Kotlin Flows Real-time Updates Explained

In Kotlin, Flow is a powerful construct for handling streams of data. When used correctly, it can become a cornerstone for building real-time applications. One powerful aspect of Kotlin Flow is the ability to combine different flows together, and that is precisely what we’re going to explore in this post. Let’s start from the basics and gradually move towards more advanced examples.

The Basics of Combining Flows

Combining flows allows you to effectively ‘merge’ multiple streams of data into a single stream. Kotlin provides several operators for this purpose, including zip, combine and flattenMerge. Let's start with a basic example using zip.

val flow1 = flowOf(1, 2, 3)
val flow2 = flowOf("a", "b", "c")
val zipped = flow1.zip(flow2) { number, letter -> "$number$letter" }

// When collected, the zipped flow will output: 1a, 2b, 3c

Combining Flows with ‘combine’ Operator

While zip only combines the corresponding values of each flow, the combine operator combines the most recently emitted value from each flow any time a new value is emitted from any of the combined flows.

val flow1 = flowOf(1, 2, 3).onEach { delay(100) } 
val flow2 = flowOf("a", "b", "c").onEach { delay(300) } 
val combined = flow1.combine(flow2) { number, letter -> "$number$letter" }

// When collected, the combined flow will output: 1a, 2a, 2b, 3b, 3c

Real-Time Updates with Combined Flows

Combined flows really shine when used for real-time updates. Consider a chat application that has separate flows for incoming messages and user status updates. By combining these flows, we can create a single stream of updates for the UI.

val messagesFlow: Flow<ChatMessage> = chatAPI.messageUpdates()
val userStatusFlow: Flow<UserStatus> = chatAPI.userStatusUpdates()

val uiUpdatesFlow: Flow<UiUpdate> = messagesFlow
    .map { UiUpdate.MessageUpdate(it) }
    .combine(userStatusFlow.map { UiUpdate.UserStatusUpdate(it) }) { messageUpdate, statusUpdate ->
        listOf(messageUpdate, statusUpdate)
    }
    .flattenMerge(concurrency = 2)

In the above example, we’re first transforming messagesFlow and userStatusFlow into flows of UiUpdate. We then combine them into a single flow of lists of UiUpdate. Finally, flattenMerge is used to flatten the flow of lists into a flow of UiUpdate, which can be collected and used to update the UI.

Conclusion

Combining Kotlin Flows is a versatile technique for handling multiple streams of data. It allows for efficient and responsive handling of real-time updates in your Kotlin applications. Just remember that like with any tool, understanding its usage and behavior is key to using it effectively.

As Alfred North Whitehead beautifully puts it, “The art of progress is to preserve order amid change, and to preserve change amid order.” With Kotlin Flows, we preserve the order of data while embracing the change in real-time. Happy coding!

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