🦉 OWL Tutorial: TodoApp 🦉

For this tutorial, we will build a very simple Todo list application. The app should satisfy the following requirements:

let the user create and remove tasks
tasks can be marked as completed
tasks can be filtered to display active/completed tasks

This project will be an opportunity to discover and learn some important Owl concepts, such as components, store, and how to organize an application.

Content

Setting up the project
Adding a first component
Displaying a list of tasks
Layout: some basic css
Extracting Task as a subcomponent
Adding tasks (part 1)
Adding tasks (part 2)
Toggling tasks
Deleting tasks
Using a store
Saving tasks in local storage
Filtering tasks
The Final Touch
Final Code

1. Setting up the project

For this tutorial, we will do a very simple project, with static files and no additional tooling. The first step is to create the following file structure:

todoapp/
    index.html
    app.css
    app.js
    owl.js

The entry point for this application is the file index.html, which should have the following content:

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>OWL Todo App</title>
    <link rel="stylesheet" href="app.css" />
    <script src="owl.js"></script>
    <script src="app.js"></script>
  </head>
  <body></body>
</html>

Then, app.css can be left empty for now. It will be useful later on to style our application. app.js is where we will write all our code. For now, let's just put the following code:

(function () {
  console.log("hello owl", owl.__info__.version);
})();

Note that we put everything inside an immediately executed function to avoid leaking anything to the global scope.

Finally, owl.js should be the last version downloaded from the Owl repository (you can use owl.min.js if you prefer).

Now, the project should be ready. Loading the index.html file into a browser should show an empty page, with the title Owl Todo App, and it should log a message such as hello owl 1.0.0 in the console.

2. Adding a first component

An Owl application is made out of components, with a single root component. Let us start by defining an App component. Replace the content of the function in app.js by the following code:

const { Component } = owl;
const { xml } = owl.tags;
const { whenReady } = owl.utils;

// Owl Components
class App extends Component {
  static template = xml`<div>todo app</div>`;
}

// Setup code
function setup() {
  const app = new App();
  app.mount(document.body);
}

whenReady(setup);

Now, reloading the page in a browser should display a message.

The code is pretty simple, but let us explain the last line in more detail. The browser tries to execute the javascript code in app.js as quickly as possible, and it could happen that the DOM is not ready yet when we try to mount the App component. To avoid this situation, we use the whenReady helper to delay the execution of the setup function until the DOM is ready.

Note 1: in a larger project, we would split the code in multiple files, with components in a sub folder, and a main file that would initialize the application. However, this is a very small project, and we want to keep it as simple as possible.

Note 2: this tutorial uses the static class field syntax. This is not yet supported by all browsers. Most real projects will transpile their code, so this is not a problem, but for this tutorial, if you need the code to work on every browser, you will need to translate each static keyword to an assignation to the class:

class App extends Component {}
App.template = xml`<div>todo app</div>`;

Note 3: writing inline templates with the xml helper is nice, but there is no syntax highlighting, and this makes it very easy to have malformed xml. Some editors support syntax highlighting for this situation. For example, VS Code has an addon Comment tagged template, which, if installed, will properly display tagged templates:

    static template = xml /* xml */`<div>todo app</div>`;

Note 4: Large applications will probably want to be able to translate templates. Using inline templates makes it slightly harder, since we need additional tooling to extract the xml from the code, and to replace it with the translated values.

3. Displaying a list of tasks

Now that the basics are done, it is time to start thinking about tasks. To accomplish what we need, we will keep track of the tasks as an array of objects with the following keys:

id: a number. It is extremely useful to have a way to uniquely identify tasks. Since the title is something created/edited by the user, it offers no guarantee that it is unique. So, we will generate a unique id number for each task.
title: a string, to explain what the task is about.
isCompleted: a boolean, to keep track of the status of the task

Now that we decided on the internal format of the state, let us add some demo data and a template to the App component:

class App extends Component {
  static template = xml/* xml */ `
    <div class="task-list">
        <t t-foreach="tasks" t-as="task" t-key="task.id">
            <div class="task">
                <input type="checkbox" t-att-checked="task.isCompleted"/>
                <span><t t-esc="task.title"/></span>
            </div>
        </t>
    </div>`;

  tasks = [
    {
      id: 1,
      title: "buy milk",
      isCompleted: true,
    },
    {
      id: 2,
      title: "clean house",
      isCompleted: false,
    },
  ];
}

The template contains a t-foreach loop to iterate through the tasks. It can find the tasks list from the component, since the component is the rendering context. Note that we use the id of each task as a t-key, which is very common. There are two css classes: task-list and task, that we will use in the next section.

