MarkerState encapsulation

Marker.kt

import androidx.compose.runtime.*
import com.google.android.gms.maps.model.LatLng
import com.google.maps.android.compose.DragState.*
import com.google.maps.android.compose.GoogleMapComposable
import com.google.maps.android.compose.Marker
import com.google.maps.android.compose.MarkerState
import kotlinx.coroutines.flow.onCompletion

@Immutable
interface DragOps {
    fun onMove(position: LatLng)
    fun onCancel()
    fun onEnd()
}

@Composable
@GoogleMapComposable
fun Marker(
    modelPosition: LatLng,
    draggable: Boolean,
    /* ... */
    onDrag: ((LatLng) -> Unit) -> DragOps
) {
    val markerState: MarkerState = remember { MarkerState() }

    Marker(
        markerState,
        draggable = draggable,
        /* ... */
    )

    var dragOps: DragOps? by remember { mutableStateOf(null) }

    // Sync marker state to model if model position changed without dragging;
    // if model position simply aligns with markerState nothing happens
    LaunchedEffect(modelPosition) {
        if (dragOps == null) markerState.position = modelPosition
    }

    val currentOnDrag: ((LatLng) -> Unit) -> DragOps by rememberUpdatedState(onDrag)

    // Naive dragging observer that would not work reliably currently
    LaunchedEffect(Unit) {
        snapshotFlow { markerState.dragState to markerState.position }
            .onCompletion {
                dragOps?.let {
                    dragOps = null
                    it.onCancel()
                }
            }
            .collect { (dragState, position) ->
                when (dragState) {
                    START -> {
                        dragOps = currentOnDrag { markerState.position = it }
                        dragOps!!.onMove(position)
                    }
                    DRAG -> dragOps!!.onMove(position)
                    END -> {
                        dragOps!!.let {
                            it.onMove(position)
                            dragOps = null
                            it.onEnd()
                        }
                    }
                }
            }
    }
}