In Kotlin Coroutines...

What is the difference between Flow, StateFlow and SharedFlow?

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[1] Flow

Flow is a cold asynchronous stream of values that can be collected by a collector. Flow is similar to the concept of an Iterator in synchronous programming.

Cold Stream: which means that it will start emitting values only when a collector starts collecting them. Once a collector stops collecting, the flow stops emitting values.

fun intFlow(): Flow<Int> = flow {
    for (i in 1..5) {
        emit(i)
    }
}

fun main() {
    intFlow().collect {
        println(it)
    }
}

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[2] StateFlow

A special kind of flow in Kotlin Coroutines that represents a state that can be observed and updated by multiple collectors.

Hot Stream: which means that it will start emitting values as soon as it is created and it will continue to emit new values even when there are no collectors.

When a new collector is added later on: State Flow will emit the current value and it will emit new values every time the value changes.

val count = StateFlow(0)

fun main() = runBlocking {
    launch {
        count.collect { value ->
            println("Count value: $value")
        }
    }

    for (i in 1..5) {
        count.value = i
    }
}

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[3] SharedFlow

Allows multiple collectors to share a single source of values, similar to a broadcast channel.

Hot Stream: which means that it will start emitting values as soon as it is created and it will continue to emit new values even when there are no collectors.

Unlike "State Flow", "𝐒𝐡𝐚𝐫𝐞𝐝 𝐅𝐥𝐨𝐰" emits values only once and it uses a buffer to store the values. So if a new collector is added later on, it will not get the current value. Instead, it will have to read the previous values from the buffer.

val sharedFlow = SharedFlow<Int>(extraBufferCapacity = 100)

fun main() = runBlocking {
    launch {
        sharedFlow
            .buffer()
            .collect { value ->
                println("Collected value: $value")
            }
    }

    for (i in 1..5) {
        sharedFlow.emit(i)
    }
}

532