Let me rephrase my question

Question

Let me rephrase my question with a runnable code example The output ```Getting value tick 100 tick 200 0 tick 300 tick 400 1 Got value tick 500 tick 600 A2 tick 700 tick 800 A3 tick 900 tick 1000 A4``` The desired output ```Getting value tick 100 tick 200 0 tick 300 tick 400 1 Got value A1 lt immediately emit and combine the value and the last emission from the upstream flow tick 500 tick 600 A2 tick 700 tick 800 A3 tick 900 tick 1000 A4```

Erik · Answer

Ping < baxter> who showed interest in my previous thread

Erik · Answer

The issue is that the code looks very inefficient and it likely is And it is bugged if the suspending function returns more quickly than the first `flow` emission

baxter · Answer

So I came up with this solution <https pl kotl in N6LC2kciE>

baxter · Answer

Basically taking the result of `foo ` and setting it on a `StateFlow` then using `combine` Since the initial value of the state flow is `null` we just emit as normal but as soon as `foo ` is complete the state flow will emit the change

baxter · Answer

You may even be able to do this exact same thing using ```val fooFlow = flow emit null emit foo ``` and combining on that flow instead of using state flow

Erik · Answer

Nice almost there Try to change `val isGetValueSlow = true` to `false` It means that `foo` is fast but you can see ticks before the first value from `foo` is collected I d like to collect the first value from `foo` ASAP if it comes first

baxter · Answer

Have your `flow` emit `null` on start and only emit the item that has a value or both

baxter · Answer

The gist of it is that you have both flows emit `null` but only return the one that has a non null value If both are non null emit your combined value If both are null just filter it out

Erik · Answer

Meanwhile I made my own implementation and I think it works but it s not very nice The magic happens in the custom operator function <https pl kotl in TpsJd1Sjq>

Erik · Answer

I ll have to look at your work in more detail too tomorrow

Erik · Answer

Thanks for looking into it

Erik · Answer

Note that I think that both your and my implementations don t work if `T` in `Flow lt T gt ` or `foo T` is nullable

Erik · Answer

In my operator that can be mitigated by using a `val x = mutableListOf lt T gt ` instead of a `lateinit var` to store the provided and last value but that s yet another inefficiency

uli · Answer

How about channelFlow Scan <https pl kotl in B7Bziuk0D>

Erik · Answer

Thanks for the help folks I ve now found a good solution based on all your insights that works well for my use case

uli · Answer

care to share