owl/doc/hooks.md

# 🦉 Hooks 🦉

## Content

- [Overview](#overview)
- [Example: Mouse Position](#example-mouse-position)
- [Example: Autofocus](#example-autofocus)
- [Reference](#reference)
  - [One Rule](#one-rule)
  - [`useState`](#usestate)
  - [`onMounted`](#onmounted)
  - [`onWillUnmount`](#onwillunmount)
  - [`onWillPatch`](#onwillpatch)
  - [`onPatched`](#onpatched)
  - [`useContext`](#usecontext)
  - [`useRef`](#useref)
  - [`useSubEnv`](#usesubenv)

## 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 works 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.

```js
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:

```js
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:

```js
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 have to be called in the
constructor (or in class fields):

```js
// 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 });
  }
}
```

### `useState`

The `useState` hook is certainly the most important hooks for Owl components:
this is what enables 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`).

```javascript
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++;
  }
}
```

### `onMounted`

`onMounted` is not an 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 an 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 an 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 an 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.

### `useContext`

See [`useContext`](context.md#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:

```xml
<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:

```js
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
  }
}
```

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

Note: if used on a component, the reference will be set in the `refs`
variable between `willPatch` and `patched`.

The `t-ref` directive also accepts dynamic values with string interpolation
(like the [`t-attf-`](qweb.md#dynamic-attributes) and
`t-component` directives). For example,

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

Here, the references needs to be set like this:

```js
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 component, but not to the whole application.
This is where the `useSubEnv` hook may be useful: it let a component add some
information to the environment in a way that only the component and its children
can access it:

```js
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.