View Controller Lifecycle

Understand the View Controller Lifecycle An object of the UIViewController class (and its subclasses) comes with a set of methods that manage its view hierarchy. iOS automatically calls these methods at appropriate times when a view controller transitions between states.

View Management

View controllers load their views lazily. Accessing the view property for the first time loads or creates the view controller’s views. There are several ways to specify the views for a view controller:

• Specify the view controller and its views in your app’s Storyboard.
• Specify the views for a view controller using a Nib file.
• Specify the views for a view controller using the loadView()method.
• In that method, create your view hierarchy programmatically and assign the root view of that hierarchy to the view controller’s view property.
• The content view is not necessarily created when the controller is first instantiated.
• Instead, it is lazily created the first time the system or any code accesses the controller’s view property.

A nib file lets you specify the views of a single view controller but does not let you define segues or relationships between view controllers. The nib file also stores only minimal information about the view controller itself.

class MyViewController: UIViewController {
    class func viewController() -> MyViewController {
        return UIStoryboard(name: "Name", bundle: nil).instantiateViewController(withIdentifier: String(describing: MyViewController.self)) as! MyViewController
    }
}

class MyView: UIView {
    override init(frame: CGRect) {
        super.init(frame: frame)
        initSetup()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
}

Handling View-Related Notifications

A view controller’s main responsibilities include the following:

• Updating the contents of the views, usually in response to changes to the underlying data.
• Responding to user interactions with views.
• Resizing views and managing the layout of the overall interface.

• awakeFromNib() - awakeFromNib is called when the associated nib file with a class is loaded . Any class that can own a nib can use it. viewDidLoad is used only by view controllers.

• viewDidLoad() — Called when the view controller’s content view (the top of its view hierarchy) is created and loaded from a storyboard. The view controller’s outlets are guaranteed to have valid values by the time this method is called.

• viewWillAppear() — Called just before the view controller’s content view is added to the app’s view hierarchy. Despite the name, just because the system calls this method, it does not guarantee that the content view will become visible. The view may be obscured by other views or hidden. This method simply indicates that the content view is about to be added to the app’s view hierarchy.

• viewWillLayoutSubviews() - Called to notify the view controller that its view is about to layout its subviews.

• viewDidLayoutSubviews() - Called to notify the view controller that its view has just laid out its subviews.

• viewDidAppear()—Called just after the view controller’s content view has been added to the app’s view hierarchy. Use this method to trigger any operations that need to occur as soon as the view is presented onscreen, such as fetching data or showing an animation. Despite the name, just because the system calls this method, it does not guarantee that the content view is visible. The view may be obscured by other views or hidden. This method simply indicates that the content view has been added to the app’s view hierarchy.

• viewWillDisappear()—Called just before the view controller’s content view is removed from the app’s view hierarchy. Use this method to perform cleanup tasks like committing changes or resigning the first responder status. Despite the name, the system does not call this method just because the content view will be hidden or obscured. This method is only called when the content view is about to be removed from the app’s view hierarchy.

• viewDidDisappear()—Called just after the view controller’s content view has been removed from the app’s view hierarchy. Use this method to perform additional teardown activities. Despite the name, the system does not call this method just because the content view has become hidden or obscured. This method is only called when the content view has been removed from the app’s view hierarchy.

class MyViewController: UIViewController {
    @IBOutlet weak var label: UILabel!

    override func awakeFromNib() {
        super.awakeFromNib()
    }

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
    }
}