Let me rephrase my question (<https://kotlinlang.slack.com/archives/C1CFAFJSK/p1619021225012300>) wi...

# coroutines

[Erik]

Let me rephrase my question (https://kotlinlang.slack.com/archives/C1CFAFJSK/p1619021225012300) with a runnable code example: https://pl.kotl.in/guoP8yGMg
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 // <-- 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]

Ping @baxter who showed interest in my previous thread

[Erik]

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]

So I came up with this solution: https://pl.kotl.in/N6LC2kciE

[baxter]

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]

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]

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]

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

[baxter]

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]

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]

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

[Erik]

Thanks for looking into it!

[Erik]

Note that I think that both your and my implementations don't work if T in Flow<T> or foo(): T is nullable

[Erik]

In my operator that can be mitigated by using a val x = mutableListOf<T>() instead of a lateinit var to store the provided and last value, but that's yet another inefficiency

[uli]

How about channelFlow/Scan?
https://pl.kotl.in/B7Bziuk0D

[Erik]

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]

care to share?