Iphone: Does application freeze when run into background?

I have an application that upload file to server using NSUrlConnection. It was placed on ViewDidLoad method. It did upload to server while the application is in foreground. Before I call the NSUrlConnection asynchronously, I create temporary file in application directory.

While the files were uploading, I clicked on the iPhone button so that the application will run in the background. First thing I thought the application freeze the upload, but it doesn't. The file still uploading during that time.

Does that mean the application still running normally at the background until the remaining time finished then the appl开发者_运维知识库ication quits?

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Once the home button is pressed, every application runs in the background for a certain amount of time(usually few seconds) before it enters suspended mode. If you are required to complete a task(in your case file upload) before entering suspended mode, you can use Task completion API.

Even if you are using task completion API, only one thread of your application is running, but not the whole application. Here is the code recommended by apple:

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

    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        [app 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.

        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    });
}