# @xstate/react

The @xstate/react package (opens new window) contains utilities for using XState (opens new window) with React (opens new window).

# Quick Start

  1. Install xstate and @xstate/react:
npm i xstate @xstate/react

Via CDN

<script src="https://unpkg.com/@xstate/react/dist/xstate-react.umd.min.js"></script>

By using the global variable XStateReact

or

<script src="https://unpkg.com/@xstate/react/dist/xstate-react-fsm.umd.min.js"></script>

By using the global variable XStateReactFSM

  1. Import the useMachine hook:
import { useMachine } from '@xstate/react';
import { createMachine } from 'xstate';

const toggleMachine = createMachine({
  id: 'toggle',
  initial: 'inactive',
  states: {
    inactive: {
      on: { TOGGLE: 'active' }
    },
    active: {
      on: { TOGGLE: 'inactive' }
    }
  }
});

export const Toggler = () => {
  const [state, send] = useMachine(toggleMachine);

  return (
    <button onClick={() => send('TOGGLE')}>
      {state.value === 'inactive'
        ? 'Click to activate'
        : 'Active! Click to deactivate'}
    </button>
  );
};

# Examples

# API

# useMachine(machine, options?)

A React hook (opens new window) that interprets the given machine and starts a service that runs for the lifetime of the component.

Arguments

  • machine - An XState machine (opens new window) or a function that lazily returns a machine:

    // existing machine
    const [state, send] = useMachine(machine);
    
    // lazily-created machine
    const [state, send] = useMachine(() =>
      createMachine({
        /* ... */
      })
    );
    
  • options (optional) - Interpreter options (opens new window) and/or any of the following machine config options: guards, actions, services, delays, immediate, context, state.

Returns a tuple of [state, send, service]:

  • state - Represents the current state of the machine as an XState State object.
  • send - A function that sends events to the running service.
  • service - The created service.

# useService(service)

Deprecated

In the next major version, useService(service) will be replaced with useActor(service). Prefer using the useActor(service) hook for services instead, since services are also actors.

Also, keep in mind that only a single argument (the event object) can be sent to send(eventObject) from useActor(...). When migrating to useActor(...), refactor send(...) calls to use only a single event object:

const [state, send] = useActor(service);
-send('CLICK', { x: 0, y: 3 });
+send({ type: 'CLICK', x: 0, y: 3 });

A React hook (opens new window) that subscribes to state changes from an existing service (opens new window).

Arguments

Returns a tuple of [state, send]:

  • state - Represents the current state of the service as an XState State object.
  • send - A function that sends events to the running service.

# useActor(actor, getSnapshot?)

A React hook (opens new window) that subscribes to emitted changes from an existing actor (opens new window).

Arguments

  • actor - an actor-like object that contains .send(...) and .subscribe(...) methods.
  • getSnapshot - a function that should return the latest emitted value from the actor.
    • Defaults to attempting to get the actor.state, or returning undefined if that does not exist.
const [state, send] = useActor(someSpawnedActor);

// with custom actors
const [state, send] = useActor(customActor, (actor) => {
  // implementation-specific pseudocode example:
  return actor.getLastEmittedValue();
});

# useInterpret(machine, options?, observer?)

A React hook that returns the service created from the machine with the options, if specified. It starts the service and runs it for the lifetime of the component. This is similar to useMachine; however, useInterpret allows for a custom observer to subscribe to the service.

The useInterpret is useful when you want fine-grained control, e.g. to add logging, or minimize re-renders. In contrast to useMachine that would flush each update from the machine to the React component, useInterpret instead returns a static reference (to just the interpreted machine) which will not rerender when its state changes.

To use a piece of state from the service inside a render, use the useSelector(...) hook to subscribe to it.

Since 1.3.0

Arguments

  • machine - An XState machine (opens new window) or a function that lazily returns a machine.
  • options (optional) - Interpreter options (opens new window) and/or any of the following machine config options: guards, actions, services, delays, immediate, context, state.
  • observer (optional) - an observer or listener that listens to state updates:
    • an observer (e.g., { next: (state) => {/* ... */} })
    • or a listener (e.g., (state) => {/* ... */})
import { useInterpret } from '@xstate/react';
import { someMachine } from '../path/to/someMachine';

const App = () => {
  const service = useInterpret(someMachine);

  // ...
};

With options + listener:

// ...

const App = () => {
  const service = useInterpret(
    someMachine,
    {
      actions: {
        /* ... */
      }
    },
    (state) => {
      // subscribes to state changes
      console.log(state);
    }
  );

  // ...
};

# useSelector(actor, selector, compare?, getSnapshot?)

A React hook that returns the selected value from the snapshot of an actor, such as a service. This hook will only cause a rerender if the selected value changes, as determined by the optional compare function.

