Function use_callback

pub fn use_callback<'hook, IN, OUT, F, D>(
    deps: D,
    f: F,
) -> impl 'hook + Hook<Output = Callback<IN, OUT>>
where
    IN: 'static + 'hook,
    OUT: 'static + 'hook,
    F: Fn(IN, &D) -> OUT + 'static + 'hook,
    D: PartialEq + 'static + 'hook,

Get a immutable reference to a memoized Callback. Its state persists across renders. It will be recreated only if any of the dependencies changes value.

Memoization means it will only get recreated when provided dependencies update/change. This is useful when passing callbacks to optimized child components that rely on PartialEq to prevent unnecessary renders.

Example

#[derive(Properties, PartialEq)]
pub struct Props {
    pub callback: Callback<String, String>,
}

#[function_component(MyComponent)]
fn my_component(props: &Props) -> Html {
    let greeting = props.callback.emit("Yew".to_string());

    html! {
        <>{ &greeting }</>
    }
}

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

    // This callback depends on (), so it's created only once, then MyComponent
    // will be rendered only once even when you click the button multiple times.
    let callback = use_callback((), move |name, _| format!("Hello, {}!", name));

    // It can also be used for events, this callback depends on `counter`.
    let oncallback = use_callback(counter.clone(), move |_e, counter| {
        let _ = **counter;
    });

    html! {
        <div>
            <button {onclick}>{ "Increment value" }</button>
            <button onclick={oncallback}>{ "Callback" }</button>
            <p>
                <b>{ "Current value: " }</b>
                { *counter }
            </p>
            <MyComponent {callback} />
        </div>
    }
}

Note

When used in function components and hooks, this hook is equivalent to:

pub fn use_callback<IN, OUT, F, D>(deps: D, f: F) -> Callback<IN, OUT>
where
    IN: 'static,
    OUT: 'static,
    F: Fn(IN, &D) -> OUT + 'static,
    D: PartialEq + 'static,
{
    /* implementation omitted */
}