Get data from network or database on entry/load of a Jetpack Compose based screen in Android

Question

I am new to Jetpack Compose and Kotlin, but I am trying to develop a mobile app using recently stable Jetpack Compose v1.0.0.0

I am using the NavController for navigation and my screens are made using composable functions.

Because my screen itself is a composable function, I am not able to understand how I can load some API/DB data on composable function entry.

I tried to read about LaunchedEffect, but don't know if that is the right approach and it also appeared complex to implement.

Here is the sample code:

1. DemoScreen.kt: Consider this as a screen that is my startDestination in NavHost.

   @Composable fun DemoScreen(navController: NavController) {

    var json = "Placeholder content, to be updated on screen load and refreshed every x seconds..."
   
    //1. Make network or DB call as soon as entering this composable and store in the "json" variable
    //makeDataLoadCallOnEntry() //something like this, and show processing indicator...
   
    //2. And then keep refreshing/updating the content of the "json" variable by calling this
    //function in a timer
   
   
   
    Box(modifier = Modifier
        .fillMaxWidth()
        .fillMaxHeight()
    ){
        //region #Region: Main content area
        Column(
            horizontalAlignment = Alignment.CenterHorizontally,
            verticalArrangement = Arrangement.Center,
            modifier = Modifier
                .fillMaxWidth()
                .padding(horizontal = 0.dp)
            //.padding(top = 120.dp)
        ) {
   
            TextField(
                value = json,
                onValueChange = { json = it },
                singleLine = false,
                modifier = Modifier
                    .fillMaxWidth()
                    .padding(5.dp)
                    .height(500.dp),
                maxLines = 10,
            )
            val coroutineScope = rememberCoroutineScope()
   
            Button(
                onClick = {
                    coroutineScope.launch {
                        withContext(Dispatchers.IO) {
                            try {
                                //do something in button click
   
                            } catch (e: Exception) {
                                // handle exception
                                Log.d("button", e.message.toString())
                            } finally {
                                //do something
                                Log.d("button", "in finally")
                            }
                        }
   
                    }
                },
                modifier = Modifier
                    .fillMaxWidth()
                    .padding(5.dp)
                    .height(100.dp)
            ) {
                Text(text = "Demo Button")
            }
        }
        //endregion
    }
   

   }

2. And my NavHost looks like this (this is working fine):

   @Composable fun NavigationMaster() { val navController = rememberNavController() val defaultScreen = Screens.DemoScreen.route

    NavHost(
        navController = navController,
        startDestination = defaultScreen){
   
        composable(
            route = Screens.DemoScreen.route
        ){
            DemoScreen(navController = navController)
        }
   
   
    }
   

   }

3. How can I call a function on my DemoScreen composable entry to have the data from my API prior to the screen is rendered.

4. If I can get the above, then I think I should be able to implement the timer part to automatically refresh the data from API using a loop and/or timer.

Here is a demo screenshot of how this screen currently looks (the Demo Button does not do anything, please ignore it):

Answer 1

You can do it using view model, something like this:

@Composable
fun DemoScreen() {
    val viewModel = viewModel<DemoScreenViewModel>()
    when (val state = viewModel.state.collectAsState().value) {
        DemoScreenViewModel.State.Loading -> {
            Text("Loading")
        }
        is DemoScreenViewModel.State.Data -> {
            DemoScreenContent(state.data)
        }
    }
}

class DemoScreenViewModel : ViewModel() {
    sealed class State {
        object Loading: State()
        data class Data(val data: String): State()
    }

    private var _state = MutableStateFlow<State>(State.Loading)
    val state = _state.asStateFlow()

    init {
        viewModelScope.launch {
            while (isActive) {
                val data = makeDataLoadCallOnEntry()
                _state.value = State.Data(data)
                // wait one minute and repeat your request
                delay(60 * 1000L)
            }
        }
    }

    suspend fun makeDataLoadCallOnEntry(): String {
        delay(1000)
        return "Hello world"
    }
}