this might be a noob question but today I realized that StateFlow doesn’t work very well (or better saying intentionally doesn’t work) with Lists. So if we have the below (very rough example)

data class ViewItem(
  val id: String,
  val title: String,
  var description: String = "" // yes a var
)

private val _items: MutableStateFlow<List<ViewItem>> = MutableStateFlow(emptyList())
val metrics: StateFlow<List<ViewItem>> = _metrics

_metrics.value = _metrics.value.toMutableList().find { // some predicate }?.description = "a new title"

The

_metrics

won’t emit the new value (which is actually a change in the

ViewItem

object. Am I right or I am missing something here? And If I am right, in order to solve the above “issue” I need to use

Channel

or is there any other way with

StateFlow

?

The last line of your snippet isn’t a valid Kotlin expression. Is this the logic you’re actually using? Or could you post the actual code that’s causing issues?

val currentList = _metrics.value.toMutableList()
currentList.find { it.id == "1" }?.description = "a new title"
_metrics.value = currentList

MutableStateFlow

is not aware to changes to your list. To trigger a new emission you have to make a new copy of your state with the list updated instead.

thanks for your quick answers! @Casey Brooks this could be more valid ?

val tempList = _metrics.value.toMutableList()
tempList.find{ // }...
_metrics.value = tempList

@Francesc like this? Becaause doesnt work. Or could you please tell me how?

StateFlow emissions are conflated with

Any.equals

. If the result of your modifications changes the equality property of the list, then it should be emitted. A List is equal to another List when it contains the same contents in the same order. So to get a “different” list, you either need to add an item, remove an item, or cause one item to become not equal to a previous one. However, mutable properties are very problematic for things like this. When you modify a

ViewItem

, it gets the new value in both the new and old lists. Even though the values of the objects are now different than they were, the two lists are still equal because they share the same instance of

ViewItem

. This is likely your problem

thanks a lot Casey for the detailed explanation and for the examples.

Yes, because Channels by default do not conflate emissions. But that’s probably not a good reason to drop down to the Channel API, which is intended for lower-level control and brings its own set of challenges, not to mention the fact that a Channel does not hold a value once it has been consumed. A

SharedFlow

also does not conflate emissions like the

StateFlow

does, but also like the Channel it does not really hold onto a value. Its “replayCache” kinda does, but it doesn’t hold the strong guarantee that a StateFlow does, so you’d have to assume the “current value” could be null.