Introduction
When working with asynchronous programming and reactive streams in Kotlin, you often need a way to manage state and handle events that are emitted over time. StateFlow and SharedFlow are two important concepts in Kotlin’s Flow API that allow you to manage state and events in a reactive and efficient manner.
In this post, we will: ✅ Understand the differences between StateFlow and SharedFlow
✅ Learn how to use StateFlow to manage state
✅ Master SharedFlow for event handling
✅ Explore best practices for using both in your Kotlin applications
By the end of this post, you’ll be well-equipped to manage state and events in your Kotlin applications with StateFlow and SharedFlow.
1. What is StateFlow?
StateFlow is a state-holder observable flow that is used to represent a state that can change over time. Unlike regular Flows, StateFlow always has a current value and can be observed for changes. It’s designed to be used for managing UI-related states in Android and other applications that require real-time state management.
Key Features of StateFlow:
- State-backed: It always holds a current state, which can be accessed at any time.
- Hot stream: StateFlow is a hot stream, meaning that it starts emitting values as soon as it is created, and the latest value is always available.
- Conflation: If multiple updates are emitted in a short time, only the latest one will be delivered.
1.1. Creating and Using StateFlow
To create a StateFlow, use the MutableStateFlow class. MutableStateFlow allows you to update the state, while StateFlow provides a read-only version that can be observed.
Example: Using StateFlow
// Creating a StateFlow with an initial value
val _stateFlow = MutableStateFlow("Initial State")
val stateFlow: StateFlow<String> = _stateFlow
// Collecting state changes
stateFlow.collect { state ->
println("State has changed: $state")
}
// Updating the state
_stateFlow.value = "Updated State"
// Output: State has changed: Updated State
In this example, _stateFlow holds the current state, and when it’s updated with a new value, the collector immediately receives the new state.
1.2. Use Case – Managing UI State
StateFlow is ideal for managing UI-related states in applications. In Android, it can be used with ViewModel to represent the state of a UI component.
Example: StateFlow in ViewModel (Android)
class MyViewModel : ViewModel() {
private val _state = MutableStateFlow("Loading")
val state: StateFlow<String> = _state
fun updateState(newState: String) {
_state.value = newState
}
}
In this example, the state is observed in the UI, and when the updateState function is called, the UI reflects the changes.
2. What is SharedFlow?
SharedFlow is a more general-purpose event stream that allows multiple consumers to collect emitted events. Unlike StateFlow, which holds the current value, SharedFlow is intended for broadcasting events that can be observed by multiple subscribers. SharedFlow is used when you need to broadcast one-time events like button clicks, network responses, or navigation events.
Key Features of SharedFlow:
- Multiple collectors: SharedFlow allows multiple collectors to observe emitted values.
- No state: Unlike StateFlow, it does not retain the latest value and only emits events.
- Configurable replay: You can configure the number of last emitted events to be replayed to new collectors.
2.1. Creating and Using SharedFlow
SharedFlow is created using MutableSharedFlow for emitting events and SharedFlow for observing them.
Example: Using SharedFlow
// Creating a SharedFlow with a buffer of 1 for replaying the last emitted value
val sharedFlow = MutableSharedFlow<Int>(replay = 1)
sharedFlow.emit(1) // Emit an event
sharedFlow.emit(2) // Emit another event
// Collecting events from SharedFlow
sharedFlow.collect { event ->
println("Event received: $event")
}
// Output: Event received: 2
In this example, only the last emitted value is retained in the SharedFlow and will be sent to new collectors.
2.2. Use Case – Broadcasting Events
SharedFlow is useful for broadcasting one-time events to multiple subscribers. For example, in an Android app, you can use SharedFlow to emit navigation events or success/error messages that multiple UI components or listeners need to observe.
Example: SharedFlow for Event Broadcasting
class MyViewModel : ViewModel() {
private val _eventFlow = MutableSharedFlow<String>()
val eventFlow: SharedFlow<String> = _eventFlow
fun triggerEvent(event: String) {
viewModelScope.launch {
_eventFlow.emit(event) // Emit event
}
}
}
In this example, the eventFlow can be observed by multiple UI components to react to events like button clicks or navigation commands.
3. Key Differences Between StateFlow and SharedFlow
| Feature | StateFlow | SharedFlow |
|---|---|---|
| State | Holds and emits the current state value. | Does not retain state, emits events. |
| Use Case | Ideal for managing and observing state. | Ideal for emitting one-time events. |
| Replay | No replay functionality. | Supports configurable replay of last N events. |
| Collectors | A single collector usually observes state. | Multiple collectors can observe events. |
3.1. When to Use StateFlow
- Managing UI state in Android (e.g., loading state, result state).
- Storing and observing single values that can change over time.
3.2. When to Use SharedFlow
- Event broadcasting like button clicks, error messages, or navigation.
- One-time events that need to be shared across multiple consumers.
4. Best Practices for Using StateFlow and SharedFlow
Best Practice 1: Use StateFlow for Single Source of Truth
When managing state, always use StateFlow as the single source of truth. It ensures that your app’s UI is always consistent with the current state and avoids unnecessary UI updates.
// Store UI state in StateFlow
val stateFlow: StateFlow<State> = _stateFlow
// Collect and update the UI based on state
stateFlow.collect { state ->
// Update UI based on state
}
Best Practice 2: Use SharedFlow for Events and Notifications
Use SharedFlow to send one-time events, especially when you want multiple consumers to respond to the same event.
// Use SharedFlow to emit events to listeners
sharedFlow.emit("Event Triggered")
Best Practice 3: Use replay with SharedFlow for Late Collectors
If your event has to be replayed for new collectors, configure the replay parameter to specify how many events should be cached and sent to new collectors.
val sharedFlow = MutableSharedFlow<String>(replay = 1) // Replay last event to new collectors
Best Practice 4: Avoid StateFlow and SharedFlow Overuse
Use StateFlow and SharedFlow only when necessary. Overusing them can lead to unnecessary complexity in your code. Use them for managing state and events that need to be shared or observed by multiple consumers.
5. Conclusion
In this post, we explored the powerful Kotlin Flow API, focusing on StateFlow and SharedFlow:
- StateFlow is ideal for managing state in reactive applications, especially when dealing with UI state in Android.
- SharedFlow is perfect for handling event-based communication, where you need to broadcast one-time events to multiple consumers.
By understanding how to use StateFlow and SharedFlow, you can manage state and events efficiently in your Kotlin applications.
🎯 Next Post: Kotlin Flow – Best Practices for Efficient Flow Management
