🦉 OWL Component 🦉

Content

Overview

OWL components are the building blocks for user interface. They are designed to be:

  1. declarative: the user interface should be described in terms of the state of the application, not as a sequence of imperative steps.

  2. composable: each component can seamlessly be created in a parent component by a simple tag or directive in its template.

  3. asynchronous rendering: the framework will transparently wait for each sub components to be ready before applying the rendering. It uses native promises under the hood.

  4. uses QWeb as a template system: the templates are described in XML and follow the QWeb specification. This is a requirement for Odoo.

OWL components are defined as a subclass of Component. The rendering is exclusively done by a QWeb template (which needs to be preloaded in QWeb). Rendering a component generates a virtual dom representation of the component, which is then patched to the DOM, in order to apply the changes in an efficient way.

Example

Let us have a look at a simple component:

const { useState } = owl.hooks;

class ClickCounter extends owl.Component {
  state = useState({ value: 0 });

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

<button t-name="ClickCounter" t-on-click="increment">
  Click Me! [<t t-esc="state.value"/>]
</button>

Note that this code is written in ESNext style, so it will only run on the latest browsers without a transpilation step.

This example shows how a component should be defined: it simply subclasses the Component class. If no static template key is defined, then Owl will use the component's name as template name. Here, a state object is defined, by using the useState hook. It is not mandatory to use the state object, but it is certainly encouraged. The result of the useState call is observed, and any change to it will cause a rerendering.

Reference

An Owl component is a small class which represents a component or some UI element. It exists in the context of an environment (env), which is propagated from a parent to its children. The environment needs to have a QWeb instance, which will be used to render the component template.

Be aware that the name of the component may be significant: if a component does not define a template key, then Owl will lookup in QWeb to find a template with the component name (or one of its ancestors).

Reactive system

OWL components are normal javascript classes. So, changing a component internal state does nothing more:

class Counter extends Component {
  static template = xml`<div t-on-click="increment"><t t-esc="state.value"/></div>`;
  state = { value: 0 };

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

Clicking on the Counter component defined above will call the increment method, but it will not rerender the component. To fix that, one could add an explicit call to render in increment:

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

However, it may be simple in this case, but it quickly become cumbersome, as a component get more complex, and its internal state is modified by more than one method.

A better way is to use the reactive system: by using the useState hook (see the hooks section for more details), one can make Owl react to state changes. The useState hook generates a proxy version of an object (this is done by an observer), which allows the component to react to any change. So, the Counter example above can be improved like this:

const { useState } = owl.hooks;

class Counter extends Component {
  static template = xml`<div t-on-click="increment"><t t-esc="state.value"/></div>`;
  state = useState({ value: 0 });

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

Obviously, we can call the useState hook more than once:

const { useState } = owl.hooks;

class Counter extends Component {
  static template = xml`
      <div>
        <span t-on-click="increment(counter1)"><t t-esc="counter1.value"/></span>
        <span t-on-click="increment(counter2)"><t t-esc="counter2.value"/></span>
      </div>`;
  counter1 = useState({ value: 0 });
  counter2 = useState({ value: 0 });

  increment(counter) {
    counter.value++;
  }
}

Note that hooks are subject to one important rule: they need to be called in the constructor.

Properties

Static Properties

There is another static property defined on the Component class: current. This property is set to the currently being defined component (in the constructor). This is the way hooks are able to get a reference to the target component.

Methods

We explain here all the public methods of the Component class.

Obviously, these methods are reserved for Owl, and should not be used by Owl users, unless they want to override them. Also, Owl reserves all method names starting with __, in order to prevent possible future conflicts with user code whenever Owl needs to change.

Lifecycle

A solid and robust component system needs useful hooks/methods to help developers write components. Here is a complete description of the lifecycle of a owl component:

Method Description
constructor constructor
willStart async, before first rendering
mounted just after component is rendered and added to the DOM
willUpdateProps async, before props update
willPatch just before the DOM is patched
patched just after the DOM is patched
willUnmount just before removing component from DOM
catchError catch errors (see error handling page)

Notes:

constructor(parent, props)

The constructor is not exactly a hook, it is the regular, normal, constructor of the component. Since it is not a hook, you need to make sure that super is called.

