How do I synchronize the dispatchAction function below so th

Question

How do I synchronize the dispatchAction function below so that it only starts sending items once there is a downstream collector onStart unfortunately get called right before collect ```class StateMachine val scope CoroutineScope private val inputActions BroadcastChannel lt Rankings Action gt = BroadcastChannel Channel BUFFERED private var isInitialized = atomic false init scope launch inputActions asFlow onStart isInitialized value = true onStart hook is called just before collect gets called collect action gt handle action CANNOT CALL FUNCTION HERE BECAUSE `collect` SUSPENDS fun dispatchAction action Rankings Action = scope launch while isInitialized value inputActions send action ```

Kiryushin Andrey · Answer

What is you intention for actions dispatched before initialization happens Do you want them to get lost or to get buffered in channel and consumed later when a collector gets attached to the flow Also I can t quite get how could `dispatchAction` get called before a downstream collector is attached for it is attached right in the class constructor

uli · Answer

< Kiryushin Andrey> a job is launched in the constructor to attach the collector This is not the same as the collector being attached in the constructor

Kiryushin Andrey · Answer

Oh right I ve overlooked that thank you for noticing

Kiryushin Andrey · Answer

Not sure though what behavior is desired for the `dispatchAction` method while initialization hasn t yet completed If it should just drop actions while a collector isn t there then simple condition check like the following will do ```if isInitialized value inputActions send action ```

Brendan Weinstein · Answer

The desired behavior is for send to suspend until the first collector has subscribed to upstream events

Brendan Weinstein · Answer

This is a code pared down a little bit from implementing an mvi redux like pattern for mobile Events are dispatched from the view and get pipelined through a scan operator that spits out a Flow of ViewState s back to the view

uli · Answer

how about making inputActions a `Deferred lt BroadcastChannel lt Rankings Action gt gt ` and then ```fun dispatchAction action Rankings Action = scope launch inputActions await send action ```

Brendan Weinstein · Answer

ooo like that idea I will give that a try and report back

Brendan Weinstein · Answer

ah sorry I was overly optimistic and responded while walking to work I guess the crux of the question is how to define a hook for when the first collector starts collecting on a flow I probably should have framed the question that way from the get go

Brendan Weinstein · Answer

The challenge is that collect is a suspending function so you must call it within a coroutine and you cannot execute any code after the collect call

Brendan Weinstein · Answer

``` InternalCoroutinesApi fun main val channel = BroadcastChannel lt Int gt Channel BUFFERED GlobalScope launch var i = 0 delay 10 000 while true delay 1000 channel send i i++ GlobalScope launch val flow = channel asFlow asBindFlow println onBind delay 5000 println adding terminal downstream collector flow collect println collected $it while true InternalCoroutinesApi fun lt T gt Flow lt T gt asBindFlow onBind gt Unit Flow lt T gt return BindFlow onBind = onBind flow = this InternalCoroutinesApi class BindFlow lt T gt val onBind gt Unit val flow Flow lt T gt Flow lt T gt private var hasBinded = AtomicBoolean false private val mutex = Mutex InternalCoroutinesApi override suspend fun collect collector FlowCollector lt T gt if hasBinded get mutex withLock if hasBinded get onBind hasBinded set true flow collect collector emit it ```

Brendan Weinstein · Answer

And we can avoid the mutex in BindFlow with compareAndSet ``` InternalCoroutinesApi class BindFlow lt T gt val onBind gt Unit val flow Flow lt T gt Flow lt T gt private var hasBinded = AtomicBoolean false InternalCoroutinesApi override suspend fun collect collector FlowCollector lt T gt if hasBinded compareAndSet false true onBind flow collect collector emit it ```