paul42/xstate-svelte

XState tools for Svelte

xstate-svelte

XState is a library for creating, interpreting, and executing finite state machines and statecharts, as well as managing invocations of those machines as actors. This package created to make interactions with XState in Svelte application in an easier way.

Install

npm i -S xstate-svelte

or

yarn add xstate-svelte

How to use

<script>
  import { Machine } from "xstate";
  import { useMachine } from "xstate-svelte";

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

  const { state, send } = useMachine(toggleMachine);
</script>

<main>
  <button on:click={() => send('TOGGLE')}>
    {$state.value === 'inactive'
        ? 'Click to activate'
        : 'Active! Click to deactivate'}
  </button>
</main>

API

useMachine(machine, options?)

A function 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 { 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 function that subscribes to state changes from an existing service.

Arguments

• service - An XState service.

Returns { state, send, service }:

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

useMachine(machine, options?) with @xstate/fsm

A function 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-svelte/fsm

Arguments

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

Returns { 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

<script>
  import { createMachine, assign } from "@xstate/fsm";
  import { useMachine } from "xstate-svelte/dist/fsm";

  const onFetch = () => new Promise(res => res("some data"));

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

  const { state, send } = useMachine(fetchMachine, {
    actions: {
      load: () => {
        onFetch().then(res => {
          send({ type: "RESOLVE", data: res });
        });
      }
    }
  });
</script>

<main>
  { #if $state.matches('idle') }
    <button on:click={() => send('FETCH')}>Fetch</button>
  { :else if $state.matches('loading') }
    <div>Loading...</div>
  { :else if $state.matches('success') }
    <div>
      Success! Data: {$state.context.data}
    </div>
  { /if }
</main>

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:

<script>
  import { createEventDispatcher } from "svelte";
  import { Machine, assign } from "xstate";
  import { useMachine } from "xstate-svelte";

  const dispatch = createEventDispatcher();

  const onFetch = () => new Promise(res => res("some data"));

  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 { state, send } = useMachine(fetchMachine, {
    actions: {
      notifySuccess: ctx => dispatch("resolve", ctx.data)
    },
    services: {
      fetchData: (_, e) => fetch(`some/api/${e.query}`).then(res => res.json())
    }
  });
</script>

<main>
  { #if $state.matches('idle') }
    <button on:click={() => send('FETCH', { query: 'something' })}>Fetch</button>
  { :else if $state.matches('loading') }
    <div>Loading...</div>
  { :else if $state.matches('success') }
    <div>
      Success! Data: {$state.context.data}
    </div>
  { :else if $state.matches('failure') }
    <p>{$state.context.error.message}</p>
    <button onClick={() => send('RETRY')}>Retry</button>
  { /if }
</main>

Persisted and Rehydrated State

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

<script>
  import { useMachine } from "xstate-svelte";

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

  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
</script>

Acknowledgments

This module inspired by @xstate/react