在ios后台运行

Building on what rckoenes stated, applications are allowed to register background tasks to be completed after the user hits the home button. There is a time limit of 10 or 15 minutes for these tasks to complete. Again, you can register a task to complete immediately after the user hits home, this does NOT allow you to execute code say an hour after they exit the app.

UIApplication*    app = [UIApplication sharedApplication];
task = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:task];
        task = 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.
        NSLog(@"Started background task timeremaining = %f", [app backgroundTimeRemaining]);
        if (connectedToNetwork) {
            // do work son...
        }

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

UPDATE: if your app supports versions of iOS previous to iOs 4, you should also check to ensure that multitasking is supported before registering a background task. Use something along the lines of:

UIDevice* device = [UIDevice currentDevice];

BOOL backgroundSupported = NO;

if ([device respondsToSelector:@selector(isMultitaskingSupported)])

   backgroundSupported = device.multitaskingSupported;

http://blog.csdn/focusjava/article/details/9271043

有空再翻译。