#[function_component]

#[function_component(_)] turns a normal Rust function into a function component. Functions with the attribute have to return Html and may take a single parameter for the type of props the component should accept. The parameter type needs to be a reference to a Properties type (ex. props: &MyProps). If the function doesn't have any parameters the resulting component doesn't accept any props.

The attribute doesn't replace your original function with a component. You need to provide a name as an input to the attribute which will be the identifier of the component. Assuming you have a function called chat_container and you add the attribute #[function_component(ChatContainer)] you can use the component like this:

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

#[function_component(ChatContainer)]
pub fn chat_container() -> Html {
    html! {
        // chat container impl
    }
}

html! {
    <ChatContainer />
};

Example

With props

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

#[derive(Properties, PartialEq)]
pub struct RenderedAtProps {
    pub time: String,
}

#[function_component(RenderedAt)]
pub fn rendered_at(props: &RenderedAtProps) -> Html {
    html! {
        <p>
            <b>{ "Rendered at: " }</b>
            { &props.time }
        </p>
    }
}

Without props

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

#[function_component(App)]
fn app() -> 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>
    }
}

Generic function components

The #[function_component(_)] attribute also works with generic functions for creating generic components.

use std::fmt::Display;
use yew::{function_component, html, Properties};

#[derive(Properties, PartialEq)]
pub struct Props<T>
where
    T: PartialEq,
{
    data: T,
}

#[function_component(MyGenericComponent)]
pub fn my_generic_component<T>(props: &Props<T>) -> Html
where
    T: PartialEq + Display,
{
    html! {
        <p>
            { &props.data }
        </p>
    }
}

// used like this
html! {
    <MyGenericComponent<i32> data=123 />
};

// or
html! {
    <MyGenericComponent<String> data={"foo".to_string()} />
};