yew/website/docs/concepts/function-components/pre-defined-hooks.md

---
title: "Pre-defined Hooks"
description: "The pre-defined Hooks that Yew comes with "
---

## `use_state`

`use_state` is used to manage state in a function component.
It returns a `UseStateHandle` object which `Deref`s to the current value
and provides a `set` method to update the value.

The hook takes a function as input which determines the initial state.
This value remains up-to-date on subsequent renders.

The setter function is guaranteed to be the same across the entire
component lifecycle. You can safely omit the `UseStateHandle` from the
dependents of `use_effect_with_deps` if you only intend to set
values from within the hook.

This hook will always trigger a re-render upon receiving a new state. See
[`use_state_eq`](#use_state_eq) if you want the component to only
re-render when the state changes.

### Example

```rust
use yew::{Callback, function_component, html, use_state};

#[function_component(UseState)]
fn state() -> Html {
    let counter = use_state(|| 0);
    let onclick = {
        let counter = counter.clone();
        Callback::from(move |_| counter.set(*counter + 1))
    };


    html! {
        <div>
            <button {onclick}>{ "Increment value" }</button>
            <p>
                <b>{ "Current value: " }</b>
                { *counter }
            </p>
        </div>
    }
}
```

:::caution

The value held in the handle will reflect the value at the time the
handle is returned by the `use_state`. It is possible that the handle
does not dereference to an up to date value if you are moving it into a
`use_effect_with_deps` hook. You can register the
state to the dependents so the hook can be updated when the value changes.

:::

## `use_state_eq`

This hook has the same effect as `use_state` but will only trigger a
re-render when the setter receives a value that `prev_state != next_state`.

This hook requires the state object to implement `PartialEq`.

## `use_ref`
`use_ref` is used for obtaining a mutable reference to a value.
Its state persists across renders.

It is important to note that you do not get notified of state changes.
If you need the component to be re-rendered on state change, consider using [`use_state`](#use_state).

### Example

```rust
use web_sys::HtmlInputElement;
use yew::{
    events::Event,
    function_component, html, use_ref, use_state,
    Callback, TargetCast,
};

#[function_component(UseRef)]
fn ref_hook() -> Html {
    let message = use_state(|| "".to_string());
    let message_count = use_ref(|| 0);

    let onclick = Callback::from(move |_| {
        let window = yew::utils::window();

        if *message_count.borrow_mut() > 3 {
            window.alert_with_message("Message limit reached").unwrap();
        } else {
            *message_count.borrow_mut() += 1;
            window.alert_with_message("Message sent").unwrap();
        }
    });

    let onchange = {
        let message = message.clone();
        Callback::from(move |e: Event| {
            let input: HtmlInputElement = e.target_unchecked_into();
            message.set(input.value());
        })
    };

    html! {
        <div>
            <input {onchange} value={(*message).clone()} />
            <button {onclick}>{ "Send" }</button>
        </div>
    }
}
```

## `use_reducer`

`use_reducer` is an alternative to [`use_state`](#use_state). It is used to handle component's state and is used
when complex actions needs to be performed on said state.

It accepts an initial state function and returns a `UseReducerHandle` that dereferences to the state,
and a dispatch function.
The dispatch function takes one argument of type `Action`. When called, the action and current value
are passed to the reducer function which computes a new state which is returned,
and the component is re-rendered.

The dispatch function is guaranteed to be the same across the entire
component lifecycle. You can safely omit the `UseReducerHandle` from the
dependents of `use_effect_with_deps` if you only intend to dispatch
values from within the hooks.

The state object returned by the initial state function is required to
implement a `Reducible` trait which provides an `Action` type and a
reducer function.

This hook will always trigger a re-render upon receiving an action. See
[`use_reducer_eq`](#use_reducer_eq) if you want the component to only
re-render when the state changes.

