How do you configure Firebase for multiple environments in a SwiftUI app using compiler flags?

Select the correct GoogleService-Info.plist at launch based on the active build configuration:

import SwiftUI
import FirebaseCore

enum AppEnvironment { case dev, sit, uat, prod }

@main
struct YourApp: App {

    init() {
        configureFirebase()
    }

    var body: some Scene {
        WindowGroup {
            NavigationStack { RootView() }
        }
    }

    private var currentEnvironment: AppEnvironment {
        #if Dev
        return .dev
        #elseif SIT
        return .sit
        #elseif UAT
        return .uat
        #else
        return .prod
        #endif
    }

    private func configureFirebase() {
        let plistName: String
        switch currentEnvironment {
        case .dev:  plistName = "GoogleService-Info-Dev"
        case .sit:  plistName = "GoogleService-Info-SIT"
        case .uat:  plistName = "GoogleService-Info-UAT"
        case .prod: plistName = "GoogleService-Info"
        }

        guard let filePath = Bundle.main.path(forResource: plistName, ofType: "plist") else {
            fatalError("❌ Could not find plist file: \(plistName).plist")
        }
        guard let options = FirebaseOptions(contentsOfFile: filePath) else {
            fatalError("❌ Could not load Firebase options from: \(plistName).plist")
        }

        FirebaseApp.configure(options: options)
        print("✅ Firebase configured for \(currentEnvironment) — Bundle ID: \(options.bundleID)")
    }
}

The fatalError guards are intentional — a misconfigured Firebase setup should fail loudly at launch during development, not silently at runtime.