Add option to continue download in background

SDWebImage/SDWebImageDownloader.h

@@ -23,7 +23,10 @@ typedef enum
      * Call completion block with nil image/imageData if the image was read from NSURLCache
      * (to be combined with `SDWebImageDownloaderUseNSURLCache`).
      */
-    SDWebImageDownloaderIgnoreCachedResponse = 1 << 3
+    SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
+
+    SDWebImageDownloaderContinueInBackground = 1 << 4
+
 } SDWebImageDownloaderOptions;
 
 typedef enum

SDWebImage/SDWebImageDownloaderOperation.m

@@ -23,6 +23,10 @@
 @property (strong, nonatomic) NSMutableData *imageData;
 @property (strong, nonatomic) NSURLConnection *connection;
 
+#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
+@property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundTaskId;
+#endif
+
 @end
 
 @implementation SDWebImageDownloaderOperation
@@ -57,6 +61,25 @@
         return;
     }
 
+#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
+    if ([self shouldContinueWhenAppEntersBackground])
+    {
+        __weak __typeof__(self) wself = self;
+        self.backgroundTaskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^
+        {
+            __strong __typeof(wself)sself = wself;
+
+            if (sself)
+            {
+                [sself cancel];
+
+                [[UIApplication sharedApplication] endBackgroundTask:sself.backgroundTaskId];
+                sself.backgroundTaskId = UIBackgroundTaskInvalid;
+            }
+        }];
+    }
+#endif
+
     self.executing = YES;
     self.connection = [NSURLConnection.alloc initWithRequest:self.request delegate:self startImmediately:NO];
 
@@ -343,5 +366,10 @@
     }
 }
 
+- (BOOL)shouldContinueWhenAppEntersBackground
+{
+    return self.options & SDWebImageDownloaderContinueInBackground;
+}
+
 
 @end