Chapter 1: Introduction to UIViewController

When developing iOS applications, whether using storyboards, XIBs, or programmatically creating views, a UIViewController is essential for overseeing the view hierarchy. This component, part of UIKit, is crucial for every iOS developer. Its primary function is to manage the lifecycle of the views it controls.

Chapter 2: The UIViewController Lifecycle

UIViewController's lifecycle begins at the initialization stage. This is where you can create custom initializers, whether with parameters or not.

1. init(): This is the default initializer.
2. init(nibName:bundle:): This method attempts to locate a nib file and link it to the UIViewController. No view is created at this point.
3. init(coder:): Utilized when views are created within a Storyboard.

The lifecycle continues with several critical methods that dictate how and when the UIViewController manages its views:

This video explains the UIViewController lifecycle and the significance of the viewDidLoad method in Swift.

loadView()

This method is responsible for creating the view managed by the controller. It can load a view from a nib or create one programmatically. If using Interface Builder, this method should not be overridden.

loadViewIfNeeded()

This method loads the view if it hasn't been set yet.

viewDidLoad()

Triggered once after the view is loaded into memory, it’s the ideal place to initialize the view, add subviews, set up constraints, or initiate network requests, although the bounds of the view are not yet defined.

viewWillAppear(_:)

Called just before the view becomes visible. This method can be invoked multiple times during the lifecycle, making it suitable for refreshing API calls or initializing animations.

viewWillLayoutSubviews()

Invoked before the layoutSubviews method, this is a great time to learn the view's bounds.

viewDidLayoutSubviews()

This method is called immediately after the layoutSubviews method, confirming that the size and position of subviews are set.

viewDidAppear(_:)

This method is executed when the view is completely visible. It's the right moment for presenting any animations.

viewWillDisappear(_:)

Triggered when the view is about to be hidden, making it suitable for saving user data or canceling tasks.

viewDidDisappear(_:)

Executed after the view has been removed from the screen.

didReceiveMemoryWarning()

This method is invoked when the system needs to free up memory, a potential opportunity for developers to release unused resources.

traitCollectionDidChange(_:)

Called when the interface environment changes, such as when the device orientation changes. Starting from iOS 13, the implementation details have evolved.

deinit

Called when memory is being deallocated, usually to remove observers.

This video discusses whether to use viewDidLoad or viewWillAppear in managing the UIViewController lifecycle.

Chapter 3: Managing Notifications and Memory

When using NSNotificationCenter for UI updates based on notifications, developers need to be cautious. Notifications can still arrive when the app is in the background, which may lead to unexpected behavior.

It’s advisable to subscribe to notifications in viewWillAppear and unsubscribe in viewWillDisappear to avoid potential crashes.

Conclusion

While UIViewController can be straightforward for simple use cases, it becomes complex when handling dynamic views that require user interactions. The methods invoked during transitions and the presence of child view controllers can further complicate the lifecycle.

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