Generator support for subscribe (stream)

[gkiely]

Use case:
Allows streaming of data while waiting for state machine to complete.

Example

const machine = createMachine({
  context: {
    streamed: [],
  }
});

const actor = createActor(machine).start();

actor.send({ type: 'fetch-stream' });

const streamedData = [];
for await (const state of actor.stream){
  // Allows access to state and waits for idle before proceeding
  streamedData.push(state.context.streamed);
  
  // perform ui updates
  render();
  
  if(state.matches('idle') break;
}

[gkiely]

Here's an example in userland.

export const createIterator = <A extends AnyActor, C extends ContextFrom<A>>(actor: A, finalState = 'idle') => {
  let p = Promise.withResolvers<{ value: C; done: boolean }>();
  const subscription = actor.subscribe((state: { context: C; matches: (s: string) => boolean }) => {
    p.resolve({ value: state.context, done: state.matches(finalState) ? true : false });
    if (state.matches(finalState)) {
      subscription.unsubscribe();
    }
  });

  return (async function* () {
    while (true) {
      const result = await p.promise;
      p = Promise.withResolvers();
      yield result.value;
      if (result.done) break;
    }
  })();
};

const iterator = createIterator(exampleActor);
exampleActor.send({ type: 'fetch' });

for await (const context of iterator) {
  const { streamed } = context;
  // Perform operations
}