React API

Reading state

observer

observer is the suggested way of consuming observables for the best performance and safety.

It makes the whole component into an observing context - it automatically tracks observables for changes when get() is called, even from within hooks or helper functions.

See Tracking for more about when it tracks.

import { observable } from "@legendapp/state"
import { observer } from "@legendapp/state/react"

const state$ = observable({ count: 0 })

const Component = observer(function Component() {
  // Accessing state automatically makes this component track changes to re-render
  const count = state$.count.get()

  // Re-renders whenever count changes
  return <div>{count}</div>
});

useSelector

useSelector computes a value and automatically listens to any observables accessed while running, and only re-renders if the computed value changes.

Props:

• selector: Observable or computation function that listens to observables accessed while running
• options: { suspense: boolean }: Enable suspense when the value is a Promise and you’re using it within React.Suspense.

import { observable } from "@legendapp/state"
import { useSelector } from "@legendapp/state/react"

const state$ = observable({ selected: 1, theme })

const Component = ({ id }) => {
    // Only re-renders if the return value changes
    const isSelected = useSelector(() => id === state$.selected.get())

    // Get the raw value of an observable and listen to it
    const theme = useSelector(state$.theme)

    ...
}

Using with React Suspense

Using { suspense: true } as the second parameter makes the component work with Suspense. If the observable is a Promise, Suspense will render the fallback until it resolves to a value.

import { useObservable, useSelector } from "@legendapp/state/react";
import { Suspense } from "react";

function Test({ state$ }) {
  const value = useSelector(state$, { suspense: true });
  return <div>{value}</div>;
}

export default function App() {
  const state$ = useObservable(
    new Promise((resolve) => {
      setTimeout(() => {
        resolve("hello");
      }, 1000);
    })
  );
  return (
    <div>
      <div>Suspense test</div>
      <Suspense fallback={<div>Loading...</div>}>
        <Test state$={state$} />
      </Suspense>
    </div>
  );
}

useObserve

useObserve creates an observe which you can use to take actions when observables change. This can be effectively similar to useEffect for observables, except that it runs when observables change and not because of a deps array changing.

Like observe, useObserve has an optional second callback parameter which will run after the selector, and does not track changes. This can be useful for observing an event or a single observable.

Note that useObserve runs during component render, not after render like useEffect. If you want an observer that runs after render, see useObserveEffect.

import { event } from "@legendapp/state";
import { useObserve, useObservable, Reactive } from "@legendapp/state/react";

const eventUpdateTitle = event();

function ProfilePage() {
  const profile$ = useObservable({ name: "" });

  // This runs whenever profile changes
  useObserve(() => {
    document.title = `${profile$.name.get()} - Profile`;
  });

  // Observe a single observable with a callback when it changes
  useObserve(profile$.name, ({ value }) => {
    document.title = `${value} - Profile`;
  });

  // Observe an event with a callback when it changes
  useObserve(eventUpdateTitle, () => {
    document.title = `${profile$.name.get()} - Profile`;
  });

  return (
    <div>
      <span>Name:</span>
      <Reactive.input $value={profile$.name} />
    </div>
  );
}

useObserveEffect

useObserveEffect is the same as useObserve except that it doesn’t run until the component is mounted.

Hooks for creating local state

useObservable

The useObservable hook can be used to create an observable within a React component. This can be useful when state is specific to the lifetime of the component, or to hold multiple values in local state.

Its observables will not be automatically tracked for re-rendering, so you can track them the same as any other observable.

As with

import { observer, useObservable } from "@legendapp/state/react"

const Component = function Component() {
    const state$ = useObservable({
        title: 'Title',
        first: '',
        last: '',
        profile: {...}
    })

    const fullname$ = useObservable(() => `${state$.fname.get()} ${state$.lname.get()}`)

    return (
        <div>
            <div>{fullname$}</div>
            <Input text={state$.first} />
            <Input text={state$.last} />
            <Profile name={fullname$} />
        </div>
    )
}

useComputed

useComputed is like useObservable and creates a computed observable.

import { observable } from "@legendapp/state";
import { useComputed } from "@legendapp/state/react";

const state$ = observable({ test: 10, test2: 20 });

function Component() {
  const sum = useComputed(() => state$.test.get() + state$.test2.get());

  return <div>Sum: {sum}</div>;
}

useObservableReducer

useObservableReducer works the same way as useReducer but sets an observable rather than triggering a render.

import { useObservableReducer } from "@legendapp/state/react"

const Component = () => {
    // Only re-renders if the return value changes
    const isSelected$ = useObservableReducer()

    // Get the value of the reducer
    const theme = isSelected$.get()

    ...
}

Using with Context

You may prefer passing local state through Context rather than (or in addition to) having a global state. To do that you can simply add the observable to your Context as usual, and consume the Context from child component. The observable itself is a stable object so changing the value of an observable will not cause a re-render.

import { createContext, useContext } from "react";
import { observer, useObservable } from "@legendapp/state/react";

const StateContext = createContext();

function App() {
  const state$ = useObservable({
    profile: {
      name: "",
    },
  });

  return (
    <StateContext.Provider value={state$}>
      <div>
        <Sidebar />
        <Main />
      </div>
    </StateContext.Provider>
  );
}

const Sidebar = function Sidebar() {
  // StateContext will never change so this will never cause a render
  const state$ = useContext(StateContext);

  // This component never re-renders, but name re-renders itself
  return (
    <div>
      Name: <Memo>{state$.profile.name}</Memo>
    </div>
  );
};

Miscellaneous hooks

useMount

Using observable hooks we generally avoid the built-in hooks and dependency arrays, so we have a useMount hook for convenience, which is just useEffect under the hood.

import { useMount } from "@legendapp/state/react";

const Component = () => {
  useMount(() => console.log("mounted"));
};

useUnmount

Like the useMount hook, useUnmount is just useEffect under the hood.

import { useUnmount } from "@legendapp/state/react";

const Component = () => {
  useUnmount(() => console.log("mounted"));
};

useEffectOnce

This is useEffect with a workaround in development mode to make sure it only runs once.

import { useEffectOnce } from "@legendapp/state/react";

const Component = () => {
  useEffectOnce(() => {
    console.log("mounted");
  }, []);
};

useMountOnce

This is useMount with a workaround in development mode to make sure it only runs once.

import { useMountOnce } from "@legendapp/state/react";

const Component = () => {
  useMountOnce(() => console.log("mounted"));
};

useUnmountOnce

This is useEffect with a workaround in development mode to make sure it only runs once.

import { useUnmountOnce } from "@legendapp/state/react";

const Component = () => {
  useUnmountOnce(() => console.log("mounted"));
};

usePauseProvider

This creates a React Context Provider with a paused$ observable. Set paused$ to true to pause all rendering from observable changes under the context, and set it false to resume. This applies to everything within Legend-State like observer, useSelector, Reactive, Memo, etc… But normal renders coming from React or other state is not affected.

import { useInterval } from "usehooks-ts"
import { Memo, usePauseProvider, useObservable } from '@legendapp/state/react'

function App() {
    const { PauseProvider, isPaused$ } = usePauseProvider()

    const int$ = useObservable(0)
    useInterval(() => {
        int$.set((val) => val + 1)
    }, 100)

    return (
        <Box center>
            <Button onClick={isPaused$.toggle}>
                <Memo>{() => (isPaused$.get() ? 'Resume' : 'Pause')}</Memo>
            </Button>
            <PauseProvider>
                <Memo>{int$}</Memo>
            </PauseProvider>
        </Box>
    )
}