Utilizar a API de Notificações

The Notifications API lets a web page or app send notifications that are displayed outside the page at the system level; this lets web apps send information to a user even if the application is idle or in the background. This article looks at the basics of using this API in your own apps.

Typically, system notifications refer to the operating system's standard notification mechanism: think for example of how a typical desktop system or mobile device brodcasts notifications.

The system notification system will vary of course by platform and browser, but this is ok, and the Notifications API is written to be general enough for compatibility with most system notification systems.

Before an app can send a notification, the user must grant the application the right to do so. This is a common requirement when an API tries to interact with something outside a web page — at least once, the user needs to specifically grant that application permission to present notifications, thereby letting the user control which apps/sites are allowed to display notifications.

You can check to see if you already have permission by checking the value of the Notification.permission read only property. It can have one of three possible values:

default

The user hasn't been asked for permission yet, so notifications won't be displayed.

granted

The user has granted permission to display notifications, after having been asked previously.

denied

The user has explicitly declined permission to show notifications.

If permission to display notifications hasn't been granted yet, the application needs to use the Notification.requestPermission() method to request this form the user. In its simplest form, as used in the Emogotchi demo, we just include the following:

Notification.requestPermission();

The method also optionally accepts a callback function that is called once the user has responded to the request to display permissions (as seen in the second else ... if block below.) Commonly, you'll ask for permission to display notifications when your app is first initialized, and before trying to instantiate any. If you wanted to be really thorough, you could use a construct like the following (see To-do List Notifications):

function notifyMe() {
  // Let's check if the browser supports notifications
  if (!("Notification" in window)) {
    alert("This browser does not support system notifications");
  }

  // Let's check whether notification permissions have already been granted
  else if (Notification.permission === "granted") {
    // If it's okay let's create a notification
    var notification = new Notification("Hi there!");
  }

  // Otherwise, we need to ask the user for permission
  else if (Notification.permission !== 'denied') {
    Notification.requestPermission(function (permission) {
      // If the user accepts, let's create a notification
      if (permission === "granted") {
        var notification = new Notification("Hi there!");
      }
    });
  }

  // Finally, if the user has denied notifications and you
  // want to be respectful there is no need to bother them any more.
}

Please note that while the Notifications API is not privileged or certified, you should still include an entry in your manifest.webapp file when including it in an Firefox OS app:

"permissions": {
  "desktop-notification": {
    "description": "Needed for creating system notifications."
  }
},
"messages": [{"notification": "path/to/your/index.html"}]

Creating a notification is easy; just use the Notification constructor. This constructor expects a title to display within the notification and some options to enhance the notification such as an icon or a text body.

For example, in the Emogotchi example we have two functions that can be called when a notification needs to be fired; which one is used depends on whether we want a set notification, or a notification that includes random body content:

function spawnNotification(theBody,theIcon,theTitle) {
  var options = {
      body: theBody,
      icon: theIcon
  }
  var n = new Notification(theTitle,options);
  setTimeout(n.close.bind(n), 5000);
}

function randomNotification() {
  var randomQuote = quoteChooser();
  var options = {
      body: randomQuote,
      icon: 'img/sad_head.png',
  }

  var n = new Notification('Emogotchi says',options);
  setTimeout(n.close.bind(n), 5000);
}

Firefox and Safari close notifications automatically after a few moments (around four seconds). This may also happen at the operating system level. Some browsers don't however, such as Chrome. To make sure that the notifications close in all browsers, at the end of the above functions, we call the Notification.close function inside a setTimeout() function to close the notification after 4 seconds. Also note the use of bind() to make sure the close() call is associated with the notification.

setTimeout(n.close.bind(n), 5000);

The notifications API spec lists two events that are triggered on the Notification instance:

click

Triggered when the user clicks on the notification.

error

Triggered if something goes wrong with the notification; this is usually because the notification couldn't be displayed for some reason.

These events can be tracked using the onclick and onerror handlers. Because Notification also inherits from EventTarget, it's possible to use the addEventListener() method on it.

There are also two other events that used to be listed in the spec, but were recently removed. These may still work in browsers for a while, but they should be treated as obsolete and not used:

close

Triggered once the notification is closed.

show

Triggered when the notification is displayed to the user.

It is usually undesirable for a user to receive a lot of notifications in a short space of time — for example, what if a messenger application notified a user for each incoming message, and they were being sent a lot? To avoid spamming the user with too many notifications, it's possible to modify the pending notifications queue, replacing single or multiple pending notifications with a new one.

To do this, it's possible to add a tag to any new notification. If a notification already has the same tag and has not been displayed yet, the new notification replaces that previous notification. If the notification with the same tag has already been displayed, the previous notification is closed and the new one is displayed.

Assume the following basic HTML:

<button>Notify me!</button>

It's possible to handle multiple notifications this way:

window.addEventListener('load', function () {
  // At first, let's check if we have permission for notification
  // If not, let's ask for it
  if (window.Notification && Notification.permission !== "granted") {
    Notification.requestPermission(function (status) {
      if (Notification.permission !== status) {
        Notification.permission = status;
      }
    });
  }

  var button = document.getElementsByTagName('button')[0];

  button.addEventListener('click', function () {
    // If the user agreed to get notified
    // Let's try to send ten notifications
    if (window.Notification && Notification.permission === "granted") {
      var i = 0;
      // Using an interval cause some browsers (including Firefox) are blocking notifications if there are too much in a certain time.
      var interval = window.setInterval(function () {
        // Thanks to the tag, we should only see the "Hi! 9" notification
        var n = new Notification("Hi! " + i, {tag: 'soManyNotification'});
        if (i++ == 9) {
          window.clearInterval(interval);
        }
      }, 200);
    }

    // If the user hasn't told if he wants to be notified or not
    // Note: because of Chrome, we are not sure the permission property
    // is set, therefore it's unsafe to check for the "default" value.
    else if (window.Notification && Notification.permission !== "denied") {
      Notification.requestPermission(function (status) {
        if (Notification.permission !== status) {
          Notification.permission = status;
        }

        // If the user said okay
        if (status === "granted") {
          var i = 0;
          // Using an interval cause some browsers (including Firefox) are blocking notifications if there are too much in a certain time.
          var interval = window.setInterval(function () {
            // Thanks to the tag, we should only see the "Hi! 9" notification
            var n = new Notification("Hi! " + i, {tag: 'soManyNotification'});
            if (i++ == 9) {
              window.clearInterval(interval);
            }
          }, 200);
        }

        // Otherwise, we can fallback to a regular modal alert
        else {
          alert("Hi!");
        }
      });
    }

    // If the user refuses to get notified
    else {
      // We can fallback to a regular modal alert
      alert("Hi!");
    }
  });
});

See the live result below:

When a user clicks on a notification generated by an app, you will be notified of this event in two different ways, depending on the circumstance:

1. A click event if your app has not closed or been put in the background between the time you create the notification and the time the user clicks on it.
2. A system message otherwise.

See this code snippet for an example of how to deal with this.

• Referência das notificações do utilizador
• Notificação dos utilziadores via APIs de Notificação e Vibração
• Notificação