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.