Remove the `maxConcurrentDownloads`, which can be set from downloader configuration. Rename the `prefetcherQueue` to `delegateQueue` to match the correct description.

Fix the delegateQueue to async dispatch, avoid immediate callback cause recursion call(Match previous behavior)

SDWebImage/SDWebImagePrefetcher.h

@@ -58,15 +58,11 @@ typedef void(^SDWebImagePrefetcherCompletionBlock)(NSUInteger noOfFinishedUrls,
 @interface SDWebImagePrefetcher : NSObject
 
 /**
- *  The web image manager
+ * The web image manager used by prefetcher to prefetch images.
+ * @note You can specify a standalone manager and downloader with custom configuration suitable for image prefetching. Such as `currentDownloadCount` or `downloadTimeout`.
  */
 @property (strong, nonatomic, readonly, nonnull) SDWebImageManager *manager;
 
-/**
- * Maximum number of URLs to prefetch at the same time. Defaults to 3.
- */
-@property (nonatomic, assign) NSUInteger maxConcurrentDownloads;
-
 /**
  * The options for prefetcher. Defaults to SDWebImageLowPriority.
  */
@@ -79,16 +75,18 @@ typedef void(^SDWebImagePrefetcherCompletionBlock)(NSUInteger noOfFinishedUrls,
 
 /**
  * Queue options for prefetcher when call the progressBlock, completionBlock and delegate methods. Defaults to Main Queue.
+ * @note The call is asynchronously to avoid blocking target queue.
+ * @note The delegate queue should be set before any prefetching start and may not be changed during prefetching to avoid thread-safe problem.
  */
-@property (strong, nonatomic, nonnull) dispatch_queue_t prefetcherQueue;
+@property (strong, nonatomic, nonnull) dispatch_queue_t delegateQueue;
 
 /**
- * The delegate for the prefetcher.
+ * The delegate for the prefetcher. Defatuls to nil.
  */
 @property (weak, nonatomic, nullable) id <SDWebImagePrefetcherDelegate> delegate;
 
 /**
- * Returns the global shared image prefetcher instance.
+ * Returns the global shared image prefetcher instance. It use a standalone manager which is different from shared manager.
  */
 @property (nonatomic, class, readonly, nonnull) SDWebImagePrefetcher *sharedImagePrefetcher;
 

SDWebImage/SDWebImagePrefetcher.m

@@ -51,20 +51,11 @@
         _manager = manager;
         _runningTokens = [NSMutableSet set];
         _options = SDWebImageLowPriority;
-        _prefetcherQueue = dispatch_get_main_queue();
-        self.maxConcurrentDownloads = 3;
+        _delegateQueue = dispatch_get_main_queue();
     }
     return self;
 }
 
-- (void)setMaxConcurrentDownloads:(NSUInteger)maxConcurrentDownloads {
-    self.manager.imageDownloader.maxConcurrentDownloads = maxConcurrentDownloads;
-}
-
-- (NSUInteger)maxConcurrentDownloads {
-    return self.manager.imageDownloader.maxConcurrentDownloads;
-}
-
 #pragma mark - Prefetch
 - (nullable SDWebImagePrefetchToken *)prefetchURLs:(nullable NSArray<NSURL *> *)urls {
     return [self prefetchURLs:urls progress:nil completed:nil];
@@ -138,9 +129,11 @@
         return;
     }
     BOOL shouldCallDelegate = [self.delegate respondsToSelector:@selector(imagePrefetcher:didPrefetchURL:finishedCount:totalCount:)];
-    dispatch_queue_async_safe(self.prefetcherQueue, ^{
+    NSUInteger finishedCount = [self tokenFinishedCount];
+    NSUInteger totalCount = [self tokenTotalCount];
+    dispatch_async(self.delegateQueue, ^{
         if (shouldCallDelegate) {
-            [self.delegate imagePrefetcher:self didPrefetchURL:url finishedCount:[self tokenFinishedCount] totalCount:[self tokenTotalCount]];
+            [self.delegate imagePrefetcher:self didPrefetchURL:url finishedCount:finishedCount totalCount:totalCount];
         }
         if (token.progressBlock) {
             token.progressBlock((NSUInteger)token->_finishedCount, (NSUInteger)token.totalCount);
@@ -153,9 +146,11 @@
         return;
     }
     BOOL shoulCallDelegate = [self.delegate respondsToSelector:@selector(imagePrefetcher:didFinishWithTotalCount:skippedCount:)] && ([self countOfRunningTokens] == 1); // last one
-    dispatch_queue_async_safe(self.prefetcherQueue, ^{
+    NSUInteger totalCount = [self tokenTotalCount];
+    NSUInteger skippedCount = [self tokenSkippedCount];
+    dispatch_async(self.delegateQueue, ^{
         if (shoulCallDelegate) {
-            [self.delegate imagePrefetcher:self didFinishWithTotalCount:[self tokenTotalCount] skippedCount:[self tokenSkippedCount]];
+            [self.delegate imagePrefetcher:self didFinishWithTotalCount:totalCount skippedCount:skippedCount];
         }
         if (token.completionBlock) {
             token.completionBlock((NSUInteger)token->_finishedCount, (NSUInteger)token->_skippedCount);