[XState] Invoke Callbacks to Send and Receive Events from a Parent XState Machine

We can invoke a callback as a service when we enter a state in XState. This gives us the ability to trigger various functionality by responding to events sent to the service, and allows us to send events back to the parent machine.

We do this by writing a "callback handler" and setting it as the src of our invoked service. A callback handler is a function that receives the current context and the event object that triggered the invocation. This function returns another function that receives two functions as arguments. A callback function to send events to the parent machine, and an onEvent function for the handler to respond to events sent to the handler.

The way events are sent to the callback handler is by utilizing the options argument of the send action creator. We identify where we send events to using the to property, and setting the value to the id of our service.

const handlerEchoCallback = (context, event) => {
  return (callback, onReceive) => {
    onReceive(e => {
      if (e.type === 'FOO') {
       callback('ECHO'); // call the 'ECHO' action
      }
    })
  }
}

const callbackMachine = Machine({
  id: 'callbackMachine',
  initial: 'listening',
  states: {
    listening: {
      invoke: {
        src: handlerEchoCallback,
        id: 'handlerEchoCallback'
      },
      on: {
        SPEAK: {
          actions: send('FOO', {
            to: 'handlerEchoCallback'
          })
        },
        ECHO: {
          actions: (context, event) => {
            console.log('echo', context, event);
          }
        }
      }
    }
  }
})