SharedFlow in Jetpack Compose: When and How to Use It

Jetpack Compose, Android’s modern toolkit for building native UIs, continues to redefine mobile app development. Among the many tools and APIs available for managing state and reactive flows in Compose, SharedFlow stands out as a versatile solution for handling events and sharing data efficiently. This article dives deep into the SharedFlow API, exploring when and how to use it effectively in Jetpack Compose applications.

What is SharedFlow?

SharedFlow is part of Kotlin’s Flow API, introduced as a cold, reactive stream for asynchronous data handling. While StateFlow is great for state-driven data, SharedFlow is designed for events that can be shared across multiple collectors. It allows you to:

  • Emit events that can be observed by multiple subscribers.

  • Replay a specified number of past emissions for new collectors.

  • Maintain control over event buffering and emission behavior.

Key Features of SharedFlow:

  • Hot Flow: Unlike Flow, which is cold and starts emitting data only when collected, SharedFlow is always active and emits data to active collectors.

  • Replay: Replays a specified number of past values for new collectors.

  • Buffering: Configurable buffering to handle scenarios where emissions exceed the consumption rate.

  • Emit-on-demand: Ensures that collectors don’t miss emissions even if they subscribe late.

These characteristics make SharedFlow a powerful tool for broadcasting events like navigation commands, user interactions, or real-time updates.

When to Use SharedFlow in Jetpack Compose

SharedFlow shines in scenarios where:

  1. One-to-Many Communication: You need to broadcast events to multiple observers, such as navigation commands or UI triggers.

  2. Transient Data: Events should not be retained after being handled (e.g., Toast messages or Snackbar events).

  3. Event Replay: New subscribers need access to recent emissions (e.g., restoring UI state after a configuration change).

  4. Non-State Data: Unlike StateFlow, SharedFlow is suitable for events that don’t represent a persistent state.

Examples of SharedFlow Use Cases:

  • Emitting Snackbar messages.

  • Triggering navigation events.

  • Broadcasting real-time updates (e.g., stock price updates).

  • Handling one-time actions like permissions requests or dialogs.

Setting Up SharedFlow in Jetpack Compose

Here’s a step-by-step guide to implementing SharedFlow in a Jetpack Compose project:

Step 1: Define a SharedFlow

You’ll typically define a SharedFlow in a ViewModel to share events between the business logic and UI layers:

class MyViewModel : ViewModel() {

    private val _eventFlow = MutableSharedFlow<String>()
    val eventFlow: SharedFlow<String> = _eventFlow

    fun sendEvent(message: String) {
        viewModelScope.launch {
            _eventFlow.emit(message)
        }
    }
}

  • Use MutableSharedFlow to emit events internally.

  • Expose the SharedFlow to make it immutable for consumers.

Step 2: Collect SharedFlow in the Composable

Use the LaunchedEffect API in Jetpack Compose to collect SharedFlow events:

@Composable
fun MyScreen(viewModel: MyViewModel) {
    val coroutineScope = rememberCoroutineScope()

    LaunchedEffect(Unit) {
        viewModel.eventFlow.collect { event ->
            // Handle the event, e.g., show a Toast
            Toast.makeText(LocalContext.current, event, Toast.LENGTH_SHORT).show()
        }
    }

    // Rest of your UI code
    Button(onClick = { viewModel.sendEvent("Button Clicked!") }) {
        Text("Click Me")
    }
}

Step 3: Configure SharedFlow Parameters (Optional)

Customize SharedFlow behavior by setting parameters like replay and extraBufferCapacity:

private val _eventFlow = MutableSharedFlow<String>(
    replay = 1, // Number of events to replay for new collectors
    extraBufferCapacity = 2, // Buffer size beyond replayed events
    onBufferOverflow = BufferOverflow.DROP_OLDEST // Handle overflow
)

Advanced Usage and Best Practices

1. Combining SharedFlow with StateFlow

While StateFlow is ideal for managing UI state, SharedFlow complements it by handling transient, one-time events. Use them together:

  • StateFlow: For persistent UI state.

  • SharedFlow: For transient events like navigation or notifications.

class MyViewModel : ViewModel() {
    private val _stateFlow = MutableStateFlow<MyState>(MyState.Initial)
    val stateFlow: StateFlow<MyState> = _stateFlow

    private val _eventFlow = MutableSharedFlow<MyEvent>()
    val eventFlow: SharedFlow<MyEvent> = _eventFlow

    fun triggerEvent(event: MyEvent) {
        viewModelScope.launch { _eventFlow.emit(event) }
    }
}

2. Handling SharedFlow in Lifecycle-Aware Manner

When collecting SharedFlow in Compose, ensure lifecycle awareness using LaunchedEffect or DisposableEffect to avoid memory leaks and redundant collectors.

3. Managing Backpressure

Choose appropriate buffering strategies to manage backpressure effectively, especially in high-frequency emission scenarios. The BufferOverflow strategies include:

  • SUSPEND: Suspend the emitter when the buffer is full.

  • DROP_OLDEST: Drop the oldest buffered item.

  • DROP_LATEST: Drop the newest item.

4. Testing SharedFlow

Write unit tests for SharedFlow using Kotlin’s coroutine test framework:

@Test
fun testSharedFlow() = runTest {
    val sharedFlow = MutableSharedFlow<String>()
    val results = mutableListOf<String>()

    val job = launch {
        sharedFlow.collect { results.add(it) }
    }

    sharedFlow.emit("Test Event")
    assertEquals(listOf("Test Event"), results)
    job.cancel()
}

Common Pitfalls and How to Avoid Them

1. Overusing SharedFlow

Avoid using SharedFlow for scenarios better suited to StateFlow or other reactive tools. For example, persistent UI states are better managed with StateFlow.

2. Ignoring Buffer Configuration

Improper buffer settings can lead to dropped or delayed events. Always configure replay and buffer capacity thoughtfully.

3. Mismanaging Collectors

Ensure collectors are properly cleaned up to prevent memory leaks, especially when using SharedFlow with non-Compose components.

Conclusion

SharedFlow is a robust solution for handling events in Jetpack Compose applications, bridging the gap between reactive streams and UI interactions. By understanding its capabilities and best practices, you can build scalable, responsive, and event-driven applications.

Whether you’re managing navigation events, broadcasting updates, or handling transient data, SharedFlow equips you with the tools to tackle advanced scenarios in modern Android development.

Mastering SharedFlow alongside StateFlow will significantly enhance your ability to manage state and events in Jetpack Compose, ensuring a seamless and efficient user experience.