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iOS NSNotificationCenter가 앱이 백그라운드에서 포그라운드로 이동했는지 확인 방법.

[objective-c]

알림 신청

[[NSNotificationCenter defaultCenter] addObserver:self

                                         selector:@selector(yourUpdateMethodGoesHere:)

                                             name:UIApplicationWillEnterForegroundNotification

                                           object:nil];

호출해야하는 코드를 구현.

- (void) yourUpdateMethodGoesHere:(NSNotification *) note {

// code

}

뷰컨트롤러 종료시 사용한 알림은 삭제 한다.

[[NSNotificationCenter defaultCenter] removeObserver:self];

[swift3]

override func viewDidAppear(_ animated: Bool) {

    super.viewDidAppear(animated)

    NotificationCenter.default.addObserver(self,

                                           selector:#selector(applicationWillEnterForeground(_:)),

                                           name:NSNotification.Name.UIApplicationWillEnterForeground,

                                           object: nil)

}

override func viewWillDisappear(_ animated: Bool) {

    super.viewWillDisappear(animated)

    NotificationCenter.default.removeObserver(self)

}

func applicationWillEnterForeground(_ notification: NSNotification) {

   ....

}

[swift4.2]

NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification

            , object: nil)

[swift4.1]

override func viewDidAppear(_ animated: Bool) {

        NotificationCenter.default.addObserver(self, selector: #selector(enterForeground), name: Notification.Name.UIApplicationWillEnterForeground, object: nil)

    }

    override func viewDidDisappear(_ animated: Bool) {

        NotificationCenter.default.removeObserver(self, name: Notification.Name.UIApplicationWillEnterForeground, object: nil)

    }

    @objc func enterForeground() {

       // Do stuff

   }