1. Introduction

Kotlin Flow, introduced as a part of the Kotlin Coroutines library, has become an essential tool for asynchronous programming in Kotlin. It provides a concise and expressive way to handle asynchronous data streams. Two commonly used functions while working with Flow for collecting data are collect() and collectLatest(). While both terminal operators serve the purpose of collecting values emitted by a flow, they have distinct characteristics and use cases.

In this tutorial, we’ll delve into the differences between collect() and collectLatest().

2. Overview of Kotlin Flows

Before diving into the specifics of collect() and collectLatest(), let’s briefly recap what Kotlin flows are. A Flow is an asynchronous stream of values that can be transformed, combined, and consumed reactively. It is part of Kotlin coroutines and offers a structured and concise approach to handling streams of asynchronous data.

When working with a Flow, we’ll need an active coroutine context or suspend function for most operations.

3. The Basics of collect()

The collect() function is a fundamental method used to consume values emitted by a flow. Calling collect() on a flow starts the collection process and blocks the coroutine until the flow completes. This means that when the flow emits multiple values, this collector processes each value sequentially. Let’s look at the usage of the collect() function, which is a suspend function, inside a runBlocking() lambda so that we have an active coroutine context:

fun main() {
    runBlocking {
        flowOf(1, 2, 3)
            .collect { value ->
                println(value)
            }
    }
}

In the above example, the collect() function will print each value “1”, “2” and “3” as emitted by the flow.

4. Understanding collectLatest()

The collectLatest() function introduces distinct behavior. Unlike collect(), collectLatest() cancels or ignores earlier values in the flow and favors the most recently emitted value only.

Like the previous example, we’ll need to enclose the usage of collectLatest() in runBlocking() to provide a coroutine context:

fun main() {
    runBlocking {
        flowOf(1, 2, 3)
            .collectLatest { value ->
                println(value)
            }
    }
}

In this example, if the flow emits values “1”, “2”, and “3” in quick succession, collectLatest() will ignore everything but the value “3”, as the processing of values “1” and “2” will be canceled. This means that only “3” will be printed. The runBlocking() lambda encapsulates our code in a coroutine context because collectLatest() is a suspend function.

5. Usage Considerations

The choice between collect() and collectLatest() can impact the performance of our application. If processing each emitted value is crucial and the order matters, collect() is the better choice. On the other hand, if we are dealing with rapidly changing data and only care about the latest value, collectLatest() can help improve efficiency by canceling unnecessary processing.

A real-world dataset to help illustrate the differences is if we were building a temperature station. We’d use collect() to get all the temperatures to show a time graph. Whereas we’d use collectLatest() to get the current temperature only.

6. Conclusion

Kotlin Flow is a powerful component of the Kotlin coroutines library. It offers an effective means of handling asynchronous data streams. The collect() function allows for sequentially processing each emitted value and blocking the coroutine until the flow completes. On the other hand, collectLatest() uniquely cancels previous values from the flow in favor of the most recent value only. This feature is particularly advantageous when prioritizing the latest emitted value and discarding unnecessary processing.

As always, the full implementation of these examples is available over on GitHub.

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