Hi all Does anyone know why `StateFlow` doesn't emit new changes of a `MutableList`? this is my code...

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[Mjahangiry75]

Hi all
Does anyone know why StateFlow doesn't emit new changes of a MutableList?
this is my code:

_selectedLanguages.value = _selectedLanguages.value.apply {
   if (changedLanguage.isSelected)
      add(changedLanguage)
   else
      remove(changedLanguage)
}

[Albert Chang]

StateFlow won't emit the same value. As you are setting the same list it won't be emitted. Use immutable list instead.

[Albert Chang]

Or if you want to stick with MutableList, use MutableSharedFlow(replay = 1,onBufferOverflow = BufferOverflow.DROP_OLDEST).

[Mjahangiry75]

The value of stateFlow is mutableList cause i want to change it on runtime, I checked mutablelist's hashcode doesn't change when I add to or remove from it

[Mjahangiry75]

I'll try the MutableSharedFlow, thanks for helping.

[gildor]

mutablelist’s hashcode is changing on content change

[gildor]

if you list is ArrayList

[Albert Chang]

Actually it has nothing to do with hashCode(). It depends on the result of equals(). See https://github.com/Kotlin/kotlinx.coroutines/blob/72238976ac6c06f7b1f6a72d51a41b0573bbc225/kotlinx-coroutines-core/common/src/flow/StateFlow.kt#L279.

[Mjahangiry75]

@gildor I've tried the code below but it doesn't hashCode too:

val a = arrayListOf(1,,2, 3)
val b = a
b.add(4)
println(a==b) // prints true

[gildor]

yep, exactly

[gildor]

@Mjahangiry75 It’;s not hashcode! It’s equals! it’s not the same

[gildor]

also problem here of course that you comparing 2 the same objects, so this solution:
flow.value = flow.value.mutate()

Will never work in this case, because when it will be compared, it will be the same object (of course with the same content)

[Mjahangiry75]

@gildor yes, why does it equals when I added an item to it?

[gildor]

because flow.value and your changed lis is the same object

[gildor]

so when you add something, value in flow.value already there, it has reference to the same object

[gildor]

you can just copy list and it will work:
flow.value = flow.value.toMutableList().mutate()

[Mjahangiry75]

Thanks men @gildor @Albert Chang, I'm trying your solutions.

[Mjahangiry75]

@gildor it works like a charm!!!!. you saved my day.

[gildor]

in general it doesn’t look as good solution to have mutable list in your Flow

[gildor]

instead I would make it read-only

[gildor]

so it would allow you just do:

_selectedLanguages.value = if (changedLanguage.isSelected)
      _selectedLanguages.value + changedLanguage
   else
      _selectedLanguages.value - changedLanguage
}

[Mjahangiry75]

Yes, you are right!

[Zach Klippenstein (he/him) [MOD]]

Also FYI there’s a #C1CFAFJSK channel

[Mjahangiry75]

@Zach Klippenstein (he/him) [MOD] In my case in need it as a stateflow to use in databinding
Thanks for information

[Zach Klippenstein (he/him) [MOD]]

This question comes up a lot, so I filed https://github.com/Kotlin/kotlinx.coroutines/issues/2516

[Mjahangiry75]

Good job.