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In Jetpack Compose, a modern UI toolkit for Android, side effects are ways to handle changes outside the UI hierarchy that need to be reflected in your Compose functions or to ensure certain actions take place as a result of state changes. Side effects help bridge the gap between the declarative nature of Compose and the imperative interactions that might be needed, such as updating a database, logging, or performing a one-time action.

Key Side Effect APIs in Jetpack Compose

Here’s a breakdown of the main side effect functions provided by Compose and how they are typically used:

1.SideEffect
What is it?

  • SideEffect is used to run code whenever the Composable function is re-composed. It ensures that code inside it will always run after the Composable function has finished rendering.

Use Case:

  • Logging or reporting analytics.
  • You want to perform some actions that are related to the UI but don’t directly affect its state.
@Composable
fun ExampleSideEffect(name: String) {
    Text("Hello, $name")
    SideEffect {
        Log.d("ExampleSideEffect", "Composed with name: $name")
    }
}

– This will log a message every time the name parameter changes, which means that the ExampleSideEffect gets recomposed.

2. LaunchedEffect

What is it?

  • LaunchedEffect is used for running suspend functions (coroutines) when a particular key changes. It runs its block of code when the Composable enters the composition and again when its keys change.
  • It cancels any previous coroutine if the key changes.

Use Case:

  • Fetching data from a network or database when a particular state changes.
  • Showing a one-time toast message when the UI is first composed.

Example:

@Composable
fun ExampleLaunchedEffect(userId: String) {
    var userName by remember { mutableStateOf("") }

    LaunchedEffect(userId) {
        // Fetch user data when userId changes.
        userName = fetchUserName(userId)
    }

    Text("User name is $userName")
}

In this example, whenever the userId changes, LaunchedEffect fetches the user name.

3. rememberUpdatedState

What is it?

  • This is used when you want a stable reference to some changing state within a side effect.
  • It ensures that the latest value is available for a side effect while keeping the side effect stable.

Use Case:

  • You are launching a coroutine or a callback, but you want the latest state to be used within that coroutine.

Example:

@Composable
fun ExampleRememberUpdatedState(onTimeout: () -> Unit) {
    val currentOnTimeout by rememberUpdatedState(onTimeout)

    LaunchedEffect(Unit) {
        delay(5000)
        currentOnTimeout() // Uses the latest version of onTimeout.
    }
}

This ensures that if onTimeout changes, the LaunchedEffect will still call the latest onTimeout even though it might have been launched with an older version.

4. DisposableEffect

What is it?

  • DisposableEffect is used when you need to perform cleanup actions when a Composable leaves the composition.
  • It is similar to LaunchedEffect, but it provides a Disposable function for cleanup when the effect should be disposed of.

Use Case:

  • Starting and stopping a listener, such as a location listener or a sensor.
  • Cleaning up resources, like closing a database connection or canceling a network request.

Example:

@Composable
fun ExampleDisposableEffect() {
    DisposableEffect(Unit) {
        val listener = MyListener()
        listener.startListening()

        onDispose {
            listener.stopListening() // Clean up when this Composable leaves the composition.
        }
    }

    Text("Listening for updates...")
}

This starts `MyListener` when the `Composable` enters the composition and stops it when the `Composable` leaves the composition.

5. produceState

What is it?

  • produceState is used to convert non-Compose state into State that can be used in Compose. It launches a coroutine and updates its State over time.

Use Case:

  • Fetching data from a repository and exposing it as a State that Composable can observe.

Example:

@Composable
fun ExampleProduceState(userId: String): State<String> {
    return produceState(initialValue = "Loading...", userId) {
        value = fetchUserName(userId)
    }
}

This creates a state that initially holds "Loading…” and updates with the user’s name once it’s fetched.

6. snapshotFlow

What is it?

  • snapshotFlow is used when you want to observe changes to a State and convert them into a Flow.

Use Case:

  • Reacting to changes in a State variable and processing them in a coroutine.

Example

@Composable
fun ExampleSnapshotFlow(counter: State<Int>) {
    LaunchedEffect(Unit) {
        snapshotFlow { counter.value }
            .collect { newValue ->
                Log.d("ExampleSnapshotFlow", "Counter changed to $newValue")
            }
    }
}

Here, any changes to counter will be collected and logged.

7. rememberCoroutineScope

What is it?

  • rememberCoroutineScope is used to create a CoroutineScope tied to the lifecycle of a Composable. It provides a CoroutineScope that can be used to launch coroutines for background work, such as network requests or animations, from inside a Composable.
  • This scope is cancelled automatically when the Composable leaves the composition, so you don’t need to manage its lifecycle manually.

Use Case:

  • Triggering an asynchronous action when the user interacts with the UI, such as starting an animation or fetching data on button clicks.

