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#[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 />};


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>    }}
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>) -> Htmlwhere    T: PartialEq + Display,{    html! {        <p>            { & }        </p>    }}
#[derive(PartialEq)]struct Foo;
impl Display for Foo {    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {        f.write_str("foo")    }}
// used like thishtml! {    <MyGenericComponent<i32> data=123 />};
// orlet foo = Foo;
html! {    <MyGenericComponent<Foo> data={foo} />};