Kotlin Flow has revolutionized how we handle asynchronous tasks in Android development, offering a robust and type-safe way to work with streams of data. This post dives into the intricacies of Kotlin Flow, its comparison with coroutines, and its application in Android apps, along with testing practices.

Understanding Kotlin Flow

Flow in Kotlin is akin to sequences in Kotlin or streams in Java, but with superpowers for asynchronous stream processing. It’s cold, meaning it doesn’t start emitting data until a collector starts collecting, making it highly efficient and suitable for handling data streams that are not immediately required.

Creating and Collecting Flow

To create a flow, you can use one of the provided builder functions, like flowOf, asFlow, or a custom flow block. Here’s a simple example:

val simpleFlow = flow {

To collect values from a flow, you use the collect function, which is a suspend function and must be called from a coroutine:

runBlocking {
    simpleFlow.collect { value ->

Switching Contexts with Flow

Flow allows you to switch the context where the code is executed using the flowOn operator. It’s crucial for performing operations on appropriate threads, for instance, fetching data from a network or database on a background thread and updating the UI on the main thread.

val ioFlow = flow {
    emit(fetchData()) // Suppose this fetches data from a network
}.flowOn(Dispatchers.IO) // Use IO dispatcher for upstream operations

StateFlow and SharedFlow

StateFlow and SharedFlow are special types of Flow for state handling and event broadcasting. StateFlow is a hot flow with a state, always holding the latest value. It’s particularly useful for representing state in your UI in a thread-safe manner.

private val _state = MutableStateFlow(InitialState)
val state: StateFlow<State> = _state.asStateFlow()

SharedFlow is more flexible, allowing configuration of how many past emissions new collectors receive, making it perfect for events that multiple components in your app might be interested in.

private val _events = MutableSharedFlow<Event>()
val events: SharedFlow<Event> = _events.asSharedFlow()

Best Practices for Android

When using Flow in Android, it’s best to adhere to structured concurrency principles and lifecycle-aware components. For instance, collecting a Flow in a ViewModel and exposing a StateFlow or LiveData to the UI ensures that your app reacts to state changes promptly and efficiently.

Additionally, leveraging lifecycle-runtime-ktx extensions like lifecycleScope.launchWhenStarted allows your coroutines to automatically pause and resume with your activity or fragment’s lifecycle, preventing memory leaks and unnecessary processing.

Testing Flows

Testing flows is straightforward with the kotlinx-coroutines-test library. By using TestCoroutineDispatcher and TestCoroutineScope, you can ensure your flow logic works as expected in a controlled environment.

fun testFlowEmissions() = runBlockingTest {
    val flow = repository.getData()
    val results = flow.toList()
    assertEquals(expected, results)


Kotlin Flow is a powerful tool in the arsenal of an Android developer, simplifying asynchronous data handling with robust, type-safe operations. By embracing Flow and its principles, developers can write more concise, maintainable, and testable code, ultimately leading to better app performance and user experience. Whether you’re managing UI state with StateFlow, broadcasting events with SharedFlow, or orchestrating complex data transformations, Kotlin Flow offers the flexibility and power to meet the challenges of modern Android development.