This is usually where you would set the initial state and the template of the component.

  constructor(parent, props) {
    super(parent, props);
    this.state = useState({someValue: true});
    this.template = 'mytemplate';
  }

Note that with ESNext class fields, the constructor method does not need to be implemented in most cases:

class ClickCounter extends owl.Component {
  state = useState({ value: 0 });

  ...
}

willStart()

willStart is an asynchronous hook that can be implemented to perform some action before the initial rendering of a component.

It will be called exactly once before the initial rendering. It is useful in some cases, for example, to load external assets (such as a JS library) before the component is rendered. Another use case is to load data from a server.

  async willStart() {
    await owl.utils.loadJS("my-awesome-lib.js");
  }

At this point, the component is not yet rendered. Note that a slow willStart method will slow down the rendering of the user interface. Therefore, some care should be made to make this method as fast as possible.

mounted()

mounted is called each time a component is attached to the DOM, after the initial rendering and possibly later if the component was unmounted and remounted. At this point, the component is considered active. This is a good place to add some listeners, or to interact with the DOM, if the component needs to perform some measure for example.

It is the opposite of willUnmount. If a component has been mounted, it will always be unmounted at some point in the future.

The mounted method will be called recursively on each of its children. First, the parent, then all its children.

It is allowed (but not encouraged) to modify the state in the mounted hook. Doing so will cause a rerender, which will not be perceptible by the user, but will slightly slow down the component.

willUpdateProps(nextProps)

The willUpdateProps is an asynchronous hook, called just before new props are set. This is useful if the component needs to perform an asynchronous task, depending on the props (for example, assuming that the props are some record Id, fetching the record data).

  willUpdateProps(nextProps) {
    return this.loadData({id: nextProps.id});
  }

This hook is not called during the first render (but willStart is called and performs a similar job).

willPatch()

The willPatch hook is called just before the DOM patching process starts. It is not called on the initial render. This is useful to read information from the DOM. For example, the current position of the scrollbar.

Note that modifying the state is not allowed here. This method is called just before an actual DOM patch, and is only intended to be used to save some local DOM state. Also, it will not be called if the component is not in the DOM.

patched(snapshot)

This hook is called whenever a component did actually update its DOM (most likely via a change in its state/props or environment).

This method is not called on the initial render. It is useful to interact with the DOM (for example, through an external library) whenever the component was patched. Note that this hook will not be called if the component is not in the DOM.

Updating the component state in this hook is possible, but not encouraged. One needs to be careful, because updates here will create an additional rendering, which in turn will cause other calls to the patched method. So, we need to be particularly careful at avoiding endless cycles.

willUnmount()

willUnmount is a hook that is called each time just before a component is unmounted from the DOM. This is a good place to remove listeners, for example.

  mounted() {
    this.env.bus.on('someevent', this, this.doSomething);
  }
  willUnmount() {
    this.env.bus.off('someevent', this, this.doSomething);
  }

This is the opposite method of mounted.

catchError(error)

The catchError method is useful when we need to intercept and properly react to (rendering) errors that occur in some sub components. See the page on error handling.

Root Component

Most of the time, an Owl component will be created automatically by a tag (or the t-component directive) in a template. There is however an obvious exception: the root component of an Owl application has to be created manually:

class App extends owl.Component { ... }

const app = new App();
app.mount(document.body);

The root component does not have a parent nor props (see note below). It will be setup with an environment (either the env defined on its class, or a default empty environment).

Note: a root component can however be given a props object in its constructor, like this: new App(null, {some: 'object'});. It will not be a true props object, managed by Owl (so, for example, it will never be updated).

Composition

The example above shows a QWeb template with a sub component. In a template, components are declared with a tagname corresponding to the class name. It has to be capitalized.

