Power management

Screen

To avoid power drainage, turn off the screen from software whenever possible. This also applies if you do not have the screen connected. Because limitations in JanOS, the graphics stack is still active even when the screen has been detached when the screen is not turned off from software, thus draining battery power. You can use the mozPower API to achieve this.

navigator.mozPower.screenEnabled = false;

// To re-enable
navigator.mozPower.screenEnabled = true;

The downside of disabling the screen is that certain sensors will be disabled, like the accelerometer and the proximity sensor. If you want to do a reading of one of these sensors, turn on the screen, wait until the measurement is in, and disable the screen again. For example, for the accelerometer:

navigator.mozPower.screenEnabled = true;

// Orientation only works when the screen is on
window.addEventListener('devicemotion', function odm(e) {
  window.removeEventListener('devicemotion', odm);

  var x = e.accelerationIncludingGravity.x;
  var y = e.accelerationIncludingGravity.y;

  // Do something with the data, and:
  navigator.mozPower.screenEnabled = false;
});

CPU

By default JanOS does not allow the CPU to go into sleep state. This is a good choice when you're doing continuous measurements, but can drain your battery too. If you do not need the CPU to stay awake, allow the CPU to sleep through the mozPower API.

navigator.mozPower.cpuSleepAllowed = true;

Wakelocks

The issue now is that the CPU might go to sleep while you are f.e. waiting for an asynchronous operation to succeed. Whenever you do not want the CPU to sleep, you will need to acquire a wakelock.

// Acquire wakelock
var lock = navigator.requestWakeLock('cpu');

// Make an async call to a web server
var x = new XMLHttpRequest({ mozSystem: true });
x.onload = function() {
  // When done, release the wakelock again
  lock.unlock();
};
x.open('GET', 'http://janos.io');
x.send();

Timers

When the CPU is asleep, normal JavaScript timers like setTimeout might no longer firing at the scheduled time. The callback will be invoked whenever the CPU seems fit. If you need to wake up the device at certain points in time, you will have to use the mozAlarms API.

// First schedule an alarm for 30 seconds from now
var r = navigator.mozAlarms.add(new Date(Date.now() + 30000), 'ignoreTimezone', {
  type: 'my-awesome-alarm'
});
r.onsuccess = () => console.log('All OK!');
r.onerror = () => console.error('Creating alarm failed!', r.error);

// Here we handle incoming alarms
navigator.mozSetMessageHandler('alarm', function(e) {
  if (e.data.type !== 'my-awesome-alarm') return;

  // Acquire wakelock
  var wl = navigator.requestWakeLock('cpu');

  // Do your thing
  doSomething(() => {
    // When done, release the wakelock again
    wl.unlock();
  });
});

Please note that the CPU will never sleep when connected to USB or power. This makes troubleshooting harder. If you run into issues when the CPU is asleep, extensively log to files instead of the console to make debugging easier.

Server communication

If you depend on a third party service to notify you when to wake up and do something, you cannot simply keep a socket open. For this purpose you can use the SimplePush API. As this requires both a client and server setup, the easiest way to deal with it, is to watch the 5 minute tutorial video.

Watch how to use push notifications