Prevent LaunchedEffect from re-running on configuration change-CodePudding

I want to run the code only once when the composable is loaded. So I am using LaunchedEffect with key as true to achieve this.

LaunchedEffect(true) {
    // do API call
}

This code is working fine but whenever there is any configuration change like screen rotation this code is executed again. How can I prevent it from running again in case of configuration change?

CodePudding user response：

The simplest solution is to store information about whether you made an API call with rememberSaveable: it will live when the configuration changes.

var initialApiCalled by rememberSaveable { mutableStateOf(false) }
if (!initialApiCalled) {
    LaunchedEffect(Unit) {
        // do API call
        initialApiCalled = false
    }
}

The disadvantage of this solution is that if the configuration changes before the API call is completed, the LaunchedEffect coroutine will be cancelled, as will your API call.

The cleanest solution is to use a view model, and execute the API call inside init:

class ScreenViewModel: ViewModel() {
    init {
        viewModelScope.launch {
            // do API call
        }
    }
}

@Composable
fun Screen(viewModel: ScreenViewModel = viewModel()) {
    
}

Passing view model like this, as a parameter, is recommended by official documentation. In the prod code you don't need to pass any parameter to this view, just call it like Screen(): the view model will be created by default viewModel() parameter. It is moved to the parameter for test/preview capability as shown in this answer.

CodePudding user response：

I assume the best way is to use the .also on the livedata/stateflow lazy creation so that you do guarantee as long as the view model is alive, the loadState is called only one time, and also guarantee the service itself is not called unless someone is listening to it. Then you listen to the state from the viewmodel, and no need to call anything api call from launched effect, also your code will be reacting to specic state.

Here is a code example

class MyViewModel : ViewModel() {
private val uiScreenState: : MutableStateFlow<WhatEverState> =
    MutableStateFlow(WhatEverIntialState).also {
        loadState()
    }

fun loadState(): StateFlow<WhatEverState>> {
    return users
}

private fun loadUsers() {
    // Do an asynchronous operation to fetch users.
}
}

When using this code, you do not have to call loadstate at all in the activity, you just listen to the observer.

You may check the below code for the listening

class MyFragment : Fragment {
override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View {
    return ComposeView(requireContext()).apply {
        setContent {
            StartingComposeTheme {
                Box(modifier = Modifier.fillMaxSize()) {
                    val state by viewModel.uiScreenState.collectAsState()
                    when (state) {
                        //do something
                    }
                }
            }
        }
    }
}

}}