🦉 Quick Overview 🦉

Owl components in an application are used to define a (dynamic) tree of components.

State: each component can manage its own local state. It is a simple ES6 class, there are no special rules:

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

  state = { value: 0 };

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

The example above shows a component with a local state. Note that since there is nothing magical to the state object, we need to manually call the render function whenever we update it. This can quickly become annoying (and not efficient if we do it too much). There is a better way: using the useState hook, which transforms an object into a reactive version of itself:

const { useState } = owl.hooks;

class Counter extends 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++;
  }
}

Note that the t-on-click handler can even be replaced by an inline statement:

    <button t-on-click="state.value++">

Props: sub components often needs some information from their parents. This is done by adding the required information to the template. This will then be accessible by the sub component in the props object. Note that there is an important rule here: the information contained in the props object is not owned by the sub component, and should never be modified.

class Child extends Component {
  static template = xml`<div>Hello <t t-esc="props.name"/></div>`;
}

class Parent extends Component {
  static template = xml`
    <div>
        <Child name="'Owl'" />
        <Child name="'Framework'" />
    </div>`;
  static components = { Child };
}

Communication: there are multiple ways to communicate information between components. However, the two most important ways are the following:

from parent to children: by using props,
from a children to one of its parent: by triggering events.

The following example illustrate both mechanisms:

class OrderLine extends Component {
  static template = xml`
    <div t-on-click="add">
        <div><t t-esc="props.line.name"/></div>
        <div>Quantity: <t t-esc="props.line.quantity"/></div>
    </div>`;

  add() {
    this.trigger("add-to-order", { line: props.line });
  }
}

class Parent extends Component {
  static template = xml`
    <div t-on-add-to-order="addToOrder">
        <OrderLine
            t-foreach="orders"
            t-as="line"
            line="line" />
    </div>`;
  static components = { OrderLine };
  orders = useState([
    { id: 1, name: "Coffee", quantity: 0 },
    { id: 2, name: "Tea", quantity: 0 },
  ]);

  addToOrder(event) {
    const line = event.detail.line;
    line.quantity++;
  }
}

In this example, the OrderLine component trigger a add-to-order event. This will generate a DOM event which will bubble along the DOM tree. It will then be intercepted by the parent component, which will then get the line (from the detail key) and then increment its quantity. See the page on event handling for more details on how events work.

Note that this example would have also worked if the OrderLine component directly modifies the line object. However, this is not a good practice: this only works because the props object received by the child component is reactive, so the child component is then coupled to the parents implementation.