--- title: ASVisibility layout: docs permalink: /docs/asvisibility.html prevPage: debug-tool-ASRangeController.html nextPage: asenvironment.html --- `ASNavigationController` and `ASTabBarController` both implement the `ASVisibility` protocol. These classes can be used even without `ASDisplayNodes`, making them suitable base classes for your inheritance hierarchy. For any child view controllers that are `ASViewControllers`, these classes know the exact number of user taps it would take to make the view controller visible (0 if currently visible). Knowing a view controller’s visibility depth allows view controllers to automatically take appropriate actions as a user approaches or leaves them. Non-default tabs in an app might preload some of their data; a controller 3 levels deep in a navigation stack might progressively free memory for images, text, and fetched data as it gets deeper. Any container view controller can implement a simple protocol to integrate with the system. For example, `ASNavigationController` will return a visibility depth of it's own `visibilityDepth` + 1 for a view controller that would be revealed by tapping the back button once. You can opt into some of this behavior automatically by enabling `automaticallyAdjustRangeModeBasedOnViewEvents` on `ASViewController`s. With this enabled, if either the view controller or its node conform to `ASRangeControllerUpdateRangeProtocol` (`ASCollectionNode` and `ASTableNode` do by default), the ranges will automatically be decreased as the visibility depth increases to save memory.