What data type should I use when I want to have a ...

# coroutines

[martmists]

What data type should I use when I want to have a mutable list of elements, and the ability to have multiple listeners waiting for new elements being added? Kind of like a flow, but which starts enumerating elements from the start of the collection rather than when the flow starts. A Channel also won't work, as it has an internal buffer rather than keeping all elements.

[Sam]

One option would be to use a MutableSharedFlow with an infinite (well, Int.MAX_VALUE) replay cache.

[Sam]

Depending on the use case, though, I'd be tempted to use a MutableStateFlow<List<T>>, and just use its update function to replace the entire list each time it changes.

[martmists]

Unfortunately since I'm adding a lot of single elements, I'm worried about the cost of creating large lists, as well as race conditions when taking the current value and adding an element to it.

[Sam]

If you're adding a lot of elements, MutableSharedFlow is probably better 👍. The state flow approach is safe since update is atomic, but you're right it could get expensive to recreate the list each time.

[Dmitry Khalanskiy [JB]]

What I would do is MutableStateFlow<PersistentList> (with PersistentList implementation from https://github.com/Kotlin/kotlinx.collections.immutable). Persistent collections support creating lists with new elements added efficiently, and there are no race conditions, as each persistent list is a separate object.

[Dmitry Khalanskiy [JB]]

Oh, if by race conditions, you mean that several threads may want to update the state flow, you need https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-mutable-state-flow/compare-and-set.html

[Dmitry Khalanskiy [JB]]

Though I'd probably wrap the persistent list into some other class to ensure that === is used to check for equality; comparing lists element-by-element may be too inefficient in your case.

[martmists]

If I use a Flow<List<T>> won't a collect receive the entire list every time instead of receiving only the newest element?

[Dmitry Khalanskiy [JB]]

It will, yes. If you're interested in updates and not snapshots, @Sam's suggestion with an infinite replay cache does seem like a good solution to me.