Problem: Click callback uses outdated data

[cwsiteplan]

Hi,

i ran into a strange issue using the marker click callback.

Pushed a sample here: https://github.com/p-lr/MapCompose/compare/master...cwsiteplan:MapCompose:bug/marker-click-callback

I'm adding a click callback to the mapState initially and inside the callback i do access data that does change over time. But somehow the callback accesses only the initially passed object.

Interestingly, moving the code outside the child composable it starts to work fine 🤔

Probably not related to your implementation at all, but would be glad if you have an idea why.

[p-lr]

You probably need to update the callback when your state changes. Try with:

@Composable
fun MyMapComposable(modifier: Modifier = Modifier, myData: MyData, mapState: MapState) {
    LaunchedEffect(myData) {
        mapState.onMarkerClick { id, x, y ->
            Log.d("Bug", "my marker state: ${myData.someList.size}")
        }
    }

    MapUI(modifier, state = mapState)
}

[cwsiteplan]

yep, that works too - but i'm not quite sure why i would need to do that .

shouldn't the lambda access the updated data anyways? (data is correctly updated outside of the callback)

[p-lr]

That's because LaunchedEffect can take keys as parameters, and whenever one keys changes of value, the composable block is recomposed. By using LaunchedEffect(Unit){..}, you are actually capturing the initial value of your state.

[cwsiteplan]

thanks, i thought i'm aware of the concept of LaunchedEffect but assumed that the created callback (lambda) would still access the current state - seems it also captures a snapshot of the current state -interesting.

[p-lr]

Another approach is to call a method from the view-model from the lambda, passing id, x and y values. Since the view-model has all necessary data, you can do necessary operations and state changes from there, without having to worry about updating the lambda.