diff --git a/SDWebImage/SDWebImageDownloader.h b/SDWebImage/SDWebImageDownloader.h
index 54224c5c..b5911aff 100644
--- a/SDWebImage/SDWebImageDownloader.h
+++ b/SDWebImage/SDWebImageDownloader.h
@@ -11,7 +11,14 @@
 #import "SDWebImageOperation.h"
 
 typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
+    /**
+     * Put the download in the low queue priority and task priority.
+     */
     SDWebImageDownloaderLowPriority = 1 << 0,
+    
+    /**
+     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
+     */
     SDWebImageDownloaderProgressiveDownload = 1 << 1,
 
     /**
@@ -45,7 +52,7 @@ typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
     SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
 
     /**
-     * Put the image in the high priority queue.
+     * Put the download in the high queue priority and task priority.
      */
     SDWebImageDownloaderHighPriority = 1 << 7,
     
diff --git a/SDWebImage/SDWebImageDownloaderOperation.m b/SDWebImage/SDWebImageDownloaderOperation.m
index 99e508ae..722645f0 100644
--- a/SDWebImage/SDWebImageDownloaderOperation.m
+++ b/SDWebImage/SDWebImageDownloaderOperation.m
@@ -14,6 +14,13 @@
 #define LOCK(lock) dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
 #define UNLOCK(lock) dispatch_semaphore_signal(lock);
 
+// iOS 8 Foundation.framework extern these symbol but the define is in CFNetwork.framework. We just fix this without import CFNetwork.framework
+#if (__IPHONE_OS_VERSION_MIN_REQUIRED && __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_9_0)
+const float NSURLSessionTaskPriorityHigh = 0.75;
+const float NSURLSessionTaskPriorityDefault = 0.5;
+const float NSURLSessionTaskPriorityLow = 0.25;
+#endif
+
 NSString *const SDWebImageDownloadStartNotification = @"SDWebImageDownloadStartNotification";
 NSString *const SDWebImageDownloadReceiveResponseNotification = @"SDWebImageDownloadReceiveResponseNotification";
 NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNotification";
@@ -176,7 +183,6 @@ typedef NSMutableDictionary<NSString *, id> SDCallbacksDictionary;
     }
 
     if (self.dataTask) {
-        [self.dataTask resume];
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunguarded-availability"
         if ([self.dataTask respondsToSelector:@selector(setPriority:)]) {
@@ -187,6 +193,7 @@ typedef NSMutableDictionary<NSString *, id> SDCallbacksDictionary;
             }
         }
 #pragma clang diagnostic pop
+        [self.dataTask resume];
         for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
             progressBlock(0, NSURLResponseUnknownLength, self.request.URL);
         }