@xstate/react

What is @xstate/react?

@xstate/react is a library that provides React hooks for using XState, a state machine and statechart library. It allows you to manage complex state logic in a more structured and predictable way by using finite state machines and statecharts.

What are @xstate/react's main functionalities?

Creating and Using State Machines

This feature allows you to create and use state machines in your React components. The code sample demonstrates a simple toggle button that switches between 'inactive' and 'active' states.

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' } }
  }
});

function ToggleButton() {
  const [state, send] = useMachine(toggleMachine);

  return (
    <button onClick={() => send('TOGGLE')}>
      {state.matches('inactive') ? 'Off' : 'On'}
    </button>
  );
}

Using Context for Extended State

This feature allows you to use context for extended state in your state machines. The code sample demonstrates a counter that increments and decrements a count value stored in the machine's context.

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

const counterMachine = createMachine({
  id: 'counter',
  initial: 'active',
  context: { count: 0 },
  states: {
    active: {
      on: {
        INCREMENT: { actions: 'increment' },
        DECREMENT: { actions: 'decrement' }
      }
    }
  }
}, {
  actions: {
    increment: (context) => context.count++,
    decrement: (context) => context.count--
  }
});

function Counter() {
  const [state, send] = useMachine(counterMachine);

  return (
    <div>
      <p>{state.context.count}</p>
      <button onClick={() => send('INCREMENT')}>Increment</button>
      <button onClick={() => send('DECREMENT')}>Decrement</button>
    </div>
  );
}

Invoking Services

This feature allows you to invoke services (e.g., API calls) within your state machines. The code sample demonstrates a state machine that fetches data from an API and handles loading, success, and failure states.

import { useMachine } from '@xstate/react';
import { createMachine, assign } from 'xstate';

const fetchMachine = createMachine({
  id: 'fetch',
  initial: 'idle',
  context: { data: null, error: null },
  states: {
    idle: { on: { FETCH: 'loading' } },
    loading: {
      invoke: {
        src: 'fetchData',
        onDone: { target: 'success', actions: assign({ data: (context, event) => event.data }) },
        onError: { target: 'failure', actions: assign({ error: (context, event) => event.data }) }
      }
    },
    success: { on: { FETCH: 'loading' } },
    failure: { on: { FETCH: 'loading' } }
  }
}, {
  services: {
    fetchData: () => fetch('/api/data').then(response => response.json())
  }
});

function FetchData() {
  const [state, send] = useMachine(fetchMachine);

  return (
    <div>
      {state.matches('idle') && <button onClick={() => send('FETCH')}>Fetch Data</button>}
      {state.matches('loading') && <p>Loading...</p>}
      {state.matches('success') && <pre>{JSON.stringify(state.context.data, null, 2)}</pre>}
      {state.matches('failure') && <p>Error: {state.context.error}</p>}
    </div>
  );
}

Other packages similar to @xstate/react

redux

Redux is a popular state management library for JavaScript applications. It provides a centralized store for state and uses actions and reducers to manage state changes. Unlike @xstate/react, which uses state machines and statecharts, Redux relies on a more traditional approach with a single global state and pure functions to handle state transitions.

mobx

MobX is a state management library that makes state observable and automatically updates the UI when the state changes. It uses reactive programming principles and provides a more flexible and less boilerplate-heavy approach compared to @xstate/react. While @xstate/react focuses on finite state machines, MobX emphasizes reactivity and simplicity.

recoil

Recoil is a state management library for React that provides a more fine-grained approach to state management. It allows you to create atoms (pieces of state) and selectors (derived state) that can be used in your components. Recoil offers a more granular and flexible way to manage state compared to the structured approach of state machines in @xstate/react.

README

@xstate/react

Quick Start

1. Install xstate and @xstate/react:

npm i xstate @xstate/react

1. Import the useMachine hook:

import { useMachine } from '@xstate/react';
import { Machine } from 'xstate';

const toggleMachine = Machine({
  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>
  );
};

API

useMachine(machine, options?)

A React hook that interprets the given machine and starts a service that runs for the lifetime of the component.

Arguments

• machine - An XState machine.
• options (optional) - Interpreter options OR one of the following Machine Config options: guards, actions, activities, services, delays, immediate, context, or 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)

A React hook that subscribes to state changes from an existing service.

Arguments

• service - An XState service.

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.

useMachine(machine) with @xstate/fsm

A React hook 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/lib/fsm

Arguments

• machine - An XState finite state machine (FSM).
• options - An optional options object.

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/lib/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 = Machine({
  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

Using a switch statement might suffice for a simple, non-hierarchical state machine, but for hierarchical and parallel machines, the state values will be objects, not strings. In this case, it's better to use state.matches(...):

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

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:

// ...

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

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

Resources

State Machines in React