Version: 0.21

Custom Hooks

Defining custom Hooks

The stateful logic of a component can be extracted into reusable functions by creating custom Hooks.

Consider that we wish to create an event listener that listens to an event on the window object.

use yew::prelude::*;
use gloo::events::EventListener;
use gloo::utils::window;
use std::mem::drop;


#[function_component(ShowStorageChanged)]
pub fn show_storage_changed() -> Html {
    let state_storage_changed = use_state(|| false);

    {
        let state_storage_changed = state_storage_changed.clone();
        use_effect(|| {
            let listener = EventListener::new(&window(), "storage", move |_| state_storage_changed.set(true));

            move || { drop(listener); }
        });
    }

    html! { <div>{"Storage Event Fired: "}{*state_storage_changed}</div> }
}

There's one problem with this code: the logic can't be reused by another component. If we build another component that listens to a different event, instead of copying the code, we can move the logic into a custom hook.

We'll start by creating a new function called use_event. The use_ prefix denotes that a function is a hook. This function will take an event target, an event type, and a callback. All hooks must be marked by #[hook] on their function definition.

use web_sys::{Event, EventTarget};
use std::borrow::Cow;
use gloo::events::EventListener;
use yew::prelude::*;

#[hook]
pub fn use_event<E, F>(target: &EventTarget, event_type: E, callback: F)
where
    E: Into<Cow<'static, str>>,
    F: Fn(&Event) + 'static,
{
    todo!()
}

This simple hook can be created by composing built-in hooks. For this example, we'll use the use_effect_with hook, so an event listener can be recreated when the hook arguments change.

use yew::prelude::*;
use web_sys::{Event, EventTarget};
use std::borrow::Cow;
use std::rc::Rc;
use gloo::events::EventListener;

#[hook]
pub fn use_event<E, F>(target: &EventTarget, event_type: E, callback: F)
where
    E: Into<Cow<'static, str>>,
    F: Fn(Event) + 'static,
{
    #[derive(PartialEq, Clone)]
    struct EventDependents {
        target: EventTarget,
        event_type: Cow<'static, str>,
        callback: Callback<Event>,
    }

    let deps = EventDependents {
        target: target.clone(),
        event_type: event_type.into(),
        callback: Callback::from(callback),
    };

    use_effect_with(
        deps,
        |deps| {
            let EventDependents {
                target,
                event_type,
                callback,
            } = deps.clone();

            let listener = EventListener::new(&target, event_type, move |e| {
                callback.emit(e.clone());
            });

            move || {
                drop(listener);
            }
        },
    );
}

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.

View the docs on docs.rs for documentation and hooks directory to see implementations of pre-defined hooks.

Defining custom Hooks​

Defining custom Hooks