iOS 程序退出之后

- (void)applicationDidEnterBackground:(UIApplication *)application
{
__block UIBackgroundTaskIdentifier background_task;
//注册一个后台任务，告诉系统我们需要向系统借一些事件
background_task = [application beginBackgroundTaskWithExpirationHandler:^ {

//不管有没有完成，结束background_task任务
[application endBackgroundTask: background_task];
background_task = UIBackgroundTaskInvalid;
}];

/*
尽量用异步，如果像这样 你会发现主线程会被阻塞
while(TRUE)
{
NSLog(@"Background time Remaining: %f",[[UIApplication sharedApplication] backgroundTimeRemaining]);
[NSThread sleepForTimeInterval:1]; //wait for 1 sec
}
*/
//异步
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

//### background task starts
NSLog(@"Running in the background\n");

static int i = 0;
while(i < 5)
{
NSLog(@"Background time Remaining: %f",[[UIApplication sharedApplication] backgroundTimeRemaining]);
[NSThread sleepForTimeInterval:1]; //wait for 1 sec
i++;
}
/*
while(TRUE)
{
NSLog(@"Background time Remaining: %f",[[UIApplication sharedApplication] backgroundTimeRemaining]);
[NSThread sleepForTimeInterval:1]; //wait for 1 sec
i++;
}
*/

//我们自己完成任务后，结束background_task任务
//如果我们用while（TRUE）的话，下面这两行代码应该不会执行
[application endBackgroundTask: background_task];
background_task = UIBackgroundTaskInvalid;
});
}