Add completion block support to `SDWebImagePrefetcher` (fix #127)

SDWebImage/SDWebImagePrefetcher.h

@@ -39,6 +39,15 @@
  */
 - (void)prefetchURLs:(NSArray *)urls;
 
+/**
+ * Assign list of URLs to let SDWebImagePrefetcher to queue the prefetching,
+ * currently one image is downloaded at a time,
+ * and skips images for failed downloads and proceed to the next image in the list
+ *
+ * @param urls list of URLs to prefetch
+ * @param completionBlock block to be called when prefetching is completed
+ */
+- (void)prefetchURLs:(NSArray *)urls completed:(void (^)(NSUInteger finishedCount, NSUInteger skippedCount))completionBlock;
 
 /**
  * Remove and cancel queued list

SDWebImage/SDWebImagePrefetcher.m

@@ -17,6 +17,7 @@
 @property (assign, nonatomic) NSUInteger skippedCount;
 @property (assign, nonatomic) NSUInteger finishedCount;
 @property (assign, nonatomic) NSTimeInterval startedTime;
+@property (SDDispatchQueueSetterSementics, nonatomic) void (^completionBlock)(NSUInteger, NSUInteger);
 
 @end
 
@@ -79,6 +80,11 @@
         else if (self.finishedCount == self.requestedCount)
         {
             [self reportStatus];
+            if (self.completionBlock)
+            {
+                self.completionBlock(self.finishedCount, self.skippedCount);
+                self.completionBlock = nil;
+            }
         }
     }];
 }
@@ -90,10 +96,16 @@
 }
 
 - (void)prefetchURLs:(NSArray *)urls
+{
+    [self prefetchURLs:urls completed:nil];
+}
+
+- (void)prefetchURLs:(NSArray *)urls completed:(void (^)(NSUInteger, NSUInteger))completionBlock
 {
     [self cancelPrefetching]; // Prevent duplicate prefetch request
     self.startedTime = CFAbsoluteTimeGetCurrent();
     self.prefetchURLs = urls;
+    self.completionBlock = completionBlock;
 
     // Starts prefetching from the very first image on the list with the max allowed concurrency
     NSUInteger listCount = self.prefetchURLs.count;