Mastering One-Time Launch Functions in Jetpack Compose Screens

When developing modern Android applications with Jetpack Compose, understanding how to handle one-time launch functions is crucial for creating efficient and bug-free UI logic. A common challenge developers face is ensuring that specific actions—like API calls or animations—are executed only once when a screen appears. Jetpack Compose offers powerful tools like LaunchedEffect to tackle this issue effectively.

What Is a One-Time Launch Function?

A one-time launch function refers to any operation that should be triggered exactly once during the lifecycle of a composable screen. These functions are often used to initialize data, fetch resources from an API, or perform navigation transitions. Because of their nature, these operations must not repeat unnecessarily as they can lead to performance issues or unexpected behavior.

How to Use LaunchedEffect for One-Time Execution

Jetpack Compose provides the LaunchedEffect composable specifically designed for launching coroutines tied to the lifecycle of a composable function . By using LaunchedEffect, you ensure that the associated code block runs only when the composable enters the composition and cancels automatically if the composable leaves the composition.

To guarantee a one-time execution, developers typically use a constant key like true, false, or Unit inside the LaunchedEffect. Here’s an example:

@Composable
fun MyScreen(viewModel: MyViewModel) {
    LaunchedEffect(true) {
        viewModel.fetchData()
    }
}

In this case, LaunchedEffect(true) ensures that fetchData() is called only once when the screen is first composed . The coroutine will not restart unless the composable itself recomposes in a new scope.

Handling ViewModel Events That Should Run Once

Sometimes, your ViewModel might emit single-shot events—for instance, showing a success message after completing an API call. In such cases, it’s essential to avoid repeating side effects caused by recompositions. You can combine LaunchedEffect with StateFlow or SharedFlow in your ViewModel to ensure these events are handled correctly.

For example, if your ViewModel exposes a OneShotEvent via a SharedFlow, you can collect it inside a LaunchedEffect block with no keys, ensuring it reacts only once per emission:

@Composable
fun MyScreen(viewModel: MyViewModel) {
    val context = LocalContext.current
    LaunchedEffect(Unit) {
        viewModel.events.collect { event ->
            Toast.makeText(context, event.message, Toast.LENGTH_SHORT).show()
        }
    }
}

This approach ensures that the Toast notification is shown only once for each event emitted from the ViewModel .

Best Practices When Using LaunchedEffect

1. Use Stable Keys: To prevent unnecessary restarts, always use stable keys (such as true, false, or Unit) when you want to execute a coroutine only once.
2. Avoid Side Effects in Composition: Never place business logic directly inside the composable function body. Always defer such work to effect handlers like LaunchedEffect.
3. Cancel Coroutines Gracefully: Since LaunchedEffect is scoped to the lifecycle of the composable, it automatically cancels its coroutine when the composable exits the composition. This ensures efficient resource management .
4. Test Recomposition Behavior: Understand how LaunchedEffect behaves under different recomposition scenarios. If you change the key argument, the previous coroutine will cancel, and a new one will start .

Conclusion

Mastering one-time launch functions in Jetpack Compose is essential for building robust, maintainable apps. By leveraging LaunchedEffect and carefully choosing key arguments, you can control when and how your side effects run, preventing redundant operations and improving overall app performance. Whether initializing screen data or handling single-event triggers from a ViewModel, proper use of effect handlers ensures clean, predictable UI behavior .