Callbacks

Callbacks are used to asynchronously communicate upwards the components tree and with other things like agents or the DOM during event handling. Internally their type is just an Fn wrapped in Rc to allow them to be cheaply cloned.

They have an emit function if you want to call them manually.

use yew::{html, Component, Context, Html, Callback};

let cb: Callback<String, String> = Callback::from(move |name: String| {
    format!("Bye {}", name)
});

let result = cb.emit(String::from("Bob")); // call the callback
// web_sys::console::log_1(&result.into()); // if uncommented will print "Bye Bob"

Passing callbacks as props

A common pattern in yew is to create a callback and pass it down as a prop.

use yew::{function_component, html, Html, Properties, Callback};

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

#[function_component]
fn HelloWorld(props: &Props) -> Html {

    props.on_name_entry.emit(String::from("Bob"));

    html! { "Hello" }
}

// Then supply the prop
#[function_component]
fn App() -> Html {
    let on_name_entry: Callback<String> = Callback::from(move |name: String| {
        let greeting = format!("Hey, {}!", name);
        // web_sys::console::log_1(&greeting.into()); // if uncommented will print
    });

    html! { <HelloWorld {on_name_entry} /> }
}

DOM Events and Callbacks

Callbacks are also used to hook into DOM events.

For example here we define a callback that will be called when the user clicks the button:

use yew::{function_component, html, Html, Properties, Callback};

#[function_component]
fn App() -> Html {
    let onclick = Callback::from(move |_| {
        let greeting = String::from("Hi there");
        // web_sys::console::log_1(&greeting.into()); // if uncommented will print
    });

    html! {
        <button {onclick}>{ "Click" }</button>
    }
}