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Discover the intricacies of iOS ViewController lifecycle events. Learn why methods like `viewDidLoad` get called even when your app launches in the background.
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Understanding the ViewController Lifecycle Events During App Launches: What to Expect?

If you're diving into iOS development, you may have encountered some confusion regarding the lifecycle events of view controllers when your app launches—particularly whether these events are triggered when the app is launched into the background. This is a common question among developers, and today we’re going to break it down.

The Core Question

When you launch an iOS app into the background, does the lifecycle of the root view controller start? More specifically, do methods like viewDidLoad, viewWillAppear, and viewDidAppear get called, even if the app isn't in the foreground? This question stems from the expectations we have about the app’s lifecycle events.

Initial Understanding

The assumption often made is that view controller lifecycle methods wouldn’t be triggered unless the app is active and in the foreground. However, you may notice that calling window?.makeKeyAndVisible() while the app is launching in the background does indeed invoke these lifecycle methods.

Is This Expected?

Yes, it is, but let’s clarify what "appear" means in this sense. The term does not imply that the user will see the view right away. It indicates that the view controller hierarchy is being built and that your view controller's view is part of it.

The Importance of Stability in Lifecycle Events

The lifecycle events of view controllers and apps need to be reliable. If they behaved erratically based on the launch state, it could lead to potential issues within your app. Here’s a simple breakdown:

App Lifecycle: Represents the overall state of the app (i.e., running, background).

View Controller Lifecycle: Focuses on the state of individual view controllers (i.e., loading views).

The Interleaving of App and View Controller Lifecycles

While both the app lifecycle and view controller lifecycle are stable, their interleaving can vary significantly across different iOS versions and architectures (like using a navigation controller).

The Evolution of Lifecycle Calls

There’s a historical context to these calls. When the app launches, the expected order of lifecycle events was traditionally:

application(_:didFinishLaunchingWithOptions:)

viewDidLoad

viewWillAppear(_:)

applicationDidBecomeActive(_:)

viewDidAppear(_:)

However, as noted by developers who have experienced changes across iOS versions, this order can shift dramatically. For example, starting with iOS 8, the order changed such that the app delegate received applicationDidBecomeActive(_:) after viewDidAppear(_:), which confused many developers who relied on these sequences.

Learning from Experience

A crucial takeaway is not to depend on the timing relationships between the lifecycle events of different classes. By doing so, you may end up encountering unexpected behavior, especially since these sequences can change from one iOS version to the next.

The Final Word

In conclusion, while the sequence of lifecycle events when the app is launched, even in the background, may differ from what you expect, it is indeed by design. Understanding these processes ensures that your app remains robust and adaptable to future iOS updates. So remember—stay flexible and avoid relying solely on rigid event sequences.

With these insights, you should be better equipped to handle the complexities of iOS app lifecycles and ensure your apps behave as intended in various scenarios.