Example main.swift

EntryPoint.swift

import Foundation
import SwiftUI

let isUITesting = /* your UI test detection here */

@main
struct EntryPoint {
    static func main() {
        if isUITesting {
            UITestApp.main()
        } else if NSClassFromString("XCTestCase") == nil {
            MyApp.main()
        } else {
            TestApp.main()
        }
    }
}

struct MyApp: App {
    var body: some Scene {
        WindowGroup { 
            //.. app here
        }
    }
}

struct UITestApp: App {
    var body: some Scene {
        WindowGroup {
            //.. prod app bootstrapped for UI tests 
        }
    }
}

struct TestApp: App {
    var body: some Scene {
        WindowGroup { Text("Running tests...") }
    }
}

main.swift

NSSetUncaughtExceptionHandler { print("🧨", $0) }

let isUITesting = /* your UI test detection here */
let isUnitTesting = NSClassFromString("XCTest") != nil
let noTests = !isUnitTesting && !isUITesting

let delegate = isUITesting
    ? NSStringFromClass(UITestAppDelegate.self)
    : noTests ? NSStringFromClass(AppDelegate.self)
    : nil

print("🚀 Using AppDelegate:", delegate as AnyObject)

let argc = CommandLine.argc
let argv = UnsafeMutableRawPointer(CommandLine.unsafeArgv)
    .bindMemory(to: UnsafeMutablePointer<Int8>?.self, capacity: Int(CommandLine.argc))

UIApplicationMain(argc, argv, nil, delegate)

The last step is to remove @UIApplicationMain from your existing application delegate

The file needs to be called main.swift if you are using the UIApplicationMain approach. The EntryPoint approach can be anywhere as it will be found via the @main attribute.