Transition Actors

Transition actors are actors whose logic is represented by a state-transition function, which is a function that returns the next state of an actor given:

• The current state of the actor
• The event that triggered the transition

This is very similar to a reducer function in libraries like Redux.

Transition actor capabilities

	Capability	Notes
✅	Receive events	Transition actors receive events to modify its state.
✅	Send events	Transition actors can send events to other actors it has reference to, such as those provided in its input. Note that doing this would be impure.
❌	Spawn actors	Transition actors currently cannot spawn new actors.
✅	Input	You can provide input to transition actors.
❌	Output	Transition actors currently do not produce output – they are active indefinitely until they are stopped or an error occurs.

Transition actor logic

You can define transition actor logic using the fromTransition(...) actor logic creator, which takes two arguments:

• A state-transition function that returns the next state of the actor
• An initial state for the actor

The actor logic creator returns actor logic that can be used to create transition actors.

import { fromTransition, createActor } from 'xstate';

const countLogic = fromTransition(
  (state, event) => {
    switch (event.type) {
      case 'increment': {
        return { count: state.count + 1 };
      }
      case 'decrement': {
        return { count: state.count - 1 };
      }
      default: {
        return state;
      }
    }
  },
  { count: 0 },
); // Initial state

const countActor = createActor(countLogic);
countActor.subscribe((snapshot) => {
  console.log(snapshot.context);
});
countActor.start();
// logs { count: 0 }

countActor.send({ type: 'increment' });
// logs { count: 1 }

countActor.send({ type: 'decrement' });
// logs { count: 0 }

Transition actor input

You can pass in input to a transition actor, which will be passed to the function that resolves the initial state.

import { fromTransition, createActor } from 'xstate';

const countLogic = fromTransition(
  (state, event) => {
    // ...
  },
  ({ input }: { input: number }) => ({
    count: input, // Initial state
  }),
);

const countActor = createActor(countLogic, {
  input: 42,
});

countActor.subscribe((snapshot) => {
  console.log(snapshot.context);
  // logs { count: 42 }
});

countActor.start();