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🦉 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

  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
  14. 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" />
  </head>
  <body>
    <script src="owl.js"></script>
    <script src="app.js"></script>
  </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). Be aware that you should download the owl.iife.js or owl.iife.min.js, because these files are built to run directly on the browser, and rename it owl.js (other files such as owl.cjs.js are built to be bundled by other tools).

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 2.x.y 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 a Root component. Replace the content of the function in app.js by the following code:

const { Component, mount, xml } = owl;

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

mount(Root, document.body);

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

The code is pretty simple: we define a component with an inline template, then mount it in the document body.

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.
  • text: 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 Root 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.text"/></span>
            </div>
        </t>
    </div>`;

  tasks = [
    {
      id: 1,
      text: "buy milk",
      isCompleted: true,
    },
    {
      id: 2,
      text: "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
// -------------------------------------------------------------------------
class Task extends Component {
  static 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.text"/></span>
    </div>`;
  static props = ["task"];
}

// -------------------------------------------------------------------------
// Root Component
// -------------------------------------------------------------------------
class Root extends Component {
  static template = xml /* xml */`
    <div class="task-list">
      <t t-foreach="tasks" t-as="task" t-key="task.id">
        <Task task="task"/>
      </t>
    </div>`;
    static components = { Task };

    tasks = [
        ...
    ];
}

// -------------------------------------------------------------------------
// Setup
// -------------------------------------------------------------------------
mount(Root, document.body, {dev: true});

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 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 last argument of the mount 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 Root component. But this input will be outside of the task list, so we need to adapt Root 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 text = ev.target.value.trim();
        ev.target.value = "";
        console.log('adding task', text);
        // 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.

We need to execute code when the Root component is ready (mounted). Let's do that using the onMounted hook. 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 { Component, mount, xml, useRef, onMounted } = owl;
// in App
setup() {
    const inputRef = useRef("add-input");
    onMounted(() => inputRef.el.focus());
}

This is a very common situation: whenever we need to perform some actions depending on the lifecycle of a component, we need to do it in the setup method, by using one of the lifecycle hook. Here, we first get a reference to the inputRef, then in the onMounted hook, we simply focus the html element.

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 text = ev.target.value.trim();
        ev.target.value = "";
        if (text) {
            const newTask = {
                id: this.nextId++,
                text: text,
                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 { Component, mount, xml, useRef, onMounted, useState } = owl;

// 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 ideally, it should not modify it. However, for now, that's what we will do (this will be improved in a later step). 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.props.task.isCompleted = !this.props.task.isCompleted;
}

9. Deleting tasks

Let us now add the possibility do delete tasks. This is different from the previous feature: deleting task has to be done on the task itself, but the actual operation need to be done on the task list. So, we need to communicate the request to the Root component. This is usually done by providing a callback in a prop.

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.text"/></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;
}
static props = ["task", "onDelete"];

deleteTask() {
    this.props.onDelete(this.props.task);
}

And now, we need to provide the onDelete callback to each tasks in the Root component:

  <Task task="task" onDelete.bind="deleteTask"/>
deleteTask(task) {
    const index = this.tasks.findIndex(t => t.id === task.id);
    this.tasks.splice(index, 1);
}

Notice that the onDelete prop is defined with a .bind suffix: this is a special suffix that makes sure the function callback is bound to the component.

10. Using a store

Looking at the code, it is apparent that all the code handling tasks is scattered all around the application. Also, it mixes UI code and business logic code. Owl does not provide any high level abstraction to manage business logic, but it is easy to do it with the basic reactivity primitives (useState and reactive).

Let us use it in our application to implement a central store. 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, mount, xml, useRef, onMounted, useState, reactive, useEnv } = owl;

// -------------------------------------------------------------------------
// Store
// -------------------------------------------------------------------------
function useStore() {
  const env = useEnv();
  return useState(env.store);
}

// -------------------------------------------------------------------------
// TaskList
// -------------------------------------------------------------------------
class TaskList {
  nextId = 1;
  tasks = [];

  addTask(text) {
    text = text.trim();
    if (text) {
      const task = {
        id: this.nextId++,
        text: text,
        isCompleted: false,
      };
      this.tasks.push(task);
    }
  }

  toggleTask(task) {
    task.isCompleted = !task.isCompleted;
  }

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

function createTaskStore() {
  return reactive(new TaskList());
}

// -------------------------------------------------------------------------
// Task Component
// -------------------------------------------------------------------------
class Task extends Component {
  static 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="() => store.toggleTask(props.task)"/>
      <span><t t-esc="props.task.text"/></span>
      <span class="delete" t-on-click="() => store.deleteTask(props.task)">🗑</span>
    </div>`;

  static props = ["task"];

  setup() {
    this.store = useStore();
  }
}

// -------------------------------------------------------------------------
// Root Component
// -------------------------------------------------------------------------
class Root extends Component {
  static 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="store.tasks" t-as="task" t-key="task.id">
          <Task task="task"/>
        </t>
      </div>
    </div>`;
  static components = { Task };

  setup() {
    const inputRef = useRef("add-input");
    onMounted(() => inputRef.el.focus());
    this.store = useStore();
  }

