I do want to filter a SnapshotStateList and than use the filtered list in a LazyColumn. How can i do...

# compose

[Timo Drick]

I do want to filter a SnapshotStateList and than use the filtered list in a LazyColumn. How can i do it?
Using this code does not work because it does not detect changes in the list and do not filter the list again:

val list = remember(snapshotStateList) {
    snapshotStateList.filter { removed.contains(it).not() }
}

[Timo Drick]

As a workaround following code would work but only if the size of the list changes:

val list = remember(snapshotStateList.size) {
    snapshotStateList.filter { removed.contains(it).not() }
}

[Paul Woitaschek]

Not at all related but I'd go for it !in removed

[Adam Powell]

Hmm, maybe something like:

val filteredList by remember(snapshotStateList, removed) { derivedStateOf { snapshotStateList.filter { it !in removed } }

?

[Timo Drick]

Yes that works. Thank you @Adam Powell
But i do not really understand why this works.

[Adam Powell]

derivedStateOf is kind of like mutableStateOf, except it returns a read-only snapshot State<T> that always contains the latest value of the lambda expression it was constructed with. It caches the result of this computation. If the derived state block reads other snapshot state, the derived state object will invalidate its cache whenever the state it had to read changes.

[Adam Powell]

snapshotStateList is this snapshot state object it reads, so any time snapshotStateList's contents change, it invalidates the derived state.

[Adam Powell]

Then we remember that derived state object and use by property delegation just like we do with mutableStateOf in composition. We use the snapshot state list and the removed list as remember keys, so we create a new derived state object if we change the whole lists it's based on.

[Timo Drick]

ok thx for the explanation 👍

[Alexander Sitnikov]

Is original example doesn't work because snapshotStateList is mutable, and when SlotTable compares remembered value with new value when snapshotStateList changes, it sees no difference, because it's the same object?

[Adam Powell]

yep. (It also doesn't consider the removed collection changing.)

[Dominaezzz]

Wow, for some reason I assumed derivedStateOf didn't need to be remembered. TIL.

[Timo Drick]

so the remember is just in case the referenced object changes right?

[Dominaezzz]

Hmm, I'm starting to question @Adam Powell's use of removed (and maybe snapshotStateList) in the remember keys, shouldn't derivedStateOf handle that already?

[Adam Powell]

The derivedStateOf lambda closes over the collection instances of snapshotStateList and removed. If the actual collection instances change, we need to create a new derivedStateOf to observe those new collections.

[Adam Powell]

All of the above therefore also presumes that removed is either another snapshot state list where the derivedStateOf can observe changes to it or immutable (so that the remember keys will result in a new derived state object if it does change)