Chapter 1: Components

This chapter introduces the Owl framework, a tailor-made component system for Flectra. The main building blocks of OWL are components and templates.

In Owl, every part of user interface is managed by a component: they hold the logic and define the templates that are used to render the user interface. In practice, a component is represented by a small JavaScript class subclassing the Component class.


The Counter class implements a component that holds the internal state of a counter and defines how it should be incremented.

const { Component, useState } = owl;

 class Counter extends Component {
     static template = "my_module.Counter";

     state = useState({ value: 0 });

     increment() {

The Counter class specifies the name of the template to render. The template is written in XML and defines a part of user interface.

<templates xml:space="preserve">
    <t t-name="my_module.Counter" owl="1">
        <p>Counter: <t t-esc="state.value"/></p>
        <button class="btn btn-primary" t-on-click="increment">Increment</button>

You maybe noticed the owl="1" temporary attribute, it allows Flectra to differentiate Owl templates from the old JavaScript framework templates.

Let us take some time to get used to Owl itself. Below, you will find a series of exercises intended to quickly understand and practice the basics of Owl.


Here is an overview of what we are going to achieve in this chapter.


The solutions for each exercise of the chapter are hosted on the official Flectra tutorials repository.

1. Displaying a counter

As a first exercise, let us implement a counter in the Playground component located in owl_playground/static/src/. To see the result, you can go to the /owl_playground/playground route with your browser.


  1. Modify playground.js so that it acts as a counter like in the example above. You will need to use the useState function so that the component is re-rendered whenever any part of the state object has been read by this component is modified.

  2. In the same component, create an increment method.

  3. Modify the template in playground.xml so that it displays your counter variable. Use t-esc to output the data.

  4. Add a button in the template and specify a t-on-click attribute in the button to trigger the increment method whenever the button is clicked.


2. Extract counter in a component

For now we have the logic of a counter in the Playground component, let us see how to create a sub-component from it.


  1. Extract the counter code from the Playground component into a new Counter component.

  2. You can do it in the same file first, but once it’s done, update your code to move the Counter in its own file.

  3. Make sure the template is in its own file, with the same name.


Don’t forget /** @flectra-module **/ in your JavaScript files. More information on this can be found here.

3. A todo component

We will create new components in owl_playground/static/src/ to keep track of a list of todos. This will be done incrementally in multiple exercises that will introduce various concepts.


  1. Create a Todo component that receive a todo object in props, and display it. It should show something like 3. buy milk.

  2. Add the Bootstrap classes text-muted and text-decoration-line-through on the task if it is done. To do that, you can use dynamic attributes

  3. Modify owl_playground/static/src/playground.js and owl_playground/static/src/playground.xml to display your new Todo component with some hard-coded props to test it first.


    setup() {
        this.todo = { id: 3, description: "buy milk", done: false };

4. Props validation

The Todo component has an implicit API. It expects to receive in its props the description of a todo object in a specified format: id, description and done. Let us make that API more explicit. We can add a props definition that will let Owl perform a validation step in dev mode. You can activate the dev mode in the App configuration

It is a good practice to do props validation for every component.


  1. Add props validation to the Todo component.

  2. Make sure it passes in dev mode which is activated by default in owl_playground. The dev mode can be activated and deactivated by modifying the dev attribute in the in the config parameter of the mount function in owl_playground/static/src/main.js.

  3. Remove done from the props and reload the page. The validation should fail.

5. A list of todos

Now, let us display a list of todos instead of just one todo. For now, we can still hard-code the list.


  1. Change the code to display a list of todos instead of just one, and use t-foreach in the template.

  2. Think about how it should be keyed with the t-key directive.


6. Adding a todo

So far, the todos in our list are hard-coded. Let us make it more useful by allowing the user to add a todo to the list.


  1. Add an input above the task list with placeholder Enter a new task.

  2. Add an event handler on the keyup event named addTodo.

  3. Implement addTodo to check if enter was pressed (ev.keyCode === 13), and in that case, create a new todo with the current content of the input as the description.

  4. Make sure it has a unique id. It can be just a counter that increments at each todo.

  5. Then, clear the input of all content.

  6. Bonus point: don’t do anything if the input is empty.


Notice that nothing updates in the UI: this is because Owl does not know that it should update the UI. This can be fixed by wrapping the todo list in a useState hook.

this.todos = useState([]);

See also

Owl: Reactivity

7. Focusing the input

Let’s see how we can access the DOM with t-ref and useRef.


  1. Focus the input from the previous exercise when the dashboard is mounted.

  2. Bonus point: extract the code into a specialized hook useAutofocus.

8. Toggling todos

Now, let’s add a new feature: mark a todo as completed. This is actually trickier than one might think. The owner of the state is not the same as the component that displays it. So, the Todo component needs to communicate to its parent that the todo state needs to be toggled. One classic way to do this is by using a callback prop toggleState.


  1. Add an input with the attribute type="checkbox" before the id of the task, which must be checked if the state done is true.

  2. Add a callback props toggleState.

  3. Add a click event handler on the input in the Todo component and make sure it calls the toggleState function with the todo id.

  4. Make it work!


9. Deleting todos

The final touch is to let the user delete a todo.


  1. Add a new callback prop removeTodo.


If you’re using an array to store your todo list, you can use the JavaScript splice function to remove a todo from it.

// find the index of the element to delete
const index = list.findIndex((elem) => === elemId);
if (index >= 0) {
      // remove the element at index from list
      list.splice(index, 1);
  1. Insert <span class="fa fa-remove"> in the template of the Todo component.

  2. Whenever the user clicks on it, it should call the removeTodo method.


10. Generic components with slots

Owl has a powerful slot system to allow you to write generic components. This is useful to factorize the common layout between different parts of the interface.


  1. Write a Card component using the following Bootstrap HTML structure:

    <div class="card" style="width: 18rem;">
        <img src="..." class="card-img-top" alt="..." />
        <div class="card-body">
        <h5 class="card-title">Card title</h5>
        <p class="card-text">
            Some quick example text to build on the card title and make up the bulk
            of the card's content.
        <a href="#" class="btn btn-primary">Go somewhere</a>
  2. This component should have two slots: one slot for the title, and one for the content (the default slot).


    Here is how one could use it:

       <t t-set-slot="title">Card title</t>
       <p class="card-text">Some quick example text...</p>
       <a href="#" class="btn btn-primary">Go somewhere</a>
  3. Bonus point: if the title slot is not given, the h5 should not be rendered at all.


11. Go further


  1. Add prop validation on the Card component.

  2. Try to express in the props validation system that it requires a default slot, and an optional title slot.