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

// -------------------------------------------------------------------------
// Setup
// -------------------------------------------------------------------------
const env = {
  store: createTaskStore(),
};
mount(Root, document.body, { dev: true, env });

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: we need to save tasks to local storage and listen to any change.

class TaskList {
  constructor(tasks) {
    this.tasks = tasks || [];
    const taskIds = this.tasks.map((t) => t.id);
    this.nextId = taskIds.length ? Math.max(...taskIds) + 1 : 1;
  }
  // ...
}

function createTaskStore() {
  const saveTasks = () => localStorage.setItem("todoapp", JSON.stringify(taskStore.tasks));
  const initialTasks = JSON.parse(localStorage.getItem("todoapp") || "[]");
  const taskStore = reactive(new TaskList(initialTasks), saveTasks);
  saveTasks();
  return taskStore;
}

The key point is that the reactive function takes a callback that will be called every time an observed value is changed. Note that we need to call the saveTasks method initially to make sure we observe all current values.

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 Root, then filter the visible tasks according to its value.

class Root extends Component {
  static 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="displayedTasks" t-as="task" t-key="task.id">
          <Task task="task"/>
        </t>
      </div>
      <div class="task-panel" t-if="store.tasks.length">
        <div class="task-counter">
          <t t-esc="displayedTasks.length"/>
          <t t-if="displayedTasks.length lt store.tasks.length">
              / <t t-esc="store.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="() => this.setFilter(f)"
            t-esc="f"/>
        </div>
      </div>
    </div>`;

  setup() {
    ...
    this.filter = useState({ value: "all" });
  }

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

  setFilter(filter) {
    this.filter.value = filter;
  }
}
.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 css class of the filter with the object syntax.

13. The Final Touch

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

  1. Add a visual feedback when the user mouse is over a task:
.task:hover {
  background-color: #def0ff;
}
  1. Make the text 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="() => store.toggleTask(props.task)"/>
<label t-att-for="props.task.id"><t t-esc="props.task.text"/></label>
  1. Strike the text 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, mount, xml, useRef, onMounted, useState, reactive, useEnv } = owl;

  // -------------------------------------------------------------------------
  // Store
  // -------------------------------------------------------------------------
  function useStore() {
    const env = useEnv();
    return useState(env.store);
  }

  // -------------------------------------------------------------------------
  // TaskList
  // -------------------------------------------------------------------------
  class TaskList {
    constructor(tasks) {
      this.tasks = tasks || [];
      const taskIds = this.tasks.map((t) => t.id);
      this.nextId = taskIds.length ? Math.max(...taskIds) + 1 : 1;
    }

    addTask(text) {
      text = text.trim();
      if (text) {
        const task = {
          id: this.nextId++,
          text: text,
          isCompleted: false,
        };
        this.tasks.push(task);
      }
    }

    toggleTask(task) {
      task.isCompleted = !task.isCompleted;
    }

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

  function createTaskStore() {
    const saveTasks = () => localStorage.setItem("todoapp", JSON.stringify(taskStore.tasks));
    const initialTasks = JSON.parse(localStorage.getItem("todoapp") || "[]");
    const taskStore = reactive(new TaskList(initialTasks), saveTasks);
    saveTasks();
    return taskStore;
  }

  // -------------------------------------------------------------------------
  // Task Component
  // -------------------------------------------------------------------------
  class Task extends Component {
    static template = xml/* xml */ `
      <div class="task" t-att-class="props.task.isCompleted ? 'done' : ''">
        <input type="checkbox"
          t-att-id="props.task.id"
          t-att-checked="props.task.isCompleted"
          t-on-click="() => store.toggleTask(props.task)"/>
        <label t-att-for="props.task.id"><t t-esc="props.task.text"/></label>
        <span class="delete" t-on-click="() => store.deleteTask(props.task)">🗑</span>
      </div>`;

    static props = ["task"];

    setup() {
      this.store = useStore();
    }
  }

  // -------------------------------------------------------------------------
  // Root Component
  // -------------------------------------------------------------------------
  class Root extends Component {
    static 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="displayedTasks" t-as="task" t-key="task.id">
            <Task task="task"/>
          </t>
        </div>
        <div class="task-panel" t-if="store.tasks.length">
          <div class="task-counter">
            <t t-esc="displayedTasks.length"/>
            <t t-if="displayedTasks.length lt store.tasks.length">
                / <t t-esc="store.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="() => this.setFilter(f)"
              t-esc="f"/>
          </div>
        </div>
      </div>`;
    static components = { Task };

    setup() {
      const inputRef = useRef("add-input");
      onMounted(() => inputRef.el.focus());
      this.store = useStore();
      this.filter = useState({ value: "all" });
    }

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

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

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

  // -------------------------------------------------------------------------
  // Setup
  // -------------------------------------------------------------------------
  const env = { store: createTaskStore() };
  mount(Root, document.body, { dev: true, env });
})();
.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;
}