Finally, notice the use of the t-att-checked attribute: prefixing an attribute by t-att makes it dynamic. Owl will evaluate the expression and set it as the value of the attribute.

4. Layout: some basic css

So far, our task list looks quite bad. Let us add the following to app.css:

.task-list {
  width: 300px;
  margin: 50px auto;
  background: aliceblue;
  padding: 10px;
}

.task {
  font-size: 18px;
  color: #111111;
}

This is better. Now, let us add an extra feature: completed tasks should be styled a little differently, to make it clearer that they are not as important. To do that, we will add a dynamic css class on each task:

    <div class="task" t-att-class="task.isCompleted ? 'done' : ''">

.task.done {
  opacity: 0.7;
}

Notice that we have here another use of a dynamic attribute.

5. Extracting Task as a subcomponent

It is now clear that there should be a Task component to encapsulate the look and behavior of a task.

This Task component will display a task, but it cannot own the state of the task: a piece of data should only have one owner. Doing otherwise is asking for trouble. So, the Task component will get its data as a prop. This means that the data is still owned by the App component, but can be used by the Task component (without modifying it).

Since we are moving code around, it is a good opportunity to refactor the code a little bit:

// -------------------------------------------------------------------------
// Task Component
// -------------------------------------------------------------------------
const TASK_TEMPLATE = xml /* xml */`
    <div class="task" t-att-class="props.task.isCompleted ? 'done' : ''">
        <input type="checkbox" t-att-checked="props.task.isCompleted"/>
        <span><t t-esc="props.task.title"/></span>
    </div>`;

class Task extends Component {
    static template = TASK_TEMPLATE;
    static props = ["task"];
}

// -------------------------------------------------------------------------
// App Component
// -------------------------------------------------------------------------
const APP_TEMPLATE = xml /* xml */`
    <div class="task-list">
        <t t-foreach="tasks" t-as="task" t-key="task.id">
            <Task task="task"/>
        </t>
    </div>`;

class App extends Component {
    static template = APP_TEMPLATE;
    static components = { Task };

    tasks = [
        ...
    ];
}

// -------------------------------------------------------------------------
// Setup code
// -------------------------------------------------------------------------
function setup() {
    owl.config.mode = "dev";
    const app = new App();
    app.mount(document.body);
}

whenReady(setup);

A lot of stuff happened here:

first, we have now a sub component Task, defined on top of the file,
whenever we define a sub component, it needs to be added to the static components key of its parent, so Owl can get a reference to it,
the templates have been extracted out of the components, to make it easier to differentiate the "view/template" code from the "script/behavior" code,
the Task component has a props key: this is only useful for validation purpose. It says that each Task should be given exactly one prop, named task. If this is not the case, Owl will throw an error. This is extremely useful when refactoring components
finally, to activate the props validation, we need to set Owl's mode to dev. This is done in the setup function. Note that this should be removed when an app is used in a real production environment, since dev mode is slightly slower, due to extra checks and validations.

6. Adding tasks (part 1)

We still use a list of hardcoded tasks. It's really time to give the user a way to add tasks himself. The first step is to add an input to the App component. But this input will be outside of the task list, so we need to adapt App template, js, and css:

<div class="todo-app">
    <input placeholder="Enter a new task" t-on-keyup="addTask"/>
    <div class="task-list">
        <t t-foreach="tasks" t-as="task" t-key="task.id">
            <Task task="task"/>
        </t>
    </div>
</div>

addTask(ev) {
    // 13 is keycode for ENTER
    if (ev.keyCode === 13) {
        const title = ev.target.value.trim();
        ev.target.value = "";
        console.log('adding task', title);
        // todo
    }
}

.todo-app {
  width: 300px;
  margin: 50px auto;
  background: aliceblue;
  padding: 10px;
}

.todo-app > input {
  display: block;
  margin: auto;
}

.task-list {
  margin-top: 8px;
}

We now have a working input, which log to the console whenever the user adds a task. Notice that when you load the page, the input is not focused. But adding tasks is a core feature of a task list, so let us make it as fast as possible by focusing the input.

