What is a SceneDelegate?

 In my SwiftUI project I see AppDelegate file as well as a SceneDelegate file.

What are the differences between them?

For example between the methods in SceneDelegate

scene(_:willConnectTo:options:)

and in the AppDelegate

application(_:didFinishLaunchingWithOptions:)

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edited Jun 7 '19 at 16:27

rmaddy

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asked Jun 7 '19 at 16:24

SwiftiSwift

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• 1
  According to the talks, they took the functionality of AppDelegate from before and split it up. The SceneDelegate is specifically responsible for managing the active window, including displaying views. – mginn Jun 7 '19 at 16:41
• @Adrian Looks like you may have missed stackoverflow.com/a/56498306/461982 – Abandoned Cart Jun 11 '19 at 1:50

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The two files are meant to split the work by what is needed to run the app as a whole and what is needed for one "instance" that would support visibly running in the background. This would be something like configuring a database once, but displaying different sets of values by window.

You could think of them as the global and private versions. One is shared and the other is limited to the individual owner. In a way, they are exactly what you would expect by the names.

Multi-window support is happening

Next time you create a new Xcode project you’ll see your AppDelegate has split in two: AppDelegate.swift and SceneDelegate.swift. This is a result of the new multi-window support that landed with iPadOS, and effectively splits the work of the app delegate in two.

From iOS 13 onwards, your app delegate should:

1. Set up any data that you need for the duration of the app.
2. Respond to any events that focus on the app, such as a file being shared with you.
3. Register for external services, such as push notifications.
4. Configure your initial scenes.

In contrast, scene delegates are there to handle one instance of your app’s user interface. So, if the user has created two windows showing your app, you have two scenes, both backed by the same app delegate.

Keep in mind that these scenes are designed to work independently from each other. So, your application no longer moves to the background, but instead individual scenes do – the user might move one to the background while keeping another open.

Courtesy of https://www.hackingwithswift.com/articles/193/whats-new-in-ios-13

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answered Jun 7 '19 at 16:43

Abandoned Cart

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  What if my app supports only iPhone? – Mohammad Zaid Pathan Jun 11 '19 at 1:13
• 2
  @ZaidPathan As the article said, it was only a result of support that was added with the release of iPadOS. – Abandoned Cart Jun 11 '19 at 1:46 
• I'm very curious if it will have an impact on how iPhone applications are designed. I can imagine a game in which there is one scene for the menu and another scene for gameplay. Another practical usage of distinct window with scenedelegate can be checkout for e-commerce apps... Or am I missing someting? :D – lyzkov Jun 11 '19 at 12:31
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  @lyzkov I think the word "scenes" may have confused you. This change divides the code from the app delegate that needs to be run with each instance from code that should only be run once for all instances. It's really that simple. – Abandoned Cart Jun 11 '19 at 14:50
• Where should user authentication logic go? Per statement about "Set up any data that you need for the duration of the app", auth should go into AppDelegate. But initial app entry logic only works if it's in SceneDelegate.scene, so I have to listen for user auth state in SceneDelegate as well – xiaolingxiao Apr 14 '20 at 13:51 

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Multiplatform

In addition to the answer of Abandoned Cart, Since Xcode 11, You have a new option called Multiplatform for choosing as a starting template. That's where you will only see a file contains:

@main
struct MyMultiplatformApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

This is how the system knows where to start the code with @main (in Swift 5.3) and it contains WindowGroup that manages multiple windows of your app in all apple platforms. So you don't need the be worry about SceneDelegate and AppDelegate anymore.

If you need it to be like the old app delegate, for example when you want to use its methods, You should subscribe for corresponding notifications or use the UIAppDelegateAdapter wrapper as I described here