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Custom Hooks

Defining custom Hooks#

Component's stateful logic can be extracted into usable function by creating custom Hooks.

Consider that we have a component which subscribes to an agent and displays the messages sent to it.

use yew::{function_component, html, use_effect, use_state, Callback};use yew_agent::Bridged;// EventBus is an implementation yew_agent::Agentuse website_test::agents::EventBus;

#[function_component(ShowMessages)]pub fn show_messages() -> Html {    let state = use_state(Vec::new);
    {        let state = state.clone();        use_effect(move || {            let producer = EventBus::bridge(Callback::from(move |msg| {                let mut messages = (*state).clone();                messages.push(msg);                state.set(messages)            }));
            || drop(producer)        });    }
    let output = state.iter().map(|it| html! { <p>{ it }</p> });    html! { <div>{ for output }</div> }}

There's one problem with this code: the logic can't be reused by another component. If we build another component which keeps track of the messages, instead of copying the code we can move the logic into a custom hook.

We'll start by creating a new function called use_subscribe. The use_ prefix conventionally denotes that a function is a hook. This function will take no arguments and return Rc<RefCell<Vec<String>>>.

use std::{cell::RefCell, rc::Rc};
fn use_subscribe() -> Rc<RefCell<Vec<String>>> {    todo!()}

This is a simple hook which can be created by combining other hooks. For this example, we'll two pre-defined hooks. We'll use use_state hook to store the Vec for messages, so they persist between component re-renders. We'll also use use_effect to subscribe to the EventBus Agent so the subscription can be tied to component's lifecycle.

use std::collections::HashSet;use yew::{use_effect, use_state, Callback};use yew_agent::Bridged;// EventBus is an implementation yew_agent::Agentuse website_test::agents::EventBus;
fn use_subscribe() -> Vec<String> {    let state = use_state(Vec::new);
    let effect_state = state.clone();
    use_effect(move || {        let producer = EventBus::bridge(Callback::from(move |msg| {            let mut messages = (*effect_state).clone();            messages.push(msg);            effect_state.set(messages)        }));        || drop(producer)    });
    (*state).clone()}

Although this approach works in almost all cases, it can't be used to write primitive hooks like the pre-defined hooks we've been using already

Writing primitive hooks#

use_hook function is used to write such hooks. View the docs on docs.rs for the documentation and hooks directory to see implementations of pre-defined hooks.