SharedFlow: A Hot Flow for Broadcasting Events
A SharedFlow is a hot stream that broadcasts values to all subscribers, like a live TV channel. It's ideal for one-to-many events, like UI updates or notifications. The main footgun: it never completes, so collect will suspend forever.
Why it exists
Sometimes you need to send a single stream of events to multiple parts of your app at the same time. A regular, "cold" Flow is unsuitable because it creates a new, independent execution for each collector. SharedFlow solves this by providing a single source that broadcasts values to all current and future collectors.
The mental model
Think of a SharedFlow as a live TV broadcast. The station (the flow) is always on and emitting a signal, regardless of how many people are watching. Subscribers (collectors) can tune in at any time and start receiving the live signal. If the flow is configured with a replay cache, a new subscriber is like rewinding a DVR to see the last few moments before catching up to the live broadcast.
How it works
A SharedFlow is created using a MutableSharedFlow() constructor or by applying the shareIn operator to a cold Flow. The common pattern is to keep a private MutableSharedFlow for emitting values and expose a public, read-only SharedFlow for collection. You can configure its behavior on creation. The replay parameter sets how many recent items are cached and sent to new subscribers. The extraBufferCapacity adds a buffer to handle slow subscribers without blocking the emitter. If the buffer fills, the onBufferOverflow strategy determines the outcome: SUSPEND (the default), DROP_OLDEST, or DROP_LATEST.
When to use it
Use SharedFlow for one-to-many event distribution where multiple, independent parts of your app need to react to the same event stream. This is perfect for broadcasting UI events (like a logout signal), receiving real-time server updates, or implementing a simple in-memory event bus for your application.
When not to use it
If you only need to represent a single, observable piece of state, use StateFlow. It's a specialized SharedFlow designed for that exact purpose. If each consumer needs its own separate, fresh execution of the data stream from the beginning, use a standard cold Flow.
One canonical example
A common use case is an event bus. You can create a class with a private MutableSharedFlow to control emissions and a public SharedFlow for subscribers. For example: private val _events = MutableSharedFlow<Event>() and val events = _events.asSharedFlow(). A public function like produceEvent(event) would then call _events.emit(event) to broadcast the event to all listeners.
Interview question
Which scenario is the most appropriate use case for a Kotlin SharedFlow?
- a.Executing a complex calculation once and providing its result to a single consumer.
- b.Broadcasting real-time user authentication status updates to all active UI components.Correct
- c.Emitting a sequence of database records, where each subscriber needs to process the entire sequence from the start.
- d.Managing a single, mutable configuration object whose latest value is always available to new observers.
Why? this is the answer
SharedFlow is designed for one-to-many event distribution, like broadcasting UI events or real-time updates to multiple parts of an application, as described in option B. Option D describes the use case for StateFlow, which is specialized for a single, observable piece of state.
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