Introduce renderAsState, a handy way to run the workflow runtime from composables.

[zach-klippenstein]

Part of #458.

renderAsState is a wrapper around the main workflow runtime entry point, renderWorkflowIn, that manages saving and restoring snapshot states as well as updating props and getting renderings and outputs from the workflow.

It's designed to follow the compose pattern for subscribing to reactive types: Flow.collectAsState(), LiveData.observeAsState(), Observable.subscribeAsState().

Example:

@Composable fun App(workflow: Workflow<…>, props: Props) {
  val scaffoldState = …

  // Run the workflow in the current composition's coroutine scope.
  val rendering by workflow.renderAsState(props, onOutput = { output ->
    // Note that onOutput is a suspend function, so you can run animations
    // and call other suspend functions.
    scaffoldState.snackbarHostState
      .showSnackbar(output.toString())
  })
  val viewEnvironment = remember {
    ViewEnvironment(mapOf(ViewRegistry to appViewRegistry))
  }

  Scaffold(…) { padding ->
    // Display the root rendering using the view environment's ViewRegistry.
    WorkflowRendering(rendering, viewEnvironment, Modifier.padding(padding))
  }
}

[steve-the-edwards]

A couple questions.

[rjrjr]

I really want to use this in prod. 🤤

[rjrjr]

Everything that was hard about bootstrapping, Compose is making seamless. Thank God you came up with making the first rendering synchronous.

[zach-klippenstein]

TBH I'm still not sure this won't blow up on us. I would be honestly quite surprised if, with all the workflows we've written internally at this point, at least one isn't doing something in its initialState method that requires being on the main thread.

[rjrjr]

Wouldn't want anyone to get bored.

[zach-klippenstein]

Should we put this in a dedicated compose-runtime module? As-is, this requires the workflow runtime to be a transitive dependency of any consumers that want to just define some ComposeViewFactorys.

This is marked as experimental, so I'm going to leave it here for now, and do the initial release this way, but if we want to move it out it won't be hard.

[zach-klippenstein]

Comments addressed, merging on green.