后台申请3分钟运行时间

有一种方式苹果文档说到任何应用都有3分钟的后台执行任务时间。好吧先看下一段代码

- (void)applicationDidEnterBackground:(UIApplication *)application {    UIApplication*   app = [UIApplication sharedApplication];    __block    UIBackgroundTaskIdentifier bgTask;    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{        dispatch_async(dispatch_get_main_queue(), ^{            if (bgTask != UIBackgroundTaskInvalid)            {                bgTask = UIBackgroundTaskInvalid;            }        }); }]；dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{        dispatch_async(dispatch_get_main_queue(), ^{            if (bgTask != UIBackgroundTaskInvalid)               {                bgTask = UIBackgroundTaskInvalid;        });     });}