looking for a flow operator that does something like input Foo output List lt Foo gt everytime the upstream flow emits the operator would emit a list of this value and any previous values that are emitted

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

10/21/2020, 1:32 AM

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

☝️ 2

william

10/21/2020, 12:29 PM

ty 🙂

ephemient

10/21/2020, 1:37 PM

FYI ^^ is O(N²)

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

Untitled

👍 1

Marc Knaup

10/21/2020, 1:50 PM

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

10/21/2020, 1:52 PM

the problem is that you need something like

.subList()

to avoid copying

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

but extending mutable list will cause any earlier

.subList()

values to break with ConcurrentModificationException

hence the manual array extending and list wrapping

Marc Knaup

10/21/2020, 1:53 PM

Ah, you’re right

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