Since App is a component, it has a mounted lifecycle method that we can implement. We will also need to get a reference to the input, by using the t-ref directive with the useRef hook:

<input placeholder="Enter a new task" t-on-keyup="addTask" t-ref="add-input"/>

// on top of file:
const { useRef } = owl.hooks;

// in App
inputRef = useRef("add-input");

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

The inputRef is defined as a class field, so it is equivalent to defining it in the constructor. It simply instructs Owl to keep a reference to anything with the corresponding t-ref keyword. We then implement the mounted lifecycle method, where we now have an active reference that we can use to focus the input.

7. Adding tasks (part 2)

In the previous section, we did everything except implement the code that actually create tasks! So, let us do that now.

We need a way to generate unique id numbers. To do that, we will simply add a nextId number in App. At the same time, let us remove the demo tasks in App:

nextId = 1;
tasks = [];

Now, the addTask method can be implemented:

addTask(ev) {
    // 13 is keycode for ENTER
    if (ev.keyCode === 13) {
        const title = ev.target.value.trim();
        ev.target.value = "";
        if (title) {
            const newTask = {
                id: this.nextId++,
                title: title,
                isCompleted: false,
            };
            this.tasks.push(newTask);
        }
    }
}

This almost works, but if you test it, you will notice that no new task is ever displayed when the user press Enter. But if you add a debugger or a console.log statement, you will see that the code is actually running as expected. The problem is that Owl has no way of knowing that it needs to rerender the user interface. We can fix the issue by making tasks reactive, with the useState hook:

// on top of the file
const { useRef, useState } = owl.hooks;

// replace the task definition in App with the following:
tasks = useState([]);

It now works as expected!

8. Toggling tasks

If you tried to mark a task as completed, you may have noticed that the text did not change in opacity. This is because there is no code to modify the isCompleted flag.

Now, this is an interesting situation: the task is displayed by the Task component, but it is not the owner of its state, so it cannot modify it. Instead, we want to communicate the request to toggle a task to the App component. Since App is a parent of Task, we can trigger an event in Task and listen for it in App.

In Task, change the input to:

<input type="checkbox" t-att-checked="props.task.isCompleted" t-on-click="toggleTask"/>

and add the toggleTask method:

toggleTask() {
    this.trigger('toggle-task', {id: this.props.task.id});
}

We now need to listen for that event in the App template:

<div class="task-list" t-on-toggle-task="toggleTask">

and implement the toggleTask code:

toggleTask(ev) {
    const task = this.tasks.find(t => t.id === ev.detail.id);
    task.isCompleted = !task.isCompleted;
}

9. Deleting tasks

Let us now add the possibility do delete tasks. To do that, we first need to add a trash icon on each task, then we will proceed just like in the previous section.

First, let us update the Task template, css and js:

<div class="task" t-att-class="props.task.isCompleted ? 'done' : ''">
    <input type="checkbox" t-att-checked="props.task.isCompleted" t-on-click="toggleTask"/>
    <span><t t-esc="props.task.title"/></span>
    <span class="delete" t-on-click="deleteTask">🗑</span>
</div>

.task {
  font-size: 18px;
  color: #111111;
  display: grid;
  grid-template-columns: 30px auto 30px;
}

.task > input {
  margin: auto;
}

.delete {
  opacity: 0;
  cursor: pointer;
  text-align: center;
}

.task:hover .delete {
  opacity: 1;
}

deleteTask() {
    this.trigger('delete-task', {id: this.props.task.id});
}

And now, we need to listen to the delete-task event in App:

<div class="task-list" t-on-toggle-task="toggleTask" t-on-delete-task="deleteTask">

deleteTask(ev) {
    const index = this.tasks.findIndex(t => t.id === ev.detail.id);
    this.tasks.splice(index, 1);
}

10. Using a store

Looking at the code, it is apparent that we now have code to handle tasks scattered in more than one place. Also, it mixes UI code and business logic code. Owl has a way to manage state separately from the user interface: a Store.

