looking for a flow operator that does something like input: Foo output: List<Foo> everytime ...

# coroutines

[william]

looking for a flow operator that does something like

input: Foo
output: List<Foo>

everytime the upstream flow emits, the operator would emit a list of this value and any previous values that are emitted

[Marc Knaup]

.scan(emptyList<Foo>()) { list, element -> list + element }

[william]

ty 🙂

[ephemient]

FYI ^^ is O(N²)

[ephemient]

this is O(N), if it matters to you (but it may not)

Untitled

[Marc Knaup]

If performance is critical I would definitely run benchmarks.
It’s a good idea to use a shared list where the elements already emitted are never replaced.

I’d use a MutableList instead of array though and let the stdlib handle the expansion. ArrayList implementation is incredibly fast.

[ephemient]

the problem is that you need something like .subList() to avoid copying

[ephemient]

otherwise all earlier flow emissions are invalidated when you mutate the list, which I believe breaks expectations

[ephemient]

but extending mutable list will cause any earlier .subList() values to break with ConcurrentModificationException

[ephemient]

hence the manual array extending and list wrapping

[Marc Knaup]

Ah, you’re right