### Example

```rust
use yew::prelude::*;
use std::rc::Rc;

/// reducer's Action
enum CounterAction {
    Double,
    Square,
}

/// reducer's State
struct CounterState {
    counter: i32,
}

impl Default for CounterState {
    fn default() -> Self {
        Self { counter: 1 }
    }
}

impl Reducible for CounterState {
    /// Reducer Action Type
    type Action = CounterAction;

    /// Reducer Function
    fn reduce(self: Rc<Self>, action: Self::Action) -> Rc<Self> {
        let next_ctr = match action {
            CounterAction::Double => self.counter * 2,
            CounterAction::Square => self.counter.pow(2)
        };

        Self { counter: next_ctr }.into()
    }
}

#[function_component(UseReducer)]
fn reducer() -> Html {
    // The use_reducer hook takes an initialization function which will be called only once.
    let counter = use_reducer(CounterState::default);

   let double_onclick = {
        let counter = counter.clone();
        Callback::from(move |_| counter.dispatch(CounterAction::Double))
    };
    let square_onclick = {
        let counter = counter.clone();
        Callback::from(move |_| counter.dispatch(CounterAction::Square))
    };

    html! {
        <>
            <div id="result">{ counter.counter }</div>

            <button onclick={double_onclick}>{ "Double" }</button>
            <button onclick={square_onclick}>{ "Square" }</button>
        </>
    }
}
```

:::caution

The value held in the handle will reflect the value of at the time the
handle is returned by the `use_reducer`. It is possible that the handle does
not dereference to an up to date value if you are moving it into a
`use_effect_with_deps` hook. You can register the
state to the dependents so the hook can be updated when the value changes.

:::

## `use_reducer_eq`

This hook has the same effect as `use_reducer` but will only trigger a
re-render when the reducer function produces a value that `prev_state != next_state`.

This hook requires the state object to implement `PartialEq` in addition
to the `Reducible` trait required by `use_reducer`.

## `use_effect`

`use_effect` is used for hooking into the component's lifecycle.
Similar to `rendered` from the `Component` trait,
`use_effect` takes a function which is called after the render finishes.

The input function has to return a closure, the destructor, which is called when the component is destroyed.
The destructor can be used to clean up the effects introduced and it can take ownership of values to delay dropping them until the component is destroyed.

### Example

```rust
use yew::{Callback, function_component, html, use_effect, use_state};

#[function_component(UseEffect)]
fn effect() -> Html {
    let counter = use_state(|| 0);

    {
        let counter = counter.clone();
        use_effect(move || {
            // Make a call to DOM API after component is rendered
            yew::utils::document().set_title(&format!("You clicked {} times", *counter));

            // Perform the cleanup
            || yew::utils::document().set_title("You clicked 0 times")
        });
    }
    let onclick = {
        let counter = counter.clone();
        Callback::from(move |_| counter.set(*counter + 1))
    };

    html! {
        <button {onclick}>{ format!("Increment to {}", *counter) }</button>
    }
}
```

### `use_effect_with_deps`

Sometimes, it's needed to manually define dependencies for [`use_effect`](#use_effect). In such cases, we use `use_effect_with_deps`.
```rust ,no_run
use yew::use_effect_with_deps;

use_effect_with_deps(
    move |_| {
        // ...
        || ()
    },
    (), // dependents
);
```

**Note**: `dependents` must implement `PartialEq`.

## `use_context`

`use_context` is used for consuming [contexts](../contexts.md) in function components.


### Example

```rust
use yew::{ContextProvider, function_component, html, use_context, use_state};


/// App theme
#[derive(Clone, Debug, PartialEq)]
struct Theme {
    foreground: String,
    background: String,
}

/// Main component
#[function_component(App)]
pub fn app() -> Html {
    let ctx = use_state(|| Theme {
        foreground: "#000000".to_owned(),
        background: "#eeeeee".to_owned(),
    });

    html! {
        // `ctx` is type `Rc<UseStateHandle<Theme>>` while we need `Theme`
        // so we deref it.
        // It derefs to `&Theme`, hence the clone
        <ContextProvider<Theme> context={(*ctx).clone()}>
            // Every child here and their children will have access to this context.
            <Toolbar />
        </ContextProvider<Theme>>
    }
}

/// The toolbar.
/// This component has access to the context
#[function_component(Toolbar)]
pub fn toolbar() -> Html {
    html! {
        <div>
            <ThemedButton />
        </div>
    }
}

/// Button placed in `Toolbar`.
/// As this component is a child of `ThemeContextProvider` in the component tree, it also has access to the context.
#[function_component(ThemedButton)]
pub fn themed_button() -> Html {
    let theme = use_context::<Theme>().expect("no ctx found");

    html! {
        <button style={format!("background: {}; color: {};", theme.background, theme.foreground)}>
            { "Click me!" }
        </button>
    }
}
```