1 通过AppDelegate 实现

App进入后台事件方法

- (void)applicationDidEnterBackground:(UIApplication *)application

APP进入前台的事件方法

- (void)applicationWillEnterForeground:(UIApplication *)application

2UIViewController类中通过对Background ，Foreground事件的通知的侦听，进行实现

举例

- (void) viewWillAppear:(BOOL)animated{  

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForegroundNotification) name:UIApplicationWillEnterForegroundNotification object:nil];  

}  

- (void) appWillEnterForegroundNotification{  

    NSLog(@"trigger event when will enter foreground.");  

}  

-(void) viewDidDisappear:(BOOL)animated{  

    [[NSNotificationCenter defaultCenter] removeObserver:self];      

}  

注意：在UIViewController类中viewDidUnload方法在ios6以后过期失效