## Hooks for reading state

### 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

```jsx
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)

    ...
}
```

### useObserve

`useObserve` creates an [observe](../reactivity#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](#useObserveEffect).

```jsx
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 = `${e.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](#useObserve) except that it doesn't run until the component is mounted.

## observer HOC

The `observer` HOC has two benefits:
1. It makes the whole component into an observing context - it automatically tracks observables for changes when `get()` is called, even from within hooks. This can be useful if you don't want to use `use()` or if you want to reuse existing functions in your code that use `get()`, to make them track for changes.
2. Performance: It groups multiple calls to `use()` and `useSelector` into a single hook, which can be a big performance optimization in heavy components with lots of state.

Although you may have heard that HOCs are slow or bad, there is actually no real downside. The performance cost of `observer` and a single `use()` or `useSelector` is exactly the same (and tiny), and `observer` makes components faster with two or more `use()` or `useSelector` calls.

 See [Tracking](../tracking) for more about when it tracks.

```jsx
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>
})
```

## 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 be [tracked for re-rendering](../react-basics) the same as any other observable, so use `use()` to track them.

As with

```jsx
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](../reactivity#computed) observable.

```jsx
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.

```jsx
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.

```jsx
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.

```jsx
import { useMount } from "@legendapp/state/react"

const Component = () => {
    useMount(() => console.log('mounted'))
}
```

### useUnmount

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

```jsx
import { useMount } from "@legendapp/state/react"

const Component = () => {
    useMount(() => console.log('mounted'))
}
```

### useEffectOnce

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

```jsx
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.

```jsx
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.

```jsx
import { useUnmountOnce } from "@legendapp/state/react"

const Component = () => {
    useUnmountOnce(() => console.log('mounted'))
}
```