AppDelegate以外にActiveになったことを通知する

例えば、UIViewControllerの派生クラスでは、このような感じで通知されるように設定できます。

- (void) viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillResignActive) name:UIApplicationWillResignActiveNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidBecomeActive) name:UIApplicationDidBecomeActiveNotification object:nil];
}

- (void) viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

- (void)applicationWillResignActive
{
    // Activeでなくなる時の処理.
}

- (void)applicationDidBecomeActive
{
    // Activeになった時の処理.
}