Jetpack Compose - How to trigger an event when a screen is composed

Question

My application uses OpenID to authenticate the user.

The first page is more of a splash screen that hands the user to a webpage to authorise if needed or just perform a background refresh of the token the navigate to the main screen.

I'm unsure of how to start the authentication flow without a button click

@Composable
fun LoginScreen(viewModel: LoginViewModel) {
    val ctx = LocalContext.current
    AppTheme {
        Screen()
    }
    viewModel.performLogin(ctx)
}       

Doing the above works, but it then gets called again when the app navigates to the main screen.

fun loginComplete(navController: NavHostController) {
    navController.navigate("main")
}

@Composable
fun MyApp(viewModel: LoginViewModel) {
    val navController = rememberNavController()
    viewModel.setOnLoginCompete(navController, ::loginComplete)
    NavHost(navController, startDestination = "login") {
        composable(route = "login") {
            LoginScreen(viewModel)
        }
        composable(route = "main") {
            MainScreen()
        }
    }
}

I don't think I'm supposed to call the performLogin function like I am in a Composable function, but I can't see another way. What am I missing?

Answer 1

You can tie your flow to lifecycle callbacks. You can create utility composable for handling lifecycle events.

@Composable
fun OnLifecycleEvent(onEvent: (owner: LifecycleOwner, event:Lifecycle.Event) -> Unit) {
    val eventHandler = rememberUpdatedState(onEvent)
    val lifecycleOwner = rememberUpdatedState(LocalLifecycleOwner.current)

    DisposableEffect(lifecycleOwner.value) {
        val lifecycle = lifecycleOwner.value.lifecycle
        val observer = LifecycleEventObserver { owner, event ->
            eventHandler.value(owner, event)
        }

        lifecycle.addObserver(observer)
        onDispose {
            lifecycle.removeObserver(observer)
        }
    }
}

And the use it like this, if you want to start logging flow when your app comes to the foreground:

@Composable
fun MyApp(viewModel: LoginViewModel) {

...
    OnLifecycleEvent { owner, event ->
        when (event) {
            Lifecycle.Event.ON_RESUME -> { viewModel.performLogin(ctx) }
            else -> { ... }
       }
    }
}

I actually used this question and its answer as the source