Since 1.3.0

Arguments

  • actor - a service or an actor-like object that contains .send(...) and .subscribe(...) methods.
  • selector - a function that takes in an actor's "current state" (snapshot) as an argument and returns the desired selected value.
  • compare (optional) - a function that determines if the current selected value is the same as the previous selected value.
  • getSnapshot (optional) - a function that should return the latest emitted value from the actor.
    • Defaults to attempting to get the actor.state, or returning undefined if that does not exist. Will automatically pull the state from services.
import { useSelector } from '@xstate/react';

// tip: optimize selectors by defining them externally when possible
const selectCount = (state) => state.context.count;

const App = ({ service }) => {
  const count = useSelector(service, selectCount);

  // ...
};

With compare function:

// ...

const selectUser = (state) => state.context.user;
const compareUser = (prevUser, nextUser) => prevUser.id === nextUser.id;

const App = ({ service }) => {
  const user = useSelector(service, selectUser, compareUser);

  // ...
};

With useInterpret(...):

import { useInterpret, useSelector } from '@xstate/react';
import { someMachine } from '../path/to/someMachine';

const selectCount = (state) => state.context.count;

const App = ({ service }) => {
  const service = useInterpret(someMachine);
  const count = useSelector(service, selectCount);

  // ...
};

# asEffect(action)

Ensures that the action is executed as an effect in useEffect, rather than being immediately executed.

Arguments

  • action - An action function (e.g., (context, event) => { alert(context.message) }))

Returns a special action function that wraps the original so that useMachine knows to execute it in useEffect.

Example

const machine = createMachine({
  initial: 'focused',
  states: {
    focused: {
      entry: 'focus'
    }
  }
});

const Input = () => {
  const inputRef = useRef(null);
  const [state, send] = useMachine(machine, {
    actions: {
      focus: asEffect((context, event) => {
        inputRef.current && inputRef.current.focus();
      })
    }
  });

  return <input ref={inputRef} />;
};

# asLayoutEffect(action)

Ensures that the action is executed as an effect in useLayoutEffect, rather than being immediately executed.

Arguments

  • action - An action function (e.g., (context, event) => { alert(context.message) }))

Returns a special action function that wraps the original so that useMachine knows to execute it in useLayoutEffect.

# useMachine(machine) with @xstate/fsm

A React hook (opens new window) that interprets the given finite state machine from [@xstate/fsm] and starts a service that runs for the lifetime of the component.

This special useMachine hook is imported from @xstate/react/fsm

Arguments

Returns a tuple of [state, send, service]:

  • state - Represents the current state of the machine as an @xstate/fsm StateMachine.State object.
  • send - A function that sends events to the running service.
  • service - The created @xstate/fsm service.

Example

import { useEffect } from 'react';
import { useMachine } from '@xstate/react/fsm';
import { createMachine } from '@xstate/fsm';

const context = {
  data: undefined
};
const fetchMachine = createMachine({
  id: 'fetch',
  initial: 'idle',
  context,
  states: {
    idle: {
      on: { FETCH: 'loading' }
    },
    loading: {
      entry: ['load'],
      on: {
        RESOLVE: {
          target: 'success',
          actions: assign({
            data: (context, event) => event.data
          })
        }
      }
    },
    success: {}
  }
});

const Fetcher = ({
  onFetch = () => new Promise((res) => res('some data'))
}) => {
  const [state, send] = useMachine(fetchMachine, {
    actions: {
      load: () => {
        onFetch().then((res) => {
          send({ type: 'RESOLVE', data: res });
        });
      }
    }
  });

  switch (state.value) {
    case 'idle':
      return <button onClick={(_) => send('FETCH')}>Fetch</button>;
    case 'loading':
      return <div>Loading...</div>;
    case 'success':
      return (
        <div>
          Success! Data: <div data-testid="data">{state.context.data}</div>
        </div>
      );
    default:
      return null;
  }
};

# Configuring Machines

Existing machines can be configured by passing the machine options as the 2nd argument of useMachine(machine, options).

