Understanding the iOS App View Lifecycle

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In one of our previous posts, we learned how the app lifecycle works and how we can manage it using its delegate methods. In this article, we’ll learn how a simple screen view works and its methods. By using these methods, we’ll also cover best practices for certain tasks. So, let’s begin.

Understanding the view lifecycle is crucial for creating smooth, efficient, and responsive user interfaces. The view lifecycle defines the sequence of events that occur when a view is displayed, updated, and removed in the app’s UI.

In this article, we’ll explore the iOS view lifecycle, its key methods, and best practices for managing your view controllers and views in different states.

What is the View Lifecycle?

The view lifecycle refers to the series of events that manage the life of a view and view controller from when it’s created to when it’s removed from memory. It’s part of the broader app lifecycle but focuses on the specific management of view controllers and their views.

The lifecycle of a view typically involves the following steps:

1. Initialization: The view is created and prepared.
2. View is about to appear: The view will appear on the screen.
3. View is on the screen: The view is fully visible and interactive.
4. View will disappear: The view is about to leave the screen.
5. Deallocation: The view is no longer in use and is deallocated from memory.

Understanding these steps can help you manage resources more efficiently, avoid unnecessary reloading, and improve performance.
Key View Lifecycle Methods

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1. viewDidLoad()

This method is called when the view controller’s view is loaded into memory. It’s a good place to initialize data or perform setup tasks that are needed only once, such as configuring UI elements or setting up data models.

override func viewDidLoad() {
    super.viewDidLoad()
    // Perform initial setup here
   // callingAPI()
   // settingUpUI()
    print("View did load!")
}

Best Practice: viewDidLoad should contain setup code that only needs to be run once. Avoid making network requests or performing resource-heavy tasks here because the view is not yet visible to the user.

2. viewWillAppear(_:)

This method is called right before the view is about to appear on the screen. It’s useful for tasks that need to be refreshed each time the view appears, such as updating data or modifying UI elements.

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    // Update data or UI before the view appears
    print("View will appear!")
}

Best Practice: Use this method to prepare your view for display. For instance, you might want to refresh data or start animations that should be in place before the user sees the view.

3. viewDidAppear(_:)

Once the view has fully appeared on the screen, viewDidAppear is called. This method is useful for triggering animations, starting timers, or beginning background tasks that should happen only after the view is fully visible.

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    // Perform actions that require the view to be visible
    print("View did appear!")
}

Best Practice: Use this method for tasks that need to be executed after the view is on screen, such as starting animations or network requests that aren’t critical to load before the view is visible.

4. viewWillDisappear(_:)

This method is called when the view is about to be removed from the screen. It’s typically used to save any data, cancel ongoing tasks, or stop animations that you don’t want to continue in the background.

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    // Stop animations or tasks
    print("View will disappear!")
}

Best Practice: This method is useful for cleaning up resources, saving state, or cancelling tasks that shouldn’t continue when the view is no longer visible.

5. viewDidDisappear(_:)

Called after the view has disappeared from the screen. This method can be used to release any resources that are no longer needed, and it’s an appropriate place to stop tasks like background updates.

override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    // Clean up any remaining tasks or data
    print("View did disappear!")
}

Best Practice: Use this method to release resources or reset any properties that need to be cleared once the view is no longer visible.

Managing View Lifecycle in iOS

It’s important to consider how your app’s resources and performance are affected during the different stages of the view lifecycle. Improper handling of these methods can lead to problems like unnecessary memory usage, long load times, or unresponsive UI.

Efficient View Management

• Avoid heavy tasks in viewDidLoad(): As viewDidLoad() is called only once, it should be used for light setup tasks. Avoid heavy network calls or data processing here, as they might slow down the app's startup time.
• Optimize data fetching in viewWillAppear(_:): If you need to fetch or update data every time the view appears, it's better to do it in viewWillAppear. This ensures that the view is always displaying fresh data.
• Clean up resources in viewWillDisappear(_:): Always ensure you’re stopping tasks or releasing resources that are no longer necessary. This helps keep your app running smoothly, especially in memory-constrained environments like older devices.
• Leverage viewDidAppear(_:) for animations: Animations should begin only when the view is fully visible. This ensures that the user sees the animations from the start without delay or flicker.

The Role of Navigation Controllers

When using navigation controllers, the lifecycle methods become even more critical. For instance, pushing or popping a view controller will trigger certain lifecycle methods on the pushed or popped view controllers. You’ll need to manage state transitions between view controllers carefully, ensuring that resources are cleaned up when necessary.

Conclusion

Understanding the iOS view lifecycle is essential for building responsive, efficient, and well-optimized apps. Each lifecycle method serves a specific purpose, from setting up the view to cleaning up resources, and using them correctly can enhance your app’s performance and user experience.