Let us use it in our application. This is a pretty large refactoring (for our application), since it involves extracting all task related code out of the components. Here is the new content of the app.js file:

const { Component, Store } = owl;
const { xml } = owl.tags;
const { whenReady } = owl.utils;
const { useRef, useDispatch, useStore } = owl.hooks;

// -------------------------------------------------------------------------
// Store
// -------------------------------------------------------------------------
const actions = {
  addTask({ state }, title) {
    title = title.trim();
    if (title) {
      const task = {
        id: state.nextId++,
        title: title,
        isCompleted: false,
      };
      state.tasks.push(task);
    }
  },
  toggleTask({ state }, id) {
    const task = state.tasks.find((t) => t.id === id);
    task.isCompleted = !task.isCompleted;
  },
  deleteTask({ state }, id) {
    const index = state.tasks.findIndex((t) => t.id === id);
    state.tasks.splice(index, 1);
  },
};
const initialState = {
  nextId: 1,
  tasks: [],
};

// -------------------------------------------------------------------------
// Task Component
// -------------------------------------------------------------------------
const TASK_TEMPLATE = xml/* xml */ `
    <div class="task" t-att-class="props.task.isCompleted ? 'done' : ''">
        <input type="checkbox" t-att-checked="props.task.isCompleted"
                t-on-click="dispatch('toggleTask', props.task.id)"/>
        <span><t t-esc="props.task.title"/></span>
        <span class="delete" t-on-click="dispatch('deleteTask', props.task.id)">🗑</span>
    </div>`;

class Task extends Component {
  static template = TASK_TEMPLATE;
  static props = ["task"];
  dispatch = useDispatch();
}

// -------------------------------------------------------------------------
// App Component
// -------------------------------------------------------------------------
const APP_TEMPLATE = xml/* xml */ `
    <div class="todo-app">
        <input placeholder="Enter a new task" t-on-keyup="addTask" t-ref="add-input"/>
        <div class="task-list">
            <t t-foreach="tasks" t-as="task" t-key="task.id">
                <Task task="task"/>
            </t>
        </div>
    </div>`;

class App extends Component {
  static template = APP_TEMPLATE;
  static components = { Task };

  inputRef = useRef("add-input");
  tasks = useStore((state) => state.tasks);
  dispatch = useDispatch();

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

  addTask(ev) {
    // 13 is keycode for ENTER
    if (ev.keyCode === 13) {
      this.dispatch("addTask", ev.target.value);
      ev.target.value = "";
    }
  }
}

// -------------------------------------------------------------------------
// Setup code
// -------------------------------------------------------------------------
function setup() {
  owl.config.mode = "dev";
  const store = new Store({ actions, state: initialState });
  App.env.store = store;
  const app = new App();
  app.mount(document.body);
}

whenReady(setup);

11-Saving tasks in local storage

Now, our TodoApp works great, except if the user closes or refresh the browser! It is really inconvenient to only keep the state of the application in memory. To fix this, we will save the tasks in the local storage. With our current codebase, it is a simple change: only the setup code needs to be updated.

function makeStore() {
  const localState = window.localStorage.getItem("todoapp");
  const state = localState ? JSON.parse(localState) : initialState;
  const store = new Store({ state, actions });
  store.on("update", null, () => {
    localStorage.setItem("todoapp", JSON.stringify(store.state));
  });
  return store;
}

function setup() {
  owl.config.mode = "dev";
  App.env.store = makeStore();
  const app = new App();
  app.mount(document.body);
}

The key point is to use the fact that the store is an EventBus which triggers an update event whenever it is updated.

12. Filtering tasks

We are almost done, we can add/update/delete tasks. The only missing feature is the possibility to display the task according to their completed status. We will need to keep track of the state of the filter in App, then filter the visible tasks according to its value.

