1、定義進入前臺時呼叫的函式： 
- (void)applicationWillEnterForeground:(NSNotification *)notification 
{ 
//進入前臺時呼叫此函式 
我們可以在這個方法裡寫上我們需要重新整理的程式碼 
} 
2、註冊呼叫上面函式的通知，在willAppear中： 
- (void)viewWillAppear:(BOOL)animated 
{ 
UIApplication *app = [UIApplication sharedApplication]; 
[[NSNotificationCenter defaultCenter] addObserver:self 
selector:@selector(applicationWillEnterForeground:) 
name:UIApplicationWillEnterForegroundNotification 
object:app]; 
}

3、反註冊通知，在willDisappear中： 
- (void)viewDidDisappear:(BOOL)animated 
{ 
[[NSNotificationCenter defaultCenter] removeObserver:self]; 
}