<div t-name="ParentComponent">
  <span>some text</span>
  <MyComponent info="13" />
</div>

class ParentComponent extends owl.Component {
    static components = { MyComponent: MyComponent};
    ...
}

In this example, the ParentComponent's template creates a component MyComponent just after the span. The info key will be added to the subcomponent's props. Each props is a string which represents a javascript (QWeb) expression, so it is dynamic. If it is necessary to give a string, this can be done by quoting it: someString="'somevalue'".

Note that the rendering context for the template is the component itself. This means that the template can access state (if it exists), props, env, or any methods defined in the component.

<div t-name="ParentComponent">
    <ChildComponent count="state.val" />
</div>

class ParentComponent {
  static components = { ChildComponent };
  state = useState({ val: 4 });
}

Whenever the template is rendered, it will automatically create the subcomponent ChildComponent at the correct place. It needs to find the reference to the actual component class in the special static components key, or the class registered in QWeb's global registry (see register function of QWeb). It first looks inside the static components key, then fallbacks on the global registry.

Props: In this example, the child component will receive the object {count: 4} in its constructor. This will be assigned to the props variable, which can be accessed on the component (and also, in the template). Whenever the state is updated, then the sub component will also be updated automatically. See the props section for more information.

CSS and style: Owl allows the parent to declare additional css classes or style for the sub component: css declared in class, style, t-att-class or t-att-style will be added to the root component element.

<div t-name="ParentComponent">
  <MyComponent class="someClass" style="font-weight:bold;" info="13" />
</div>

Warning: there is a small caveat with dynamic class attributes: since Owl needs to be able to add/remove proper classes whenever necessary, it needs to be aware of the possible classes. Otherwise, it will not be able to make the difference between a valid css class added by the component, or other custom code, and a class that need to be removed. This is why we only support the explicit syntax with a class object:

<MyComponent t-att-class="{a: state.flagA, b: state.flagB}" />

Form Input Bindings

It is very common to need to be able to read the value out of an html input (or textarea, or select) in order to use it (note: it does not need to be in a form!). A possible way to do this is to do it by hand:

class Form extends owl.Component {
  state = useState({ text: "" });

  _updateInputValue(event) {
    this.state.text = event.target.value;
  }
}

<div>
  <input t-on-input="_updateInputValue" />
  <span t-esc="state.text" />
</div>

This works. However, this requires a little bit of plumbing code. Also, the plumbing code is slightly different if you need to interact with a checkbox, or with radio buttons, or with select tags.

To help with this situation, Owl has a builtin directive t-model: its value should be an observed value in the component (usually state.someValue). With the t-model directive, we can write a shorter code, equivalent to the previous example:

class Form extends owl.Component {
  state = { text: "" };
}

<div>
  <input t-model="state.text" />
  <span t-esc="state.text" />
</div>

The t-model directive works with <input>, <input type="checkbox">, <input type="radio">, <textarea> and <select>:

<div>
    <div>Text in an input: <input t-model="state.someVal"/></div>
    <div>Textarea: <textarea t-model="state.otherVal"/></div>
    <div>Boolean value: <input type="checkbox" t-model="state.someFlag"/></div>
    <div>Selection:
        <select t-model="state.color">
            <option value="">Select a color</option>
            <option value="red">Red</option>
            <option value="blue">Blue</option>
        </select>
    </div>
    <div>
        Selection with radio buttons:
        <span>
            <input type="radio" name="color" id="red" value="red" t-model="state.color"/>
            <label for="red">Red</label>
        </span>
        <span>
            <input type="radio" name="color" id="blue" value="blue" t-model="state.color" />
            <label for="blue">Blue</label>
        </span>
    </div>
</div>

Like event handling, the t-model directive accepts the following modifiers:

Modifier Description
.lazy update the value on the change event (default is on input event)
.number try to parse the value to a number (using parseFloat)
.trim trim the resulting value

For example:

<input t-model.lazy="state.someVal" />

These modifiers can be combined. For instance, t-model.lazy.number will only update a number whenever the change is done.

Note: the online playground has an example to show how it works.

References

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. See the hooks section for more detail.

