Why does viewWillAppear not get called when an app comes back from the background?

Question

I'm writing an app and I need to change the view if the user is looking at the app while talking on the phone.

I've implemented the following method:

- (void)viewWillAppear:(BOOL)animated {     [super viewWillAppear:animated];     NSLog(@"viewWillAppear:");     _sv.frame = CGRectMake(0.0, 0.0, 320.0, self.view.bounds.size.height); } 

But it's not being called when the app returns to the foreground.

I know that I can implement:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(statusBarFrameChanged:) name:UIApplicationDidChangeStatusBarFrameNotification object:nil]; 

but I don't want to do this. I'd much rather put all my layout information in the viewWillAppear: method, and let that handle all possible scenarios.

I've even tried to call viewWillAppear: from applicationWillEnterForeground:, but I can't seem to pinpoint which is the current view controller at that point.

Does anybody know the proper way to deal with this? I'm sure I'm missing an obvious solution.

Answer 1

Swift

Short answer

Use a NotificationCenter observer rather than viewWillAppear.

override func viewDidLoad() {     super.viewDidLoad()      // set observer for UIApplication.willEnterForegroundNotification     NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)  }  // my selector that was defined above @objc func willEnterForeground() {     // do stuff } 

---

Long answer

To find out when an app comes back from the background, use a NotificationCenter observer rather than viewWillAppear. Here is a sample project that shows which events happen when. (This is an adaptation of this Objective-C answer.)

import UIKit class ViewController: UIViewController {      // MARK: - Overrides      override func viewDidLoad() {         super.viewDidLoad()         print("view did load")          // add notification observers         NotificationCenter.default.addObserver(self, selector: #selector(didBecomeActive), name: UIApplication.didBecomeActiveNotification, object: nil)         NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)      }      override func viewWillAppear(_ animated: Bool) {         print("view will appear")     }      override func viewDidAppear(_ animated: Bool) {         print("view did appear")     }      // MARK: - Notification oberserver methods      @objc func didBecomeActive() {         print("did become active")     }      @objc func willEnterForeground() {         print("will enter foreground")     }  } 

On first starting the app, the output order is:

view did load view will appear did become active view did appear 

After pushing the home button and then bringing the app back to the foreground, the output order is:

will enter foreground did become active  

So if you were originally trying to use viewWillAppear then UIApplication.willEnterForegroundNotification is probably what you want.

Note

As of iOS 9 and later, you don't need to remove the observer. The documentation states:

If your app targets iOS 9.0 and later or macOS 10.11 and later, you don't need to unregister an observer in its dealloc method.