Example

@Composable
fun ExampleRememberCoroutineScope() {
    val coroutineScope = rememberCoroutineScope()
    var text by remember { mutableStateOf("Click to load") }

    Button(onClick = {
        coroutineScope.launch {
            text = loadDataFromNetwork()
        }
    }) {
        Text(text)
    }
}

suspend fun loadDataFromNetwork(): String {
    delay(2000) // Simulate a network delay
    return "Data loaded"
}

In this example, rememberCoroutineScope allows us to launch a coroutine when the button is clicked, making the UI reactive to the asynchronous data loading without needing to manage the scope manually.

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8. rememberUpdatedState

What is it?

  • rememberUpdatedState provides a way to remember the most recent value of a state without triggering recomposition. It’s particularly useful in LaunchedEffect or DisposableEffect to ensure that these side effects always have access to the latest state values even when they aren’t recomposed themselves.
  • It ensures that a reference used inside a side effect block (e.g., a coroutine or listener) always points to the updated value of the variable.

Use Case:

  • When you have a long-running coroutine or callback and you want to ensure that it always refers to the latest state or value without restarting the coroutine.

Example:

@Composable
fun ExampleRememberUpdatedState(onTimeout: () -> Unit) {
    val currentOnTimeout by rememberUpdatedState(onTimeout)

    LaunchedEffect(Unit) {
        delay(5000)
        currentOnTimeout() // Uses the latest version of onTimeout.
    }
}

Here, even if onTimeout changes while the coroutine is still waiting during the delaycurrentOnTimeout will always refer to the latest version of onTimeout. This is useful in scenarios where you have changing callbacks but don’t want to restart the side effect every time the callback changes.

9. derivedStateOf

What is it?

  • derivedStateOf is used to create a State that is derived from other states, helping to optimize recompositions. It wraps a calculation or transformation of a state, and the derived state only gets recomposed when its dependencies change.
  • This can reduce unnecessary recompositions by ensuring that the derivedState only changes when the underlying state it depends on changes.

Use Case:

  • Calculating values that are derived from other states, like filtering a list based on some condition or computing a derived value from an existing state.

Example:

@Composable
fun ExampleDerivedStateOf() {
    var inputText by remember { mutableStateOf("") }
    val isInputValid by derivedStateOf {
        inputText.length > 3
    }

    Column {
        TextField(
            value = inputText,
            onValueChange = { inputText = it },
            label = { Text("Enter text") }
        )

        if (isInputValid) {
            Text("Input is valid")
        } else {
            Text("Input is too short")
        }
    }
}

In this example, isInputValid is derived from inputText using derivedStateOf. This means that isInputValid will only update when inputText changes, making the recomposition more efficient. It won’t trigger unnecessary recompositions if inputText stays the same length.

Real-Time Example Using Multiple Side Effect APIs

Imagine a chat application where we need to fetch messages when a user logs in and keep listening for new messages. We also need to clean up resources when the user logs out.

@Composable
fun ChatScreen(userId: String, onLogout: () -> Unit) {
    var messages by remember { mutableStateOf(listOf<Message>()) }
    var isListening by remember { mutableStateOf(false) }

    // Fetch messages whenever the userId changes
    LaunchedEffect(userId) {
        messages = fetchChatMessages(userId)
    }

    // Start listening to new messages when we enter composition.
    DisposableEffect(Unit) {
        val listener = MessageListener { newMessage ->
            messages = messages + newMessage
        }
        listener.startListening()

        onDispose {
            listener.stopListening() // Stop listening when ChatScreen leaves the composition.
        }
    }

    // Log when messages change using SideEffect.
    SideEffect {
        Log.d("ChatScreen", "New messages count: ${messages.size}")
    }

    // Display messages.
    LazyColumn {
        items(messages) { message ->
            Text(text = message.content)
        }
    }
}

In this example:

  • LaunchedEffect fetches the initial messages when the userId changes.
  • DisposableEffect starts a listener when ChatScreen enters the composition and stops it when it leaves.
  • SideEffect logs each time the messages change.

This approach ensures that each task (fetching, listening, logging) is handled appropriately based on the lifecycle of the `ChatScreen` Composable, making the code cleaner and more maintainable.

Conclusion

Side effects in Jetpack Compose provide a powerful way to interact with the outside world and handle actions that are not directly related to the UI hierarchy. By understanding and effectively using the various side effect APIs, you can create more robust and responsive Compose applications.

Key takeaways:

  • Side effects bridge the gap between declarative UI and imperative actions.
  • Choose the right API based on your specific requirements (e.g., LaunchedEffect for coroutines, DisposableEffect for cleanup, SideEffect for general actions).
  • Leverage rememberUpdatedState, derivedStateOf, and snapshotFlow for more efficient and optimized state management.
  • Consider the lifecycle of your Composable when using side effects.

By mastering side effects, you can create Compose applications that not only look great but also perform efficiently and interact seamlessly with the environment.

I hope this article was helpful to you. You can write me back at karishma.agr1996@gmail.com if you want me to improve something in upcoming articles. Your feedback is valuable.

Also, follow me on Medium and Linkedin

Your claps are appreciated to help others find this article 😃 .

This article is previously published on proandroiddev.com

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