Example: the 'fetchData' service and 'notifySuccess' action are both configurable:

const fetchMachine = createMachine({
  id: 'fetch',
  initial: 'idle',
  context: {
    data: undefined,
    error: undefined
  },
  states: {
    idle: {
      on: { FETCH: 'loading' }
    },
    loading: {
      invoke: {
        src: 'fetchData',
        onDone: {
          target: 'success',
          actions: assign({
            data: (_, event) => event.data
          })
        },
        onError: {
          target: 'failure',
          actions: assign({
            error: (_, event) => event.data
          })
        }
      }
    },
    success: {
      entry: 'notifySuccess',
      type: 'final'
    },
    failure: {
      on: {
        RETRY: 'loading'
      }
    }
  }
});

const Fetcher = ({ onResolve }) => {
  const [state, send] = useMachine(fetchMachine, {
    actions: {
      notifySuccess: (ctx) => onResolve(ctx.data)
    },
    services: {
      fetchData: (_, e) =>
        fetch(`some/api/${e.query}`).then((res) => res.json())
    }
  });

  switch (state.value) {
    case 'idle':
      return (
        <button onClick={() => send('FETCH', { query: 'something' })}>
          Search for something
        </button>
      );
    case 'loading':
      return <div>Searching...</div>;
    case 'success':
      return <div>Success! Data: {state.context.data}</div>;
    case 'failure':
      return (
        <>
          <p>{state.context.error.message}</p>
          <button onClick={() => send('RETRY')}>Retry</button>
        </>
      );
    default:
      return null;
  }
};

# Matching States

When using hierarchical (opens new window) and parallel (opens new window) machines, the state values will be objects, not strings. In this case, it is best to use state.matches(...) (opens new window).

We can do this with if/else if/else blocks:

// ...
if (state.matches('idle')) {
  return /* ... */;
} else if (state.matches({ loading: 'user' })) {
  return /* ... */;
} else if (state.matches({ loading: 'friends' })) {
  return /* ... */;
} else {
  return null;
}

We can also continue to use switch, but we must make an adjustment to our approach. By setting the expression of the switch to true, we can use state.matches(...) (opens new window) as a predicate in each case:

switch (true) {
  case state.matches('idle'):
    return /* ... */;
  case state.matches({ loading: 'user' }):
    return /* ... */;
  case state.matches({ loading: 'friends' }):
    return /* ... */;
  default:
    return null;
}

A ternary statement can also be considered, especially within rendered JSX:

const Loader = () => {
  const [state, send] = useMachine(/* ... */);

  return (
    <div>
      {state.matches('idle') ? (
        <Loader.Idle />
      ) : state.matches({ loading: 'user' }) ? (
        <Loader.LoadingUser />
      ) : state.matches({ loading: 'friends' }) ? (
        <Loader.LoadingFriends />
      ) : null}
    </div>
  );
};

# Persisted and Rehydrated State

You can persist and rehydrate state with useMachine(...) via options.state:

// ...

// Get the persisted state config object from somewhere, e.g. localStorage
const persistedState = JSON.parse(localStorage.getItem('some-persisted-state-key')) || someMachine.initialState;

const App = () => {
  const [state, send] = useMachine(someMachine, {
    state: persistedState // provide persisted state config object here
  });

  // state will initially be that persisted state, not the machine's initialState

  return (/* ... */)
}

# Services

The service created in useMachine(machine) can be referenced as the third returned value:

//                  vvvvvvv
const [state, send, service] = useMachine(someMachine);

You can subscribe to that service's state changes with the useEffect hook (opens new window):

// ...

useEffect(() => {
  const subscription = service.subscribe((state) => {
    // simple state logging
    console.log(state);
  });

  return subscription.unsubscribe;
}, [service]); // note: service should never change

# Migration from 0.x

  • For spawned actors created using invoke or spawn(...), use the useActor() hook instead of useService():

    -import { useService } from '@xstate/react';
    +import { useActor } from '@xstate/react';
    
    -const [state, send] = useService(someActor);
    +const [state, send] = useActor(someActor);
    

# Resources

State Machines in React (opens new window)

Last Updated: 11/30/2021, 10:58:48 PM