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 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.