Difference between SceneDelegate and AppDelegate

Question

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

What are the differences between them?

For example be

Answer 1

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

Answer 2

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