// on top of file, readd useState:
const { useRef, useDispatch, useState, useStore } = owl.hooks;

// in App:
filter = useState({value: "all"})

get displayedTasks() {
    switch (this.filter.value) {
        case "active": return this.tasks.filter(t => !t.isCompleted);
        case "completed": return this.tasks.filter(t => t.isCompleted);
        case "all": return this.tasks;
    }
}

setFilter(filter) {
    this.filter.value = filter;
}

Finally, we need to display the visible filters. We can do that, and at the same time, display the number of tasks in a small panel below the main list:

<div class="todo-app">
    <input placeholder="Enter a new task" t-on-keyup="addTask" t-ref="add-input"/>
    <div class="task-list">
        <t t-foreach="displayedTasks" t-as="task" t-key="task.id">
            <Task task="task"/>
        </t>
    </div>
    <div class="task-panel" t-if="tasks.length">
        <div class="task-counter">
            <t t-esc="displayedTasks.length"/>
            <t t-if="displayedTasks.length lt tasks.length">
                / <t t-esc="tasks.length"/>
            </t>
            task(s)
        </div>
        <div>
            <span t-foreach="['all', 'active', 'completed']"
                t-as="f" t-key="f"
                t-att-class="{active: filter.value===f}"
                t-on-click="setFilter(f)"
                t-esc="f"/>
        </div>
    </div>
</div>

.task-panel {
  color: #0088ff;
  margin-top: 8px;
  font-size: 14px;
  display: flex;
}

.task-panel .task-counter {
  flex-grow: 1;
}

.task-panel span {
  padding: 5px;
  cursor: pointer;
}

.task-panel span.active {
  font-weight: bold;
}

Notice here that we set dynamically the class of the filter with the object syntax: each key is a class that we want to set if its value is truthy.

13. The Final Touch

Our list is feature complete. We can still add a few extra details to improve the user experience.

Add a visual feedback when the user mouse is over a task:

.task:hover {
  background-color: #def0ff;
}

Make the title of a task clickable, to toggle its checkbox:

<input type="checkbox" t-att-checked="props.task.isCompleted"
    t-att-id="props.task.id"
    t-on-click="dispatch('toggleTask', props.task.id)"/>
<label t-att-for="props.task.id"><t t-esc="props.task.title"/></label>

Strike the title of completed task:

.task.done label {
  text-decoration: line-through;
}

Final code

Our application is now complete. It works, the UI code is well separated from the business logic code, it is testable, all under 150 lines of code (template included!).

For reference, here is the final code:

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>OWL Todo App</title>
    <link rel="stylesheet" href="app.css" />
    <script src="owl.js"></script>
    <script src="app.js"></script>
  </head>
  <body></body>
</html>

(function () {
  const { Component, Store } = owl;
  const { xml } = owl.tags;
  const { whenReady } = owl.utils;
  const { useRef, useDispatch, useState, useStore } = owl.hooks;

  // -------------------------------------------------------------------------
  // Store
  // -------------------------------------------------------------------------
  const actions = {
    addTask({ state }, title) {
      title = title.trim();
      if (title) {
        const task = {
          id: state.nextId++,
          title: title,
          isCompleted: false,
        };
        state.tasks.push(task);
      }
    },
    toggleTask({ state }, id) {
      const task = state.tasks.find((t) => t.id === id);
      task.isCompleted = !task.isCompleted;
    },
    deleteTask({ state }, id) {
      const index = state.tasks.findIndex((t) => t.id === id);
      state.tasks.splice(index, 1);
    },
  };

  const initialState = {
    nextId: 1,
    tasks: [],
  };

  // -------------------------------------------------------------------------
  // Task Component
  // -------------------------------------------------------------------------
  const TASK_TEMPLATE = xml/* xml */ `
    <div class="task" t-att-class="props.task.isCompleted ? 'done' : ''">
        <input type="checkbox" t-att-checked="props.task.isCompleted"
            t-att-id="props.task.id"
            t-on-click="dispatch('toggleTask', props.task.id)"/>
        <label t-att-for="props.task.id"><t t-esc="props.task.title"/></label>
        <span class="delete" t-on-click="dispatch('deleteTask', props.task.id)">🗑</span>
    </div>`;

  class Task extends Component {
    static template = TASK_TEMPLATE;
    static props = ["task"];
    dispatch = useDispatch();
  }

