# Hooking into App’s Lifecycle  within UIViewController

# Requirement
I’m building an application in which I would like to play an animation when the (root) view controller comes into the foreground and pause it when it (app) goes to the background.

# The Platform
iOS exposes the Application Delegate that provides callback methods to hook into in response to the application’s foreground/background transition. This is great if all you want to do is write application-wide logic. In this article, I discuss how I handled a view controller specific logic in response to the application’s lifecycle events.

# Approach

Luckily for me, the platform sends notifications in response to a change in app’s lifecycle events as highlighted in the [documentation](https://developer.apple.com/documentation/uikit/uiapplicationdelegate):

*   **didBecomeActiveNotification**: Posted when the app becomes active.
*   **didEnterBackgroundNotification**: Posted when the app enters the background.
*   **willEnterForegroundNotification**: Posted when the app is entering the foreground.
*   **willResignActiveNotification**: Posted when the app is no longer active.
*   **willTerminateNotification**: Posted when the app is about to terminate.

A typical usage registers the containing view controller as an observer to the desired notification perhaps in the _init_ method. The registration also takes an objc method to callback when the notification is received.


```
NotificationCenter.default.addObserver(self, selector: #selector(onAppDidEnterBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
```


Your callback method in the view controller will have such a signature:


```
@objc func onAppDidEnterBackground() {
   // some awesome logic
}
```


# Conclusion

This works for me since the view controller is the root controller. For modally presented view controllers, I can, and probably would, use the **view*** lifecycle callbacks to achieve the same e.g. _viewWillDisappear()_.