If you create your app using one of Xcode’s built-in templates, you’ll automatically get an
AppDelegate class in AppDelegate.swift, which comes with a handful of empty methods.
This class is supposed to be there to handle application lifecycle events - i.e., responding to the app being launched, backgrounded, foregrounded, receiving data, and so on. However, in practice
AppDelegate is often abused as an easy dumping group for shared data – any thing that is used in several view controllers often gets thrown into the app delegate, but that’s nearly always the wrong place for it.
You may also sometimes see folks creating their initial user interface inside
AppDelegate. This is broadly a bad idea unless you’re just starting out – if you’ve done this, consider splitting off that layout code either into a coordinator or into a view containment subclass.
Available from iOS 8.0
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