Introduction

When developing iOS applications, you might expect certain lifecycle methods, such as viewDidAppear(), to be invoked every time a view appears on the screen. However, a common issue arises when reopening an app from the background: viewDidAppear() might not be called as anticipated. In this article, we'll explore why this happens, how the iOS application lifecycle operates, and provide technical insights to address this issue.

Understanding the iOS Application Lifecycle

The iOS application lifecycle is governed by several states: Not Running, Inactive, Active, Background, and Suspended. These states determine the current status of an app and influence which methods are triggered in the UIViewController lifecycle. Understanding these states gives you a significant advantage in predicting when a method will be called.

Application States:

• Not Running: The app is not launched or has been terminated by the system.
• Inactive: The app is in the foreground but not receiving events. This is a temporary state.
• Active: The app is in the foreground and receiving events.
• Background: The app is in the background, executing code.
• Suspended: The app is in the background but not executing code.

The Role of viewDidAppear()

The viewDidAppear() method is a part of the UIViewController lifecycle that is typically called after a view has been loaded into memory, rendered, added to a view hierarchy, and made visible on the screen. This method is ideal for tasks like animations, starting video playback, or data fetching for a new screen view.

Why viewDidAppear() May Not Be Called When Opening from Background

Key Points:

• State Transition: When an app goes from the background to the foreground, the transition typically moves from Background or Suspended to Active state. During this transition, viewDidAppear() might not be invoked if the view controller is already presented on the screen before backgrounding.
• View Controller's State: If the current view of the application did not change when entering the background and returns to the same state when reopened, then viewDidAppear() won’t be re-triggered automatically. The viewWillAppear() method usually handles the necessary updates instead.
• Behavioral Design: iOS has been designed to maintain efficiency and optimize performance, thus avoids calling methods unnecessarily. If the view is still valid and visible, there is no need to refresh it with viewDidAppear() if no state change occurred.

Example:

1override func viewDidAppear(_ animated: Bool) {
2    super.viewDidAppear(animated)
3    // Code here will not execute if the app was backgrounded and returned
4    print("viewDidAppear was called!")
5}
6
7@objc func applicationDidBecomeActive(notification: NSNotification) {
8    // Handle coming back to the foreground
9    print("App has returned from background")
10}

Solutions and Best Practices

1. Use NotificationCenter: To counteract the issue with viewDidAppear() not firing, listen for the UIApplicationDidBecomeActiveNotification to execute necessary updates when coming from background.

1    override func viewDidLoad() {
2        super.viewDidLoad()
3        
4        NotificationCenter.default.addObserver(self, 
5            selector: #selector(applicationDidBecomeActive(notification:)), 
6            name: UIApplication.didBecomeActiveNotification, 
7            object: nil)
8    }

1. Override Background Methods: You can override methods like applicationDidBecomeActive within AppDelegate to refresh the UI or fetch necessary data.
2. Decouple Logic from viewDidAppear(): If critical tasks are dependent on viewDidAppear(), consider decoupling them into standalone functions that can be independently called when the app transitions back to the foreground.
3. Check View State: Always verify the current state or parameters of the views to prevent unnecessary execution of logic that is not required every time the view appears.

Summary Table

Here’s a table summarizing key points to remember about iOS lifecycle states and viewDidAppear() behavior:

Conclusion

Encountering viewDidAppear() not being called while reopening an iOS app from the background might seem puzzling at first. However, by understanding the application lifecycle, developers can implement solutions that address this behavior effectively. Key approaches involve leveraging the Notification Center or decoupling vital logic from viewDidAppear(), ensuring smooth transitions and updates during state changes for your iOS applications.