What would the ideal primitive be if I wanted something that

Question

What would the ideal primitive be if I wanted something that had the properties of 1 Multicast like BroadcastChannel 2 Being buffered All subscribers should never miss a value like ArrayBroadcastChannel 3 Being able to provide an initial value to new subscribers like StateFlow

louiscad · Answer

< gpeal> Your subscribers rely on receiving a value preceding the last one Why not keep any required history in the value that a `StateFlow` would expose

gpeal · Answer

Just the latest one prior to the subscription

gpeal · Answer

It seems that the conflated behavior of MutableStateFlow can cause subscribers to miss events if it is updated very quickly

gpeal · Answer

Here is a sample snippet demonstrating 2 ``` Test fun testStateFlow = runBlocking val flow = MutableStateFlow 0 flow onEach println it launchIn this delay 100 repeat 10 flow value = it ``` This prints 0 and 9 and misses 1 8

gpeal · Answer

``` val stateChannel = BroadcastChannel lt S gt capacity = Channel BUFFERED var state = initialState val flow Flow lt S gt get = channelFlow send state stateChannel consumeEach send it distinctUntilChanged ``` This seems to be working thinking face

louiscad · Answer

< gpeal> Beware you re not updating `state` in your snippet so it keeps the `initialState`

Sinan Kozak · Answer

For first test you need to add yield after changing state Otherwise it will update value in same cycle without continue any other continuation in runBlocking scope

Sinan Kozak · Answer

``` Test fun testStateFlow = runBlocking val flow = MutableStateFlow 0 val job = flow onEach println it launchIn this delay 100 repeat 10 flow value = it yield job cancel ```

Sinan Kozak · Answer

this print every value

Sinan Kozak · Answer

or you can launchIn dirent scope ```private val scope = CoroutineScope Dispatchers Unconfined Test fun testStateFlow = runBlocking val flow = MutableStateFlow 0 val job = flow onEach println it launchIn scope delay 100 repeat 10 flow value = it job cancel ```

Sinan Kozak · Answer

this also print every number

bezrukov · Answer

`yield` just slightly improves behavior but doesn t fix it completely because if consumer is slow values will be loosed anyway e g adding `delay 1 ` after println Unconfined dispatcher will work but with a few trade offs 1 the whole flow chain must be in unconfined dispatcher because switching to another will lead to loosing values won t work if you wanna switch to main thread in android while stateflow is updated from background thread 2 no suspend calls allowed if you add `delay 1 ` after println it will be the same in the chain 3 It prevent fast collectors from fast collection due slow collector e g if there are 9 collectors able to collect with 10hz and one with 1hz all collectors will collect at 1hz rate Seems ```stateFlow buffer buffersize flowOn Dispatchers Unconfined could be used in any scope suspends allowed doesn t slow down other collectors ``` will work without mentioned trade offs but it will create separate buffer for each collector which is less effective than ArrayBroadcastChannel

Sinan Kozak · Answer

I used unconfined because it is test It was an example to point different scope ddoesnt need yield I do agree you points On the otherhand I just want to point that using scope of runBlocking will make test almost single track execution

gpeal · Answer

< louiscad> that was just for brevity but you can assume that it would be updated at the same time as items are added to the channel

gpeal · Answer

< Sinan Kozak> In our me and < bezrukov> case the consumers are users of a library so we can t control where and how the values will be consumed so it has to be robust against bad actors and all dispatchers