🦉 Hooks 🦉

Content

Overview
Example: Mouse Position
Example: Autofocus
Reference

Overview

Hooks were popularised by React as a way to solve the following issues:

help reusing stateful logic between components
help organizing code by feature in complex components
use state in functional components, without writing a class.

Owl hooks serve the same purpose, except that they work for class components (note: React hooks do not work on class components, and maybe because of that, there seems to be the misconception that hooks are in opposition to class. This is clearly not true, as shown by Owl hooks).

Hooks work beautifully with Owl components: they solve the problems mentioned above, and in particular, they are the perfect way to make your component reactive.

Example: mouse position

Here is the classical example of a non trivial hook to track the mouse position.

const { useState, onMounted, onWillUnmount } = owl.hooks;

// We define here a custom behaviour: this hook tracks the state of the mouse
// position
function useMouse() {
  const position = useState({ x: 0, y: 0 });

  function update(e) {
    position.x = e.clientX;
    position.y = e.clientY;
  }
  onMounted(() => {
    window.addEventListener("mousemove", update);
  });
  onWillUnmount(() => {
    window.removeEventListener("mousemove", update);
  });

  return position;
}

// Main root component
class App extends owl.Component {
  static template = xml`
    <div t-name="App">
      <div>Mouse: <t t-esc="mouse.x"/>, <t t-esc="mouse.y"/></div>
    </div>`;

  // this hooks is bound to the 'mouse' property.
  mouse = useMouse();
}

Note that we use the prefix use for hooks, just like in React. This is just a convention.

Example: autofocus

Hooks can be combined to create the desired effect. For example, the following hook combines the useRef hook with the onPatched and onMounted functions to create an easy way to focus an input whenever it appears in the DOM:

function useAutofocus(name) {
  let ref = useRef(name);
  let isInDom = false;
  function updateFocus() {
    if (!isInDom && ref.el) {
      isInDom = true;
      ref.el.focus();
    } else if (isInDom && !ref.el) {
      isInDom = false;
    }
  }
  onPatched(updateFocus);
  onMounted(updateFocus);
}

This hook takes the name of a valid t-ref directive, which should be present in the template. It then checks whenever the component is mounted or patched if the reference is not valid, and in this case, it will focus the node element. This hook can be used like this:

class SomeComponent extends Component {
  static template = xml`
    <div>
        <input />
        <input t-ref="myinput"/>
    </div>`;

  constructor(...args) {
    super(...args);
    useAutofocus("myinput");
  }
}

Reference

One rule

There is only one rule: every hook for a component has to be called in the constructor (or in class fields):

// ok
class SomeComponent extends Component {
  state = useState({ value: 0 });
}

// also ok
class SomeComponent extends Component {
  constructor(...args) {
    super(...args);
    this.state = useState({ value: 0 });
  }
}

// not ok: this is executed after the constructor is called
class SomeComponent extends Component {
  async willStart() {
    this.state = useState({ value: 0 });
  }
}

As you can see, the useState hook does not need to be given a reference to the component. This is possible because there is a way to get a reference to the current component: the Component.current static property is the reference to the component instance that is currently being created.

Hooks need to be called in the constructor to ensure that this reference is properly set. This is also a good thing for performance reasons (Owl can use this to optimize its implementation), and for a clean architecture (this makes it easier for developers to understand what is really happening in a component).

`useState`

The useState hook is certainly the most important hook for Owl components: this is what allows a component to be reactive, to react to state change.

The useState hook has to be given an object or an array, and will return an observed version of it (using a Proxy).

const { useState } = owl.hooks;

class Counter extends owl.Component {
  static template = xml`
    <button t-on-click="increment">
        Click Me! [<t t-esc="state.value"/>]
    </button>`;

  state = useState({ value: 0 });

  increment() {
    this.state.value++;
  }
}

It is important to remember that useState only works with objects or arrays. It is necessary, since Owl needs to react to a change in state.

`onMounted`

onMounted is not a user hook, but is a building block designed to help make useful abstractions. onMounted registers a callback, which will be called when the component is mounted (see example on top of this page).

`onWillUnmount`

onWillUnmount is not a user hook, but is a building block designed to help make useful abstractions. onWillUnmount registers a callback, which will be called when the component is unmounted (see example on top of this page).

`onWillPatch`

onWillPatch is not a user hook, but is a building block designed to help make useful abstractions. onWillPatch registers a callback, which will be called just before the component patched.

`onPatched`

onPatched is not a user hook, but is a building block designed to help make useful abstractions. onPatched registers a callback, which will be called just after the component patched.

`onWillStart`

onWillStart is an asynchronous hook. This means that the function registered in the hook will be run just before the component is first rendered and can return a promise, to express the fact that it is an asynchronous operation.

Note that if there are more than one onWillStart registered callback, then they will all be run in parallel.

