Android Developers
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Android Developers

Things to know about Flow’s shareIn and stateIn operators

The Flow.shareIn and Flow.stateIn operators convert cold flows into hot flows: they can multicast the information that comes from a cold upstream flow to multiple collectors. They’re often used to improve performance, add a buffer when collectors are not present, or even as a caching mechanism.

Note: Cold flows are created on-demand and emit data when they’re being observed. Hot flows are always active and can emit data regardless of whether or not they’re being observed.

In this blog post, you’ll become familiar with the shareIn and stateIn operators by example. You’ll learn how to configure them to perform certain use cases and avoid common pitfalls you might encounter.

The underlying flow producer

Continuing with the example from my previous blog post, the underlying flow producer that we’re using emits location updates. It’s a cold flow, as it’s implemented using a callbackFlow. Every new collector will trigger the flow producer block, and a new callback will be added to the FusedLocationProviderClient.

Let’s see how we can use the shareIn and stateIn operators to optimize the locationsSource flow for different use cases.

shareIn or stateIn?

The first topic we’ll cover is the difference between shareIn and stateIn. The shareIn operator returns a SharedFlow instance whereas stateIn returns a StateFlow.

Note: To learn more about StateFlow and SharedFlow, check out our documentation.

StateFlow is a specialized configuration of SharedFlow optimized for sharing state: the last emitted item is replayed to new collectors, and items are conflated using Any.equals. You can read more about this in the StateFlow documentation.

The main difference between these APIs is that the StateFlow interface allows you to access the last emitted value synchronously by reading its value property. That’s not the case with SharedFlow.

Improving performance

These APIs can improve performance by sharing the same instance of the flow to be observed by all collectors instead of creating new instances of the same flow on-demand.

In the following example, LocationRepository consumes the locationsSource flow exposed by the LocationDataSource and applies the shareIn operator to make everyone interested in the user’s location collect from the same instance of the flow. Only one instance of the locationsSource flow is created and shared for all collectors:

The WhileSubscribed sharing policy is used to cancel the upstream flow when there are no collectors. In this way, we avoid wasting resources when no one is interested in location updates.

Tip for Android apps! You can use WhileSubscribed(5000) most of the time to keep the upstream flow active for 5 seconds more after the disappearance of the last collector. That avoids restarting the upstream flow in certain situations such as configuration changes. This tip is especially helpful when upstream flows are expensive to create and when these operators are used in ViewModels.

Buffering events

For this example, our requirements have changed, and now we’re asked to always listen for location updates and display the last 10 locations on the screen when the app comes from the background:

We use a replay value of 10 to keep the last 10 emitted items in memory and re-emit those every time a collector observes the flow. To keep the underlying flow active all the time and emitting location updates, use the SharingStarted.Eagerly policy to listen for updates even if there are no collectors.

Caching data

Our requirements have changed again, and in this case, we don’t need to be always listening for location updates if the app is in the background. However, we need to cache the last emitted item so that the user always sees some data on the screen, even if stale, while getting the current location. For this case, we can use the stateIn operator.

Flow.stateIn caches and replays the last emitted item to a new collector.

WATCH OUT! Do not create new instances on each function call

NEVER use shareIn or stateIn to create a new flow that’s returned when calling a function. That’d create a new SharedFlow or StateFlow on each function invocation that will remain in memory until the scope is cancelled or is garbage collected when there are no references to it.

Flows that require input

Flows that require input, like a userId, cannot be shared easily using shareIn or stateIn. Taking as an example the iosched open-source project — Google I/O’s Android app — the flow to get user events from Firestore is implemented using a callbackFlow, as you can see in the source code. As it takes the userId as a parameter, this flow cannot be reused easily using the shareIn or stateIn operators.

Optimizing this use case depends on the requirements of your app:

  • Do you allow receiving events from multiple users at the same time? You might need to create a map of SharedFlow/StateFlow instances, and remove the reference and cancel the upstream flow when the subscriptionCount reaches zero.
  • If you allow only one user, and all collectors need to update to the new user, you could emit event updates to a common SharedFlow/StateFlow for all collectors and use the common flow as a variable in the class.

The shareIn and stateIn operators can be used with cold flows to improve performance, add a buffer when collectors are not present, or even as a caching mechanism! Use them wisely, and don’t create new instances on each function call — it won’t work as you’d expect!

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