第一步:
创建2个NSNotificationCenter监听

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillResignActive:)
name:UIApplicationWillResignActiveNotification object:nil]; //监听是否触发home键挂起程序.

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidBecomeActive:)
name:UIApplicationDidBecomeActiveNotification object:nil]; //监听是否重新进入程序程序.

第二步:
实现2个NSNotificationCenter所触发的事件方法

- (void)applicationWillResignActive:(NSNotification *)notification

{
    printf("按理说是触发home按下\n");
}

- (void)applicationDidBecomeActive:(NSNotification *)notification
{
    printf("按理说是重新进来后响应\n");
}

注: 在home键触发后,AppDelegate响应的方法为:

- (void)applicationDidEnterBackground:(UIApplication *)application
{
/*
Use this method to release shared resources, save user data, invalidate timers,

and store enough application state information to restore your application to its current state in case it is terminated later.
If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
*/
}