What is the mobile app lifecycle, and what are the main states an app goes through from launch to termination?

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The mobile app lifecycle describes the sequence of states an app transitions through from the moment it launches to when it is terminated. Understanding these states is essential because they determine when your app can run code, when it should save data, and when the operating system might reclaim its resources.

Both iOS and Android apps share a similar high-level lifecycle with some platform-specific differences. The main states are inactive or not running, active or foreground, background, and suspended or terminated.

When the user taps the app icon, the app launches and moves through an initialization phase. On iOS, this triggers the application didFinishLaunchingWithOptions method in AppDelegate, or the init of your App struct in SwiftUI. On Android, the Application class onCreate runs first, followed by the launcher Activity's onCreate, onStart, and onResume methods. During this phase, you set up dependencies, configure the root view, and restore any saved state.

The active or foreground state is when the app is visible and receiving user input. This is the primary state where the user interacts with your app. The app has full access to the screen and system resources. On Android, an Activity is considered fully active after onResume is called.

When the user switches to another app, presses the home button, or an interruption like a phone call occurs, the app transitions to the background. On iOS, applicationDidEnterBackground is called, and you typically have about five seconds to complete critical tasks like saving data. On Android, the Activity goes through onPause and onStop. In the background, the app should minimize resource usage. You should save any unsaved user data, pause ongoing animations or media playback, and release resources that can be recreated later.

The suspended state occurs when the operating system freezes a backgrounded app to reclaim resources. The app remains in memory but is not executing code. If the device needs more memory, the system may terminate suspended apps without notification. On iOS, there is no callback when the app is terminated from the suspended state. On Android, the system may call onDestroy, but this is not guaranteed. This is why saving data during the background transition is critical rather than deferring it.

In React Native, the AppState module provides access to these lifecycle changes. You can listen for active, background, and inactive state changes. In Flutter, WidgetsBindingObserver with the didChangeAppLifecycleState callback provides AppLifecycleState values including resumed, inactive, paused, and detached.

A practical implication is handling what happens when the user returns to the app. If the app was suspended and is still in memory, it resumes quickly from where the user left off. If it was terminated, it must go through the full launch process again. Saving and restoring state properly ensures the user experience is seamless in both cases.