URLCallbacks = NSMutableDictionary.new;
+        _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT);
     }
     return self;
 }
@@ -82,71 +84,85 @@ NSString *const kCompletedCallbackKey = @"completed";
 - (NSOperation *)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(void (^)(NSUInteger, long long))progressBlock completed:(void (^)(UIImage *, NSError *, BOOL))completedBlock
 {
     __block SDWebImageDownloaderOperation *operation;
+    __weak SDWebImageDownloader *wself = self;
 
-    dispatch_async(dispatch_get_main_queue(), ^ // NSDictionary isn't thread safe
+    [self addProgressCallback:progressBlock andCompletedBlock:completedBlock forURL:url createCallback:^
     {
-        BOOL performDownload = NO;
+        // 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];
+        request.HTTPShouldHandleCookies = NO;
+        request.HTTPShouldUsePipelining = YES;
+        [request addValue:@"image/*" forHTTPHeaderField:@"Accept"];
+        operation = [SDWebImageDownloaderOperation.alloc initWithRequest:request options:options progress:^(NSUInteger receivedSize, long long expectedSize)
+        {
+            if (!wself) return;
+            SDWebImageDownloader *sself = wself;
+            NSArray *callbacksForURL = [sself removeAndReturnCallbacksForURL:url];
+            for (NSDictionary *callbacks in callbacksForURL)
+            {
+                SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
+                if (callback) callback(receivedSize, expectedSize);
+            }
+        }
+        completed:^(UIImage *image, NSError *error, BOOL finished)
+        {
+            if (!wself) return;
+            SDWebImageDownloader *sself = wself;
+            NSArray *callbacksForURL = [sself removeAndReturnCallbacksForURL:url];
+            for (NSDictionary *callbacks in callbacksForURL)
+            {
+                SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
+                if (callback) callback(image, error, finished);
+            }
+        }
+        cancelled:^
+        {
+            if (!wself) return;
+            SDWebImageDownloader *sself = wself;
+            [sself removeAndReturnCallbacksForURL:url];
+        }];
+        [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;
-            performDownload = YES;
+            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)
         {
-            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)
-        {
-            // 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];
-            request.HTTPShouldHandleCookies = NO;
-            request.HTTPShouldUsePipelining = YES;
-            [request addValue:@"image/*" forHTTPHeaderField:@"Accept"];
-            operation = [SDWebImageDownloaderOperation.alloc initWithRequest:request options:options progress:^(NSUInteger receivedSize, long long expectedSize)
-            {
-                dispatch_async(dispatch_get_main_queue(), ^
-                {
-                    NSMutableArray *callbacksForURL = self.URLCallbacks[url];
-                    for (NSDictionary *callbacks in callbacksForURL)
-                    {
-                        SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
-                        if (callback) callback(receivedSize, expectedSize);
-                    }
-                });
-            }
-            completed:^(UIImage *image, NSError *error, BOOL finished)
-            {
-                dispatch_async(dispatch_get_main_queue(), ^
-                {
-                    NSMutableArray *callbacksForURL = self.URLCallbacks[url];
-                    [self.URLCallbacks removeObjectForKey:url];
-                    for (NSDictionary *callbacks in callbacksForURL)
-                    {
-                        SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
-                        if (callback) callback(image, error, finished);
-                    }
-                });
-            }
-            cancelled:^
-            {
-                dispatch_async(dispatch_get_main_queue(), ^
-                {
-                    [self.URLCallbacks removeObjectForKey:url];
-                });
-            }];
-            [self.downloadQueue addOperation:operation];
+            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