Can I get a sanity check as to why my UI won’t recompose? I’ve got a simple 1 activity setup with corresponding

ViewModel

, along with a

data class MyUIState

. I’m initializing my property as such:

var uiState by mutableStateOf(MyUiState.empty)

. , where

empty

is just a default initializer with default values. I’m then updating individual properties of

uiState

as events happen, for instance

uiState.loading = true

The `ViewModel`s is passed into my composable

MyScreen

as such from

MyActivity

setContent { MyScreen(viewModel) }

Finally, inside of my composable

MyScreen

I'm doing

val uiState = viewModel.uiState

and driving changes off of those values, for instance

if (uiState.loading)

- however, this is the point at which they aren't recomposed. Most of the examples I can find use a very simple value like

mutableStateOf(true)

, so I'm assuming there is something going on where just updating a property isn't having the right effect.

Properties of an object are not automatically made observable, just because it's held in an observable

mutableStateOf

container. Mutable objects, in general, are going to cause lots of problems for you in Compose. Instead of having your state have

var

properties, they should all be val (thus making the class immutable), and you copy the class to change its values, re-assigning it to

uiState

. Or better yet, use

StateFlow

to hold your state in the ViewModel and use

.collectAsState()

to avoid coupling the ViewModel directly to Compose

that makes total sense, but I'm a little confused as to how I'm erring in contrast to the "basic" examples provided straight from the google developer docs

The docs example is not showing how

uiState

is changed. Presumably, the

somethingRelatedToBusinessLogic()

function is doing what Casey said ("_copy the class to change its values, re-assigning it to `uiState`_").

That is missing a lot of stuff and definitely won't compile, and it looks like it's emphasizing the boilerplate to getting a handle to a ViewModel, rather than proper setup and usage of the ViewModel itself. This snippet might help you get an idea of how to handle the internals of the ViewModel, setting and updating the state. https://kotlinlang.slack.com/archives/C01D6HTPATV/p1644259697620799?thread_ts=1644257413.142249&cid=C01D6HTPATV

mutableStateOf()

and

StateFlow

in the ViewModel will do the same thing, functionally. It's more a matter of semantics, keeping the ViewModel as something that is isolated from the UI and doesn't even know about anything Compose, so that it will still work in something like a Unit Test that isn't running Compose.

StateFlow

is a more general concept and offers a few extra APIs like

.update { }

that make it more suitable in a ViewModel, when it may get updated from multiple coroutines/threads concurrently

yeah @Rick Regan my main point is that I'm just following the boilerplate example - calls to

uiState.loading = true

or

uiState = uiState.copy(loading = true)

don't trigger a recomposition

That snippet doesn't show setting/changing any properties of the state, though. Heck, it doesn't even include reading any properties from the state object itself... Really not a very good snippet, is it? 😂

hahah yeah hence the barrange of random questions on my end 😆 - very interesting approach to completely leave the

copy

function out of this documentation

I guess it's not clear what you're following if it's not in the code example. In any case, I'd expect 

uiState = uiState.copy(loading = true)

to trigger recomposition. You might have to show more of your code.

Yeah,

uiState = uiState.copy(loading = true)

should trigger recomposition, because you're changing the value of the

mutableStateOf

or

StateFlow

itself. But

uiState.loading = true

will not, because it's only mutating the state object in-place, which neither Compose nor

StateFlow

is tracking the individual properties of

okay - so, you guys are right, copy does the trick, and it was working before, but my code is still broken lol

You say that

uiState

 is getting updated with new state that has

loading = true

but it's hard to reconcile that with

LoadingDialog()

not running. If you could post a minimal example with the exact way you update maybe we could figure it out ...

i will work on it - either way thanks to you both for all your help, it is much appreciated