Added background clean disk to UIApplicationDidEnterBackgroundNotification #306

SDWebImage/SDImageCache.m

@@ -71,6 +71,11 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
                                                  selector:@selector(cleanDisk)
                                                      name:UIApplicationWillTerminateNotification
                                                    object:nil];
+        
+        [[NSNotificationCenter defaultCenter] addObserver:self
+                                                 selector:@selector(backgroundCleanDisk)
+                                                     name:UIApplicationDidEnterBackgroundNotification
+                                                   object:nil];
 #endif
     }
 
@@ -402,7 +407,29 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
     });
 }
 
--(unsigned long long)getSize
+- (void)backgroundCleanDisk
+{
+    UIApplication *application = [UIApplication sharedApplication];
+    __block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^
+    {
+        // Clean up any unfinished task business by marking where you
+        // stopped or ending the task outright.
+        [application endBackgroundTask:bgTask];
+        bgTask = UIBackgroundTaskInvalid;
+    }];
+    
+    // Start the long-running task and return immediately.
+    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
+    {
+        // Do the work associated with the task, preferably in chunks.
+        [self cleanDisk];
+        
+        [application endBackgroundTask:bgTask];
+        bgTask = UIBackgroundTaskInvalid;
+    });
+}
+
+- (unsigned long long)getSize
 {
     unsigned long long size = 0;
     NSDirectoryEnumerator *fileEnumerator = [[NSFileManager defaultManager] enumeratorAtPath:self.diskCachePath];