The next thing after learning Application Lifecycle, the view lifecycle holds an essential position in every iOS developer’s life.
Each view is created either using storyboards, xibs, or using a programmatic approach. Independent from the approach, truly understanding when the view is created, loaded, appeared, or destroyed helps us to understand our approach deeply while developing the screen designs, animations, and even business logic.
Ok, but why do we need to know
When we think about the application lifecycle, we easily see that certain things happen as the application goes to the background, suspended, etc. Whenever the user interacts with the app, some specific action happens, and the app's state may change. However, the view's state may change with any user interaction or any change in the app's data.
On the other hand, we might need to manipulate the user interface when there is a particular state transition in the view. For example, if we would like to use custom views and set them to UIViewController's view, we have to do it in a loadView method. Or, if our design requires us to have minor differences between landscape and portrait mode, we must know when we can make this change in the code. Therefore, understanding all lifecycle methods not only helps us in code but also enables us to be more creative and implement an advanced user interface.
What do we need to know?
To use the view in proper positions, we must know when the view is created, loaded, appeared, changed, disappeared, and terminated. Subclasses of UIViewController is responsible for managing the views (aka. UIViews). View controller has one root view, which is a UIView instance. The important thing here is how they work together.
UIViewController handles all the magic behind UIView while UIView just represents the screen and some content to the user. UIViewController tells the root UIView object when to come to the screen. First, the view controller creates its root view and loads it. After loading, it tells the view to appear on the screen and disappear when necessary.
Furthermore, a view controller might be a child of another view controller. So, the view controller's lifecycle not only cares about its root view but also includes the relationship with other view controllers, like whether it moved to the parent view controller or not.
Lastly, the view controller’s root view (aka. UIView) has its own content and lifecycle. The root view has subviews like buttons, labels, switches, or other UIView subclasses. The lifecycle of the subviews is, in most cases, not considered by every developer. But there is one common case which is embedding a view controller into another. In this case, we need to imitate the lifecycle actions for both view controllers’ views and call proper methods when necessary.
How to get notified about lifecycle events
UIViewController has a lot of methods to inform us when some particular action happens. For example, we can override the viewDidLoad method to do some actions right after loading the view. Or use the loadView method to replace UIViewController’s root UIView with our custom UIView class.
We’ll look at the relationships between the combination of views and view controllers to learn the current state. Instead of focusing too much on what each method does, we’ll look at what is important for each method and when to use them.
UIView - UIViewController Relationship
loadView
This is where the root view of the view controller is loaded. The view property of UIViewController is nil at the moment. We need to override this method if we want to create a custom view and set it to the view property. If the Interface Builder is used to create the view and initialize the view controller, this method must not be overridden. Once we override this method, we have to initialize our custom view and set it to view property of UIViewController and never call super.loadView because it creates the view by itself and assigns it to view. Lastly, we shouldn’t call this method directly.
viewDidLoad
This is called only once after creating the view and loading it into memory. But the bounds of the view is not defined yet. We generally override this method to initialize the objects which the view controller will use. We shouldn’t forget to call super when we override.
viewWillAppear
This method is called right before the view appears on the screen. When this is called, the view’s bounds are defined, but the orientation is not set yet. We need to be careful while using appear methods since they will be called every time the view comes to the screen. Similarly, we shouldn’t forget to call super.
viewDidAppear
This method is called right after the view is visible to the user. It’s a good place to start animations. Same as before, we shouldn’t forget to call super.
viewWillLayoutSubviews
This is the first place to learn the bounds of the view in the lifecycle. This is also called right before layoutSubviews method is triggered in UIView. This is also called when the subviews of the root view are loaded. For example, this is called when the cells of the collection view are loaded.
viewDidLayoutSubviews
This is called right after layoutSubviews called. The subviews have been set, and size, position, and constraints are applied. viewWillLayoutSubviews and viewDidLayoutSubviews are also called when the orientation of the screen changes between portrait and landscape modes. So the key point is whenever the bounds of the view are updated, or the view layout is recalculated, both methods will be called. Similarly, we shouldn’t forget to call super at some point in these methods.
viewWillDisappear
This is called when the view is about to disappear from the screen. One thing we can do here is to save the user data to not lose anything important. Another thing we might be canceling is network requests. We can override this method also if we want to hand over first responder jobs to another view when the view disappears.
viewDidDisappear
This is called when the view is disappeared from the screen. The view is removed from the view hierarchy at the moment.
