How to use beginBackgroundTaskWithExpirationHandler for already running task in iOS

Question

In my app, I am uploading images to server from iPhone. While sync if user press home button, App will close.

I want app must be running in background till sync fini

Answer 1

Here's a slight variation on the other answers which I'm using when the app is going to the background and I need to do something on the main thread:

- (void)applicationWillResignActive:(UIApplication *)application
{
     UIApplication *app = [UIApplication sharedApplication];

    // Register auto expiring background task
    __block UIBackgroundTaskIdentifier bgTaskId =
        [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:bgTaskId];
        bgTaskId = UIBackgroundTaskInvalid;
    }];

    // Execute background task on the main thread
    dispatch_async( dispatch_get_main_queue(), ^{
        // ------------------
        // DO SOMETHING INVOLVING THE WEBVIEW - WHICH MUST OCCUR ON THE MAIN THREAD!
        // ------------------
        [app endBackgroundTask:bgTaskId];
        bgTaskId = UIBackgroundTaskInvalid;
    });
}