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Battery API: threshold attribute proposal

From: Poussa, Sakari <sakari.poussa@intel.com>
Date: Tue, 8 Nov 2011 06:31:16 +0000
To: "public-device-status@w3.org" <public-device-status@w3.org>
Message-ID: <CADE0CB3.E23A%sakari.poussa@intel.com>
Hey,

Here comes the proposal for the threshold attribute for the battery API as
discussed DAP F2F meeting last week and captured on [1].

Background: During the F2F it was raised as a concern that the Battery API
does not have the dedicated low and critical events anymore. Instead, the
level and other events comes in on unspecified (implementation dependent)
rate. Finally, there was no way for WebApp to be a good power management
citizen by limiting the number (wakeup) events it was consuming. On
battery powered devices (e.g. phones), every wake-up counts. The less
wake-ups you have the better you battery life is.

Proposal: the discussed solution was to define a 'threshold' parameter.
When set, the API doesn't level events if the battery level is above the
threshold. This way the WebApp can specify when it starts to receive
battery events. When not set, the events will behave like now (without the
threshold parameter).

[NoInterfaceObject]
  interface BatteryManager : EventTarget {	
  readonly attribute boolean   charging;
  readonly attribute double    chargingTime;
  readonly attribute float     level;
  attribute float     levelThreshold; /* NEW */
  readonly attribute double    dischargingTime;
  attribute Function? onchargingchange;
  attribute Function? onchargingtimechange;
  attribute Function? onlevelchange;
  attribute Function? ondischargingtimechange;
};

attribute levelThreshold : When set (scale 0 - 1.0), indicates the level
threshold when the the system should start sending the 'onlevelchange'
events. When not set (equals to zero), the rate of the events is left to
implementation.

Below is an example which uses the new attribute to avoid any messages
before the battery level drops below 20%.

<script>
  // Power friendly app which only start receiving
  // any events when the battery level drops below 20%

  var battery = navigator.battery;
  
  // Start tracking the level only when it drops below 20%
  battery.levelThreshold = 0.2;

  battery.onlevelchange = function () {
    // Now we have only 20% left, start tracking the remaining time as well
    battery.ondischargingtimechange = function () {
      console.log("Battery level: " + battery.level + ", Remaining time
(sec): " + battery.dischargingTime);
    };
  };
</script>


Any comments?



-sakari

[1] http://www.w3.org/2009/dap/track/actions/459
Received on Tuesday, 8 November 2011 06:38:33 GMT

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