As a short example, here is how we could set the focus on a given input:

<div>
    <input t-ref="input"/>
    <button t-on-click="focusInput">Click</button>
</div>

import { useRef } from "owl/hooks";

class SomeComponent extends Component {
  inputRef = useRef("input");

  focusInput() {
    this.inputRef.el.focus();
  }
}

The useRef hook can also be used to get a reference to an instance of a sub component rendered by Owl. In that case, we need to access it with the comp property instead of el:

<div>
    <SubComponent t-ref="sub"/>
    <button t-on-click="doSomething">Click</button>
</div>

import { useRef } from "owl/hooks";

class SomeComponent extends Component {
  static components = { SubComponent };
  subRef = useRef("sub");

  doSomething() {
    this.subRef.comp.doSomeThingElse();
  }
}

Note that these two examples uses the suffix ref to name the reference. This is not mandatory, but it is a useful convention, so we do not forget to access it with the el or comp suffix.

Dynamic sub components

It is not common, but sometimes we need a dynamic component name. In this case, the t-component directive can also be used to accept dynamic values with string interpolation (like the t-attf- directive):

<div t-name="ParentComponent">
    <t t-component="ChildComponent{{id}}" />
</div>

class ParentComponent {
  static components = { ChildComponent1, ChildComponent2 };
  state = { id: 1 };
}

There is an even more dynamic way to use t-component: its value can be an expression evaluating to an actual component class. In that case, this is the class that will be used to create the component:

class A extends Component<any, any, any> {
  static template = xml`<span>child a</span>`;
}
class B extends Component<any, any, any> {
  static template = xml`<span>child b</span>`;
}
class App extends Component<any, any, any> {
  static template = xml`<t t-component="myComponent" t-key="state.child"/>`;

  state = { child: "a" };

  get myComponent() {
    return this.state.child === "a" ? A : B;
  }
}

In this example, the component App selects dynamically the concrete sub component class.

Note that the t-component directive can only be used on <t> nodes.

Functional Components

Owl does not exactly have functional components. However, there is an extremely close alternative: calling sub templates.

A stateless functional component in react is usually some kind of function that maps props to a virtual dom (often with jsx). So, basically, almost like a template rendered with props. In Owl, this can be done by simply defining a template, that will access the props object:

const Welcome = xml`<h1>Hello, {props.name}</h1>`;

class MyComponent extends Component {
  static template = xml`
        <div>
            <t t-call=${Welcome}/>
            <div>something</div>
        </div>
    `;
}

The way this works is that sub templates are inlined, and have access to the ambient context. They can therefore access props, and any other part of the caller component.

SVG Components

Owl components can be used to generate dynamic SVG graphs:

class Node extends Component {
  static template = xml`
        <g>
            <circle t-att-cx="props.x" t-att-cy="props.y" r="4" fill="black"/>
            <text t-att-x="props.x - 5" t-att-y="props.y + 18"><t t-esc="props.node.label"/></text>
            <t t-set="childx" t-value="props.x + 100"/>
            <t t-set="height" t-value="props.height/(props.node.children || []).length"/>
            <t t-foreach="props.node.children || []" t-as="child">
                <t t-set="childy" t-value="props.y + child_index*height"/>
                <line t-att-x1="props.x" t-att-y1="props.y" t-att-x2="childx" t-att-y2="childy" stroke="black" />
                <Node x="childx" y="childy" node="child" height="height"/>
            </t>
        </g>
    `;
  static components = { Node };
}

class RootNode extends Component {
  static template = xml`
        <svg height="180">
            <Node node="graph" x="10" y="20" height="180"/>
        </svg>
    `;
  static components = { Node };
  graph = {
    label: "a",
    children: [
      { label: "b" },
      { label: "c", children: [{ label: "d" }, { label: "e" }] },
      { label: "f", children: [{ label: "g" }] },
    ],
  };
}

This RootNode component will then display a live SVG representation of the graph described by the graph property. Note that there is a recursive structure here: the Node component uses itself as a subcomponent.

Note that since SVG needs to be handled in a specific way (its namespace needs to be properly set), there is a small constraint for Owl components: if an owl component is supposed to be a part of an svg graph, then its root node needs to be a g tag, so Owl can properly set the namespace.