Should the automatic recomposition feature of `@Mo...

# compose

[Mark Murphy]

Should the automatic recomposition feature of @Model be working in the current Compose code?

I created this based on the @Model KDoc:

@Model
    data class AgreementViewState(var terms: Boolean = true, var privacy: Boolean = false) {
        val canProceed = terms && privacy
    }

    @Composable
    private fun Agreement() {
        Padding(8.dp) {
            val viewState = +model { AgreementViewState() }

            Column(mainAxisAlignment = MainAxisAlignment.Start, crossAxisAlignment = CrossAxisAlignment.Start) {
                Row(mainAxisAlignment = MainAxisAlignment.Start) {
                    Checkbox(
                        checked = viewState.terms,
                        onCheckedChange = { viewState.terms = it }
                    )
                    Text(text = "I agree to the terms and conditions")
                }
                Row(mainAxisAlignment = MainAxisAlignment.Start) {
                    Checkbox(
                        checked = viewState.privacy,
                        onCheckedChange = { viewState.privacy = it }
                    )
                    Text(text = "I agree to the privacy policy")
                }
                Text(text = if (viewState.canProceed) "You may proceed" else "Please indicate your agreement")
            }
        }
    }

The initial state works (the top Checkbox is checked). And via logging I can confirm that my onCheckedChange lambdas are being called. However, the UI is not being recomposed as I update the AgreementViewState, so the checkboxes and text remain fixed.

By contrast, this works with automatic recomposition as I update the `checkboxState`:

@Composable
    private fun SimpleCheckbox() {
        Padding(8.dp) {
            val checkboxState = +state { true }

            Checkbox(
                checked = checkboxState.value,
                onCheckedChange = { checkboxState.value = it }
            )
        }
    }

I'm trying to determine if there's something that I'm doing wrong or if this is just a current limitation. Thanks!

[matvei]

Hey!

Is this @model class defined as nested or it's top level?

[matvei]

There's an issue right now that only top level classes annotated as @model will work, as far as I remember

[Mark Murphy]

ah, it's nested -- let me try moving it

[Mark Murphy]

Well, the model is now behaving differently... but it is crashing right away, apparently in generated code. But, that's probably still progress -- thanks for the help!

[matvei]

No problem, I can imagine how annoying this little problems might be.

Wild guess on the crashes: if there's "not in a frame" error -- it means that you're reading or writing @model fields from non-ui thread, which we for now don't fully support.

[Mark Murphy]

No, I was getting an NPE-equivalent about an r value not being set.

Then, I mistakenly tried cleaning the project, in case I had lingering code generated from the previous (nested) location. That completely broke the project, so now I need to try to convince the Compose custom Studio build to work again. 😣

[Sean McQuillan [G]]

Looking at the code, it appears canProceed should be a getter (or extension property).

[Sean McQuillan [G]]

Though I think this can/should be working as coded (with canProceed being a derived prop of the other observables). Do you see a similar issue if you switch it to a getter instead of a property initializer?

[Sean McQuillan [G]]

If so - definitely file a bug to note your expectation that properties work this way!

[Mark Murphy]

I can confirm that both a function and an extension property work. I filed https://issuetracker.google.com/issues/139653744 to document the derived property not working. Let me know if you think more stuff is needed in that issue.

Many thanks!

[Luca Nicoletti]

Also declaring it as var works fine for me

[Luca Nicoletti]

And plus, having it as a val and assigning it the constructor will lead to never be able to update the UI, right?

[Mark Murphy]

Ummm... good point. I'm used to thinking of immutable view states, where canProceed would be a derived property. @Model is all about the mutability, though.

[Luca Nicoletti]

Not saying a crash is expected, but how can it be updated?
Text(text = if (viewState.canProceed) "You may proceed" else "Please indicate your agreement") will never print You may proceed if you have canProceed as a val initialized by default as false

[Luca Nicoletti]

The best solution, imho, is using a get(), as I would do even not using Compose 🙂