Understanding the Flutter Lifecycle: App States (Part 1)

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Introduction

Introduction

Building a Flutter app that feels seamless and responsive, no matter whether it's in the foreground, background, or even paused during a phone call, requires more than just good design. It requires a deep understanding of the Flutter app lifecycle. What does that lifecycle look like? And how can you make sure your app reacts to these changes effectively?

In this article, we'll explore the Flutter lifecycle, from its app states to widget lifecycle methods, and help you learn when and how to leverage these phases for optimal performance and user experience.

Flutter App Life Cycle

Have you ever pressed the "home" button on your phone, switched to another app, and then returned to find that the app you left behind hadn't been terminated? Instead, it picked up right where you left off, resuming its previous state as if nothing had changed. This seamless transition is made possible by how Flutter manages its app lifecycle. Flutter provides several states that we can monitor during app development. Here are the states in the app lifecycle:

App Lifecycle States in Flutter:

1. resumed: The app is in the foreground and interacting with the user. The app is fully active. Best used for:
   a. Interacting with the user: This is the most active state, during which the app should be fully responsive. It's the ideal time to handle user interactions and refresh UI elements.
   b. Starting or resuming activities: When an app resumes after being paused (e.g., after a phone call), you might refresh data or resume video playback.
2. inactive: The app is in the foreground but temporarily unable to process user input. This typically happens during transitions (e.g., the app is paused while the user is on a phone call). Best used for:
   - Handling interruptions: You might use this state to prepare your app for being paused or sent to the background, like stopping timers or pausing animations that depend on user interactions.
   - Managing system events: It helps handle activities like pausing video playback during a phone call or handling notifications that the app receives while the user is interacting with another app.
3. hidden: The app enters the hidden state when it is no longer visible to the user. This typically happens when the app is minimized (e.g., when the user presses the home button), or when another app fully covers it. While the app is hidden, it is still running in the background, but it is not actively interacting with the user. Best used for:
   - Reducing resource usage: When the app is in this state, you should pause or release resources that are not needed, like stopping animations, suspending network requests, or releasing memory-intensive operations.
4. paused: The paused state occurs when the app is sent to the background and is no longer visible to the user. While in this state, the app is not executing any code (except for background tasks or services) and does not interact with the user. Best used for:
   - Suspending UI activities: This is the time to stop any UI updates, animations, or time-consuming operations (such as network requests) that are not essential in the background.
   - Saving user data: You can use this state to save the app's state and data (e.g., unsaved form inputs or a partially completed quiz), and prevent data loss when the app is eventually resumed.
5. detached: The detached state indicates that the app is no longer connected to the Flutter framework and is about to be terminated. It happens when the app is removed from memory completely Best used for:
   - Performing cleanup tasks: This is the final chance for the app to release any resources, cancel background tasks, and perform any necessary cleanup, such as saving data to disk or closing open files.

Using the App Lifecycle:

How do we know which app lifecycle state is currently active? Flutter provides a listener called AppLifecycleListener. It can be used to monitor lifecycle changes in the app. You simply need to initialize this listener in a widget. Inside it, there are several parameters that you can use to monitor state changes.

class _MyAppState extends State<MyApp> {
 late final AppLifecycleListener _listener;

 @override
 void initState() {
   super.initState();
   _listener = AppLifecycleListener(
     onDetach: () => debugPrint('app-detached'),
     onInactive: () => debugPrint('app-inactive'),
     onPause: () => debugPrint('app-paused'),
     onResume: () => debugPrint('app-resumed'),
     onRestart: () => debugPrint('app-restarted'),
     onShow: () => debugPrint("app-showed"),
     onHide: () => debugPrint("app-hide"),
   );
 }

 @override
 void dispose() {
   _listener.dispose();
   super.dispose();
 }

 @override
 Widget build(BuildContext context) {
   ...
 }
}

In Summary:

Understanding the Flutter app lifecycle is essential for creating responsive and efficient apps. Flutter's lifecycle consists of several key states:

• Resumed: The app is active and interacting with the user, making it the ideal time for user engagement and UI updates.
• Inactive: The app is in the foreground but cannot process input due to interruptions like a phone call. It's important to pause or prepare the app for background transitions.
• Hidden: The app is no longer visible but still running in the background. This state is used to reduce resource consumption by stopping non-essential operations.
• Paused: The app is in the background and not executing code, except for background tasks. It's important to suspend unnecessary activities and save user data to prevent loss.
• Detached: The app is about to be terminated and removed from memory, making it the last chance to release resources and perform cleanup tasks.

Flutter provides an AppLifecycleListener to monitor these states and ensure your app reacts appropriately to changes, allowing for better resource management and a smoother user experience. By understanding and utilizing the app lifecycle, developers can create apps that remain responsive and efficient under varying conditions.