添加程序进入后台，进入前台的通知

//页面将要进入前台，开启定时器-(void)viewWillAppear:(BOOL)animated{        //注册程序进入前台通知    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector (startTimer) name: UIApplicationWillEnterForegroundNotification object:nil];        //注册程序进入后台通知    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector (pauseTimer) name: UIApplicationDidEnterBackgroundNotification object:nil];            [super viewWillAppear:animated];}/** *  这个方法里移除通知 */-(void) viewWillDisappear:(BOOL)animated {    //解除程序进入前台通知    [[NSNotificationCenter defaultCenter] removeObserver:self name: UIApplicationWillEnterForegroundNotification object:nil];        //解除程序进入后台通知    [[NSNotificationCenter defaultCenter] removeObserver:self name: UIApplicationDidEnterBackgroundNotification object:nil];    [super viewWillDisappear:animated];}