Kotlin Side Effects

Jetpack Compose is a modern toolkit for building native Android UIs. It simplifies and accelerates UI development on Android with less code, powerful tools, and intuitive Kotlin APIs. One of the key concepts in Compose is managing side effects. Side effects are actions that happen outside the scope of a composable function, such as making a network request, updating a database, or navigating to a new screen.

In this blog post, we’ll dive deep into side effects in Jetpack Compose, explore the different APIs available to handle them, and provide practical examples to help you understand how to use them effectively.

What Are Side Effects in Compose?

In Compose, a side effect is any change to the state of the application that happens outside the scope of a composable function. Since composable functions are supposed to be idempotent (producing the same result for the same input), side effects can break this principle if not managed properly.

For example:

  • Launching a coroutine to fetch data.
  • Updating a shared state or database.
  • Navigating to a new screen.
  • Subscribing to a flow or observable.

Compose provides several APIs to handle side effects in a controlled and predictable way. Let’s explore them one by one.

Key Side Effect APIs in Compose

1. LaunchedEffect

LaunchedEffect is used to launch a coroutine that is tied to the lifecycle of a composable. It runs the coroutine when the composable enters the composition and cancels it when the composable leaves the composition.

Example: Fetching Data

@Composable
fun FetchDataScreen(viewModel: MyViewModel) {
    val data by viewModel.data.collectAsState()

    LaunchedEffect(Unit) {
        viewModel.fetchData()
    }

    if (data.isLoading) {
        LoadingIndicator()
    } else {
        DataList(data.items)
    }
}

In this example, LaunchedEffect is used to trigger a data fetch when the composable is first composed. The coroutine is automatically canceled if the composable is removed from the composition.

2. rememberCoroutineScope

rememberCoroutineScope provides a coroutine scope that is tied to the lifecycle of the composable. Unlike LaunchedEffect, it allows you to launch coroutines in response to user events or other triggers.

Example: Handling Button Clicks

@Composable
fun CounterScreen() {
    var count by remember { mutableStateOf(0) }
    val scope = rememberCoroutineScope()

    Column {
        Text("Count: $count")
        Button(onClick = {
            scope.launch {
                delay(1000) // Simulate a long-running task
                count++
            }
        }) {
            Text("Increment")
        }
    }
}

Here, rememberCoroutineScope is used to launch a coroutine when the button is clicked. The coroutine increments the count after a delay.

3. DisposableEffect

DisposableEffect is used for side effects that require cleanup when the composable leaves the composition. It takes a key parameter, and if the key changes, the previous effect is disposed, and a new one is created.

Example: Registering and Unregistering a Listener

@Composable
fun SensorListenerScreen(sensorManager: SensorManager) {
    DisposableEffect(Unit) {
        val listener = object : SensorEventListener {
            override fun onSensorChanged(event: SensorEvent?) {
                // Handle sensor data
            }

            override fun onAccuracyChanged(sensor: Sensor?, accuracy: Int) {}
        }

        sensorManager.registerListener(listener, sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER), SensorManager.SENSOR_DELAY_NORMAL)

        onDispose {
            sensorManager.unregisterListener(listener)
        }
    }
}

In this example, DisposableEffect is used to register a sensor listener when the composable enters the composition and unregister it when the composable leaves.

4. SideEffect

SideEffect is used to execute non-suspending side effects that should run after every successful recomposition. It is useful for updating state outside of Compose, such as logging or analytics.

Example: Logging State Changes

@Composable
fun LoggingScreen(viewModel: MyViewModel) {
    val state by viewModel.state.collectAsState()

    SideEffect {
        println("Current state: $state")
    }

    // UI content
}

Here, SideEffect is used to log the current state whenever the composable is recomposed.

5. produceState

produceState is used to convert a non-Compose state into Compose state. It launches a coroutine and produces a State object that can be observed by the UI.

Example: Fetching Data and Converting to State

@Composable
fun FetchDataScreen(viewModel: MyViewModel) {
    val dataState = produceState(initialValue = DataState.Loading, viewModel) {
        val data = viewModel.fetchData()
        value = data
    }

    when (val state = dataState.value) {
        is DataState.Loading -> LoadingIndicator()
        is DataState.Success -> DataList(state.items)
        is DataState.Error -> ErrorMessage(state.message)
    }
}

In this example, produceState is used to fetch data and convert it into a Compose state that the UI can observe.

6. derivedStateOf

derivedStateOf is used to create a state object that depends on other state objects. It recalculates its value whenever any of the dependent states change.

Example: Filtering a List

@Composable
fun FilteredListScreen(items: List<String>, filter: String) {
    val filteredItems by remember(items, filter) {
        derivedStateOf {
            items.filter { it.contains(filter, ignoreCase = true) }
        }
    }

    LazyColumn {
        items(filteredItems) { item ->
            Text(item)
        }
    }
}

Here, derivedStateOf is used to filter a list based on a search query. The filtered list is recalculated whenever the items or filter state changes.

Best Practices for Managing Side Effects

  1. Keep Side Effects Minimal: Avoid performing side effects directly inside composable functions. Use the appropriate side effect APIs instead.
  2. Use Keys Wisely: Many side effect APIs take a key parameter. Use it to control when the effect should be restarted or disposed.
  3. Clean Up Resources: Always clean up resources (e.g., listeners, subscriptions) when they are no longer needed to avoid memory leaks.
  4. Test Side Effects: Write tests to ensure that side effects behave as expected, especially when dealing with asynchronous operations.

Conclusion

Side effects are an essential part of building dynamic and interactive UIs in Jetpack Compose. By using the appropriate side effect APIs, you can ensure that your composable functions remain predictable, efficient, and easy to maintain.

Whether you’re fetching data, handling user input, or managing resources, Compose provides powerful tools to manage side effects effectively. With the examples and best practices outlined in this post, you should be well-equipped to handle side effects in your Compose-based applications.

Happy composing! 🚀