如何通过组合将 Kotlin 流事件发送到现有流 stream 中？

Question

I have two flows.

flow1() emits a stream of integers every second.

flow2() emits an integer when a button is clicked.

Both flows are combined with combinedFlows() and the combined results are collected and ouput in the textView .

Problem: The textview is not updating the flow2 click count when I click on the button . For some reason the combine{ } operator is not collecting flow2() on click. How can I emit flow2() into the existing flow stream and collect the results in the text view?

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        CoroutineScope(Dispatchers.Main).launch {
            combineFlows().collect {
                binding.textView.text = "flow1: ${it.first} and flow2 click count: ${it.second}"
            }
        }

        binding.button.setOnClickListener {
            CoroutineScope(Dispatchers.Main).launch {
                flow2().collect()
            }
        }
    }

    fun flow1() = (1..1000).asFlow().onEach { delay(1000) }

    fun flow2() = flow {
        var i = 0
        i++
        emit("$i")
    }

    fun combineFlows() = combine(flow1(), flow2()) { a, b -> Pair(a, b) }
}

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="170dp"
        android:layout_height="35dp"
        android:textSize="11sp"
        android:text="Emit Flow"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

flow {} builder is a cold flow, meaning that it executes when a terminal operator is called. When flow {} builder is invoked it creates a new instance of Flow . Therefore, calling flow2() two times in combine(flow1(), flow2()) { a, b -> Pair(a, b) } and flow2().collect() will create two different instances of Flow .

Button clicks should be regarded as a hot flow - all collectors don't get all events, they receive only last event (or only a couple of last events depending on how the hot flow is configured).

SharedFlow and StateFlow are hot flows, that can be user for such purpose:

private val counter = MutableStateFlow(0)

binding.button.setOnClickListener {
    counter.update { count -> count + 1 }
}

fun combineFlows() = combine(flow1(), counter) { a, b -> Pair(a, b) }