We took a look at the most common relationship, view and view controller relationship which was also a default one implemented by UIKit. But the view controller not only gets in a relationship with a view but also with other view controllers like a child and parent.
UIViewController - UIViewController Relationship
Understanding this relationship and the lifecycle of the view controller is important if we embed one view controller into another. Because we might need to manually call some lifecycle methods to inform the system about the state, let’s have an example for adding and removing child view controllers.
Adding a ViewController as a child to Another ViewController
let childViewController = UIViewController()
let parentViewController = UIViewController()
parentViewController.addChild(childViewController)
parentViewController.view.addSubview(childViewController.view)
// Time to setup auto-layout constraints for childViewController.view
// Example:
// NSLayoutConstraint.activate([
// childViewController.view.topAnchor.constraint(equalTo: parentViewController.view.topAnchor)
// ])
childViewController.didMove(toParent: parentViewController)
Let’s examine what we are doing about the lifecycle. After creating view controllers, we add the child to the parent. This addition automatically calls childViewController.willMove(toParent: parentViewController) for us. So, we don’t need to call it manually. Then, we need to add the child view controller’s view to the parent’s view to have a child-parent relationship for views. The system will load both views and will add one to another with our call addSubview. Then we inform the system by calling didMove(toParent:). The system will handle the rest of displaying the view controller and calling the view controller’s necessary methods in its lifecycle like viewDidLoad, viewDidAppear, etc.
Removing the Child ViewController from the Parent ViewController
childViewController.willMove(toParent: nil)
childViewController.view.removeFromSuperview()
childViewController.removeFromParent()
While removing the child view controller from the parent, we have to call willMove(toParent:) method with nil to inform the system that the child view controller will be removed from the parent. Then, we remove its view and later itself from the parent. As we see, we don’t need to call didMove(toParent:) method while we’re removing it. removeFromParent method automatically calls that method for us.
Additionally, we can override both willMove(toParent:) and didMove(toParent:). If we need to know in a view controller that it’s been added as a child to another view controller, we can override these methods and implement our custom actions.
Parent and child view controller relationships are common. It’s important to understand the lifecycle of both view controllers in order to call the necessary method at a certain point. If we forget to call one method, we might lead to some memory problems inside the app. Now, we came to the last point - the relationship between view and its subviews and lifecycle methods.
View - View (Subviews) Relationship
In this relationship, most of the lifecycle is handled automatically by the system and no need for calling some methods like parent and child view controllers relationship. For example, even if we try to call willRemoveSubview method, it does nothing. The default implementation is empty. The system calls the below functions only to inform us when something is changed. There are not so many things to talk about. Learning that we can detect when the subview is added, the view is added as a subview or moved to the window opens up opportunities for implementing extra actions during those phases.
Here are the mentioned lifecycle methods for UIView:
didAddSubview(_:)
willRemoveSubview(_:)
willMove(toSuperView:)
didMoveToSuperview()
willMove(toWindow:)
didMoveToWindow()
The only important thing to know here is removeFromSuperview method. We can call this if we want to remove the view from its parent. Calling this method removes all constraints in the view and in its subtrees. We shouldn’t call this method inside the view’s draw(_:) method.
Last Words
Only knowing one of the application, view controller, and view lifecycles are not enough to understand the whole picture and develop a great overview of iOS applications. Grasping the use of lifecycle methods is important to take appropriate actions in certain places, especially working with custom implementations of UIKit classes. Now we know the lifecycles, we can truly understand what is going on behind the scenes in the system and intervene when necessary.
Further reading: