Using MutableState, MutableStateFlow or MutableLiv...

# compose

[Francois Morvillier]

Using MutableState, MutableStateFlow or MutableLiveData (are there others?) in a @Composable function will automatically trigger recomposition when the underlying data changes. Say I have my own DataHolder to some data (a graph) and that my DataHolder receives notifications when the underlying graph data changes. It also notifies whoever is observing it. Is there a way to use my DataHolder in @Composable functions in the same way we would use MutableState so that it will trigger recomposition? For example implementing the State<T> interface?

[Adam Powell]

Yes, all of these work the same way:
val result = remember { mutableStateOf(initialValue) }
Use a LaunchedEffect or DisposableEffect to manage a subscription to an external observable type
return result

[Adam Powell]

The effect should set the value of result to the new value of the external observable whenever it changes.

[Francois Morvillier]

Sorry, I'm not sure I quite understand. I was hoping something along these lines would be possible:

class DataHolder: State<Graph> { // extend my current DataHolder in some way so that it works like a MutableState. Implementing State<Graph> would be ideal but I suppose it's not that simple
    //...
}

@Composable 
fun DataView(dataHolder: DataHolder) {
    Text("graph size: ${dataHolder.graph.getSize()}")
}

// ...
graph.addNode(...) // notifies dataHolder and triggers recomposition

[Adam Powell]

Yes, this is possible. Mutable objects implemented using snapshot state types will be transitively observable. For example:

class Graph {
  private val nodes = mutableStateListOf<Node>()

  fun addNode(node: Node) {
    nodes += node
  }

  fun getSize(): Int = nodes.size
}

the above class is observable to compose, since its internal mutable state is implemented using the SnapshotStateList returned by mutableStateListOf

[Adam Powell]

the implementation of mutable internals is what matters, not the State<T> type

[Francois Morvillier]

I wouldn't want to make my Graph class dependent on Compose though, just my DataHolder (which is subscribed to Graph). I'm not being intentionally difficult 😅

[Adam Powell]

I think the biggest challenge you're likely to run into then is once we start doing more multithreaded composition. We rely on a combination of snapshots and immutability as a thread safety mechanism