viewIsAppearing(_:)
Notifies the view controller that the system is adding the view controller’s view to a view hierarchy.
Declaration
func viewIsAppearing(_ animated: Bool)Parameters
- animated:
If True, the system is adding the view to the window using an animation.
Mentioned in
Discussion
The system calls this method once each time a view controller’s view appears after the viewWillAppear(_:) call. In contrast to viewWillAppear(_:), the system calls this method after it adds the view controller’s view to the view hierarchy, and the superview lays out the view controller’s view. By the time the system calls this method, both the view controller and its view have received updated trait collections and the view has accurate geometry.
You can override this method to perform custom tasks associated with displaying the view. For example, you might use this method to configure or update views based on the trait collections of the view or view controller. Or, because computing a scroll position relies on the view’s size and geometry, you might programmatically scroll a collection or table view to ensure a selected cell is visible when the view appears.
If you override this method, you need to call super at some point in your implementation.
Choosing the appropriate callback
Although the system calls this method after viewWillAppear(_:), both callbacks occur within the same CATransaction. This means that changes you make in either method become visible to the user at the same time.
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The traits and geometry aren’t up to date when the system calls viewWillAppear(_:), but they are when the system calls viewIsAppearing(_:), so use viewIsAppearing(_:) to update your views.
Use viewWillAppear(:_) only when:
You need a callback before the view transition begins, such as when accessing the transitionCoordinator to add alongside animations. Alongside animations are animations that you direct the framework to perform concurrently with the view controller transition animations.
You need balanced callbacks to do something that doesn’t depend on the view controller or view traits, hierarchy, or geometry. Use cases include registering for database notifications in viewWillAppear(_:) and unregistering in viewDidDisappear(_:).
For all other cases, use viewIsAppearing(_:) to update your views.
State at callback time |
|
|
|---|---|---|
Transition coordinator available for adding alongside animations | ✓ | ✘ |
View added to hierarchy | ✘ | ✓ |
View controller and view trait collections updated | ✘ | ✓ |
View geometry (size, safe area, and so forth) is accurate | ✘ | ✓ |
The system calls layout methods, such as viewWillLayoutSubviews() and viewDidLayoutSubviews(), whenever the view runs layoutSubviews(), which can happen multiple times during the transition, or at any time while the view is visible. However, the system calls viewIsAppearing(_:) only once during the appearance transition, and calls it even if the view doesn’t require laying out when it appears.
For more information about how a view controller adds views to view hierarchies, and the sequence of messages that occur, see Displaying and managing views with a view controller.