Alarm API

The Alarm API allows applications to schedule actions to be run in the future. For example, some applications like alarm-clock, calendar or auto-update might need to utilize the Alarm API to trigger particular device behaviors at specified time points.

By itself, the Alarm API just allows to schedule alarms. An alarm is dispatched to applications through the System Message API, so applications which want to react to alarms have to register themselves to the alarm messages.

Alarms are set using the window.navigator.mozAlarms object which is an instance of the MozAlarmsManager interface.

Schedule alarms

The first things to do when using alarm is to schedule alarms. There are two kind of alarms based on the respect of the time zone. In both case it's done using the MozAlarmsManager.add method.

Alarms ignoring time zones

Those kind of alarms is dispatched based on the local time of the device. If the user of the device changes its time zone, the alarm will be dispatched based on the new time zone. For example, if a user is in Paris and sets an alarm that should be dispatched at 12 PM CET (Central European Time) and that user travels to San Francisco, the alarm will be dispatched at 12 PM PDT (Pacific Daylight Time).

// This the date to schedule the alarm
var myDate  = new Date("May 15, 2012 16:20:00");
// This is arbitrary data pass to the alarm
var data    = {
  foo: "bar"
}
// The "ignoreTimezone" string is what make the alarm ignoring it
var request = navigator.mozAlarms.add(myDate, "ignoreTimezone", data);
request.onsuccess = function () {
  console.log("The alarm has been scheduled");
};
request.onerror = function () { 
  console.log("An error occurred: " + this.error.name);
};

Alarms honoring time zones

Those kind of alarms are dispatched based on the time in the time zone that defines when the alarm has been scheduled. If for some reason, the user of the device changes its time zone the alarm will be dispatched based on the original time zone. For example, if a user is in Paris and set an alarm that should be dispatched at 12pm CET (Central European Time) and if that user travel to San Francisco, the alarm will be dispatched at 3 AM PDT (Pacific Daylight Time).

// This the date to schedule the alarm
var myDate  = new Date("May 15, 2012 16:20:00");
// This is arbitrary data pass to the alarm
var data    = {
  foo: "bar"
}
// The "honorTimezone" string is what make the alarm honoring it
var request = navigator.mozAlarms.add(myDate, "honorTimezone", data);
request.onsuccess = function () {
  console.log("The alarm has been scheduled");
};
request.onerror = function () { 
  console.log("An error occurred: " + this.error.name);
};

Managing alarms

Once an alarm is scheduled, it's still possible to manage it.

The MozAlarmsManager.getAll method will return the complete list of alarms currently scheduled by the application. This list is an Array of mozAlarm objects.

mozAlarm

Those objects are anonymous JavaScript objects with the following properties:

id

A number representing the id of the alarm

date

A Date object representing the scheduled time for the alarm

respectTimezone

A string indicating if the alarm must respect or ignore the timezone information of the date object. Its value can be ignoreTimezone or honorTimezone

data

A JavaScript object contaning any data that were stored with the alarm

var request = navigator.mozAlarms.getAll();
request.onsuccess = function () {
  this.result.forEach(function (alarm) {
    console.log('Id: ' + alarm.id);
    console.log('date: ' + alarm.date);
    console.log('respectTimezone: ' + alarm.respectTimezone);
    console.log('data: ' + JSON.stringify(alarm.data));
  });
};
request.onerror = function () { 
  console.log("An error occurred: " + this.error.name);
};

The MozAlarmsManager.remove method is used to unschedule an existing alarm.

var alarmId;
// Set an alarm and store it's id
var request = navigator.mozAlarms.add(new Date("May 15, 2012 16:20:00"), "honorTimezone");
request.onsuccess = function () {
  alarmId = this.result;
}
// ...
// Later on, removing the alarm if it exists
if (alarmId) {
  navigator.mozAlarms.remove(alarmId);
}

Handling alarms

Any application can react when an alarm is dispatched by the system. In order to be able to handle any alarms, an application must register itself as an alarm handler. This is done through the System Messaging API in two steps:

First, the applications must include alarm to the messages property of its application manifest with the URL to the document which registers the callback function to be used when an alarm is dispatched.

"messages": [
  { "alarm": "/index.html" }
]

Second, the application must bind a callback function with the alarm message. This is done using the navigator.mozSetMessageHandler method. This callback function will receive a mozAlarm object containing the data attached to the alarm.

navigator.mozSetMessageHandler("alarm", function (mozAlarm) { 
  alert("alarm fired: " + JSON.stringify(mozAlarm.data)); 
});

If an application wants to know if there is a pending alarm at the system level, it's possible to use the navigator.mozHasPendingMessage method with the value alarm.

navigator.mozHasPendingMessage("alarm"); 

Permissions for the Alarm API

Please note that while the Alarm API is not privileged or certified, you should still include permissions and messages entries in your manifest.webapp file when including it in an installable open Web app.

"permissions": {
    "alarms": {
      "description": "Required to schedule alarms"
    }
  },
  "messages": [
    { "alarm": "/index.html" }
  ]

Specifications

Browser compatibility

See also

• Using Alarms to notify users
• navigator.mozAlarms
• MozAlarmsManager
• window.navigator.mozSetMessageHandler