New to compose, but I expected something like this to work but it does not. My activity has a field ...

# compose

[Colton Idle]

New to compose, but I expected something like this to work but it does not. My activity has a field myTodoList mutableListOf<String> and it has items added to it by another source (right now I have a post delayed handler that waits for 5 seconds, and then adds ten items) and I expect the items to update this Screen composable, but nothing shows.

I'm assuming it's because the state can't live in the activity? That actually doesn't really make sense I guess. Just want someone to look this over. Thank you!

@Composable
private fun MyActivityScreen(items: List<String>) {
    val myThings = remember { mutableStateOf(items) }
    Column() {
        myThings.value.forEach {
            Text(text = it)
        }
    }
}

[allan.conda]

myThings is not necessary I think. Use items directly.

[Kane Shih]

LiveData.observeAsState() ?

[Colton Idle]

@allan.conda using items directly didn't work.

[Colton Idle]

@Kane Shih Do I have to use LiveData? What if that's a dependency that I don't want to pull in and I just want to do this with pure compose?

[Kane Shih]

1 find a way to call myThings.setValue
2 Kotlin Flow or RxJava -> to State

[Colton Idle]

Hm. So I need to use Flow or Rx to use compose?

[Kane Shih]

or find a way to trigger recomposition

[Kane Shih]

but I think LiveData is still the best choice in your use case

[Colton Idle]

That's what I thought val myThings = remember { mutableStateOf(items) } is used for? @Leland Richardson [G] am I doing something wrong here?

[flosch]

Your problem lies not in compose but with your list in your activity. The list needs to be observable if you want your composables to recompose on a change of the items in the list. Either use a Flow<List>, LiveData<List> or mutableStateOf(List), hand that over to your MyActivityScreen and observe it in there.

[Colton Idle]

@flosch so in my activitiy the field is defined as

val listOfItems = mutableListOf<String>()

so if I just change that to mutableStateOf I should be good? I will try that now and let you know what happens. Thank you! I was not aware that I couldn't just use a regular list and pass it into a composable and then that composable converts it to an observable (aka. thats what I thought I was doing with my val myThings = remember { mutableStateOf(items) } statement

[flosch]

Yes that is what it does but only in your composable scope. If you change the value of myThings then it will be recomposed, but you said you changed items outside of your composable; that will not work

[Colton Idle]

Oooh. Okay yeah that makes sense. Let me give it a shot. Appreciate your time and explanation @flosch

[Colton Idle]

@flosch still does not trigger a recomposition it seems. Here is the composable

@Composable
private fun MyActivityScreen(items: MutableState<MutableList<String>>) {
    Column() {
        items.value.forEach {
            Text(text = it)
        }
    }
}

And my activity defines a field

val listOfItems = mutableStateOf(mutableListOf<String>())

and I add to the listOfItems via

listOfItems.value.add("adsf")

[flosch]

Yea you will probably have to change the value of MutableState and not of the MutableList

val listOfItems = mutableStateOf(listOf<String>())

@Composable
private fun MyActivityScreen(items: State<List<String>>) {
    Column() {
        items.value.forEach {
            Text(text = it)
        }
    }
}

listOfItems.value = listOf("a")
listOfItems.value = listOf("a", "b" )

You could also use mutableStateListOf (LINK)

[Colton Idle]

@flosch That worked! So basically mutable state can't use a mutable class/obj/reference

Wish there was a lint check for that maybe? Also wish there was an easy way to take a list and copy or duplicate it. To get around it for now I started doing list.toMutableList() then call add(new value), then .toList(). Seems inefficient?

[flosch]

data class State(val items: List<String>, ...)
val state = ...
state.copy(items = ...)

Immutable objects are the way to go with data driven architectures

[Colton Idle]

Oooh. So just wrap it in a data class. Okay. I just thought it was interesting there was no .copy on a List. 👍

Thank you @flosch!