  // -------------------------------------------------------------------------
  // App Component
  // -------------------------------------------------------------------------
  const APP_TEMPLATE = xml/* xml */ `
    <div class="todo-app">
        <input placeholder="Enter a new task" t-on-keyup="addTask" t-ref="add-input"/>
        <div class="task-list">
            <Task t-foreach="displayedTasks" t-as="task" t-key="task.id" task="task"/>
        </div>
        <div class="task-panel" t-if="tasks.length">
            <div class="task-counter">
                <t t-esc="displayedTasks.length"/>
                <t t-if="displayedTasks.length lt tasks.length">
                    / <t t-esc="tasks.length"/>
                </t>
                task(s)
            </div>
            <div>
                <span t-foreach="['all', 'active', 'completed']"
                    t-as="f" t-key="f"
                    t-att-class="{active: filter.value===f}"
                    t-on-click="setFilter(f)"
                    t-esc="f"/>
            </div>
        </div>
    </div>`;

  class App extends Component {
    static template = APP_TEMPLATE;
    static components = { Task };

    inputRef = useRef("add-input");
    tasks = useStore((state) => state.tasks);
    filter = useState({ value: "all" });
    dispatch = useDispatch();

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

    addTask(ev) {
      // 13 is keycode for ENTER
      if (ev.keyCode === 13) {
        this.dispatch("addTask", ev.target.value);
        ev.target.value = "";
      }
    }

    get displayedTasks() {
      switch (this.filter.value) {
        case "active":
          return this.tasks.filter((t) => !t.isCompleted);
        case "completed":
          return this.tasks.filter((t) => t.isCompleted);
        case "all":
          return this.tasks;
      }
    }
    setFilter(filter) {
      this.filter.value = filter;
    }
  }

  // -------------------------------------------------------------------------
  // Setup code
  // -------------------------------------------------------------------------
  function makeStore() {
    const localState = window.localStorage.getItem("todoapp");
    const state = localState ? JSON.parse(localState) : initialState;
    const store = new Store({ state, actions });
    store.on("update", null, () => {
      localStorage.setItem("todoapp", JSON.stringify(store.state));
    });
    return store;
  }

  function setup() {
    owl.config.mode = "dev";
    App.env.store = makeStore();
    const app = new App();
    app.mount(document.body);
  }

  whenReady(setup);
})();

.todo-app {
  width: 300px;
  margin: 50px auto;
  background: aliceblue;
  padding: 10px;
}

.todo-app > input {
  display: block;
  margin: auto;
}

.task-list {
  margin-top: 8px;
}

.task {
  font-size: 18px;
  color: #111111;
  display: grid;
  grid-template-columns: 30px auto 30px;
}

.task:hover {
  background-color: #def0ff;
}

.task > input {
  margin: auto;
}

.delete {
  opacity: 0;
  cursor: pointer;
  text-align: center;
}

.task:hover .delete {
  opacity: 1;
}

.task.done {
  opacity: 0.7;
}
.task.done label {
  text-decoration: line-through;
}

.task-panel {
  color: #0088ff;
  margin-top: 8px;
  font-size: 14px;
  display: flex;
}

.task-panel .task-counter {
  flex-grow: 1;
}

.task-panel span {
  padding: 5px;
  cursor: pointer;
}

.task-panel span.active {
  font-weight: bold;
}

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tutorial_todoapp.md

tutorial_todoapp.md

🦉 OWL Tutorial: TodoApp 🦉

Content

1. Setting up the project

2. Adding a first component

3. Displaying a list of tasks

4. Layout: some basic css

5. Extracting Task as a subcomponent

6. Adding tasks (part 1)

7. Adding tasks (part 2)

8. Toggling tasks

9. Deleting tasks

10. Using a store

11-Saving tasks in local storage

12. Filtering tasks

13. The Final Touch

Final code

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tutorial_todoapp.md

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tutorial_todoapp.md

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🦉 OWL Tutorial: TodoApp 🦉

Content

1. Setting up the project

2. Adding a first component

3. Displaying a list of tasks

4. Layout: some basic css

5. Extracting Task as a subcomponent

6. Adding tasks (part 1)

7. Adding tasks (part 2)

8. Toggling tasks

9. Deleting tasks

10. Using a store

11-Saving tasks in local storage

12. Filtering tasks

13. The Final Touch

Final code