Awaiting asynchronous execution

Awaiting asynchronous execution

In a scenario where it’s necessary to await the machine to enter a final state (a state which has no transitions), the onChange callback (the second argument to interpret) can be used to resolve a promise. The promise can be externally awaited.

let resolve;
let promise = new Promise(_resolve => {
  resolve = _resolve;
})

service = interpret(machine, () => {
  if(machine.state.value.final) {
    resolve();
  }
});

await promise;
// All done!

This is particularly useful for state machines which consist entirely of immediate and invoke states: these model execution flows which do not depend on external events for their execution and state transition.