Updated `storeImageDataToDisk:forKey:` asserting when called from other queue th…

…an ioQueue. Updated the header comments

@@ -124,7 +124,9 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
 - (void)storeImage:(nullable UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(nullable NSData *)imageData forKey:(nullable NSString *)key toDisk:(BOOL)toDisk;
 
 /**
- * Store image NSData into disk cache at the given key.
+ * Synchronously store image NSData into disk cache at the given key.
+ *
+ * @warning This method is synchronous, make sure to call it from the ioQueue
  *
  * @param imageData  The image data to store
  * @param key        The unique image cache key, usually it's image absolute URL

@@ -209,11 +209,14 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 }
 
 - (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
-    
-    if (!imageData) {
+    if (!imageData || !key) {
         return;
     }
     
+    const char *currentQueueLabel = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
+    const char *ioQueueLabel = dispatch_queue_get_label(self.ioQueue);
+    NSAssert(strcmp(currentQueueLabel, ioQueueLabel) == 0, @"This method should be called from the ioQueue");
+    
     if (![_fileManager fileExistsAtPath:_diskCachePath]) {
         [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
     }