It can be used to load some initial data. For example, the following hook will automatically load some data from the server, and return an object that will be ready whenever the component is rendered:

function useLoader() {
  const component = Component.current;
  const record = useState({});
  onWillStart(async () => {
    const recordId = component.props.id;
    Object.assign(record, await fetchSomeRecord(recordId));
  });
  return record;
}

Note that this example does not update the record value whenever props are updated. For that situation, we need to use the onWillUpdateProps hook.

`onWillUpdateProps`

Just like onWillStart, onWillUpdateProps is an asynchronous hook. It is designed to be run whenever the component props are updated. This could be useful to perform some asynchronous task such as fetching updated data.

function useLoader() {
  const component = Component.current;
  const record = useState({});

  async function updateRecord(id) {
    Object.assign(record, await fetchSomeRecord(id));
  }

  onWillStart(() => updateRecord(component.props.id));
  onWillUpdateProps((nextProps) => updateRecord(nextProps.id));

  return record;
}

Note that if there are more than one onWillUpdateProps registered callback, then they will all be run in parallel.

`useContext`

See useContext for reference documentation.

`useRef`

The useRef hook is useful when we need a way to interact with some inside part of a component, rendered by Owl. It can work either on a DOM node, or on a component, tagged by the t-ref directive:

<div>
    <div t-ref="someDiv"/>
    <SubComponent t-ref="someComponent"/>
</div>

In this example, the component will be able to access the div and the component SubComponent using the useRef hook:

class Parent extends Component {
  subRef = useRef("someComponent");
  divRef = useRef("someDiv");

  someMethod() {
    // here, if component is mounted, refs are active:
    // - this.divRef.el is the div HTMLElement
    // - this.subRef.comp is the instance of the sub component
    // - this.subRef.el is the root HTML node of the sub component (i.e. this.subRef.comp.el)
  }
}

As shown by the example above, html elements are accessed by using the el key, and components references are accessed with comp.

Notes:

if used on a component, the reference will be set in the refs variable between willPatch and patched,
on a component, accessing ref.el will get the root node of the component.

The t-ref directive also accepts dynamic values with string interpolation (like the t-attf- and t-component directives). For example,

<div t-ref="component_{{someCondition ? '1' : '2'}}"/>

Here, the references need to be set like this:

this.ref1 = useRef("component_1");
this.ref2 = useRef("component_2");

References are only guaranteed to be active while the parent component is mounted. If this is not the case, accessing el or comp on it will return null.

`useSubEnv`

The environment is sometimes useful to share some common information between all components. But sometimes, we want to scope that knowledge to a subtree.

For example, if we have a form view component, maybe we would like to make some model object available to all sub components, but not to the whole application. This is where the useSubEnv hook may be useful: it lets a component add some information to the environment in a way that only the component and its children can access it:

class FormComponent extends Component {
  constructor(...args) {
    super(...args);
    const model = makeModel();
    useSubEnv({ model });
  }
}

The useSubEnv takes one argument: an object which contains some key/value that will be added to the parent environment. Note that it will extend, not replace the parent environment. And of course, the parent environment will not be affected.

`useExternalListener`

The useExternalListener hook helps solve a very common problem: adding and removing a listener on some target whenever a component is mounted/unmounted. For example, a dropdown menu (or its parent) may need to listen to a click event on window to be closed:

useExternalListener(window, "click", this.closeMenu);

`useStore`

The useStore hook is the entry point for a component to connect to the store. See the store documentation for more information.

`useDispatch`

The useDispatch hook is the way for components to get a reference to the store dispatch function. See the store documentation for more information.

`useGetters`

The useGetters hook is the way for components to get a reference to the store getters. See the store documentation for more information.

Making customized hooks

Hooks are a wonderful way to organize the code of a complex component by feature instead of by lifecycle methods. They are like mixins, except that they can be easily composed together.

But, like every good things in life, hooks should be used with moderation. They are not the solution to every problem.

they may be overkill: if your component needs to perform some action specific to itself (so, the specific code does not need to be shared), there is nothing wrong with a simple class method:

// maybe overkill
class A extends Component {
  constructor(...args) {
    super(...args);
    useMySpecificHook();
  }
}

// ok
class B extends Component {
  constructor(...args) {
    super(...args);
    this.performSpecificTask();
  }
}

Note that the second solution is easier to extend in sub components.

they may be harder to test: if a customized hook injects some external side effect dependency, then it is harder to test without doing some non obvious manipulation. For example, assume that we want to give a reference to a router in a useRouter hook. We could do this:
```
const router = new Router(...);

function useRouter() {
    return router;
}
```
As you can see, this does not hook into the internal of the component. It simply returns a global object, which is difficult to mock.

A better way would be to do something like this: get the reference from the environment.
```
function useRouter() {
  return Component.current.env.router;
}
```
This means that we give control to the application developer to create the router, which is good, so they can set it up, subclass it, ... And then, to test our components, we can just add a mock router in the environment.

Note: the code above makes use of the Component.current property. This is the way hooks are able to get a reference to the component currently being created.