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Network Signal JavaScript component for WRT

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Created: jappit (04 Jun 2009)
Last edited: hamishwillee (05 Jul 2012)

This article explains how to create and use a JavaScript Network signal component in a Web Runtime widget. This example uses WRT 1.0 SystemInfo Service API, and is so compatible with both WRT 1.0 and 1.1 versions.

Wrt networksignal widget.png


NetworkSignal component: how to use it

The following steps are necessary in order to use the Network Signal component in a WRT Widget:

<script language="javascript" type="text/javascript" src="networksignal_component.js"></script>
  • Define a parent element within your WRT HTML code, that will contain the component itself:
<div id="signal_holder"></div>
  • Embed the WRT 1.0 SysInfo Service API plugin HTML code snippet in your widget HTML code:
<embed type="application/x-systeminfo-widget" hidden="yes"></embed>
  • Add to your WRT project 2 images of the same size, one representing zero network signal, the other one representing the full network signal. Since the network signal values can go from 0 to 7, it's good practice to create signal images with these 7 values clearly visible. Sample images used in this article are the following:

Wrt signal component empty.png Wrt signal component full.png

  • Initialize a NetworkSignal instance, passing these arguments:
    • the zero signal image path
    • the full signal image path
    • the images width
    • the images height

Once instantiated, append the NetworkSignal instance to the parent element defined above:

var signal = new NetworkSignal('images/signal_empty.png', 'images/signal_full.png', 72, 168);

Source code: the NetworkSignal component implementation

NetworkSignal constructor and DOM structure

Let's start implementing the NetworkSignal constructor, and a first method that will build up the component DOM structure.

* @param {String} emptySignalSrc
* @param {String} fullSignalSrc
* @param {Number} signalWidth
* @param {Number} signalHeight

function NetworkSignal(emptySignalSrc, fullSignalSrc, signalWidth, signalHeight)
this.signalHeight = signalHeight;
this.signalWidth = signalWidth;
this.emptySignalElement = null;
this.domElement = null;
this.interval = null;
this.init(emptySignalSrc, fullSignalSrc);
NetworkSignal.prototype.init = function(emptySignalSrc, fullSignalSrc)
var el = document.createElement('div'); = 'relative';
var fullImage = document.createElement('img'); = 'absolute';
fullImage.src = fullSignalSrc;
this.emptySignalElement = document.createElement('div'); = 'absolute'; = 'hidden'; = this.signalWidth + 'px'; = '0px';
var emptyImage = document.createElement('img');
emptyImage.src = emptySignalSrc;
this.domElement = el;

Apart from calling the init() method, that creates the component DOM structure, the constructor stores the images width and height for later usage, and defines an interval variable that will be used by methods defined below.

WRT 1.0 SystemInfo Service initialization

To access SystemInfo Service API objects and properties, the following HTML code snippet must be embedded in the Widget's HTML code:

<embed type="application/x-systeminfo-widget" hidden="yes"></embed>

Done this, it is possible to use SystemInfo Service API.

  • The appendTo() method appends the component DOM element to the parent element passed as argument, and then call the startService() method.
  • The startService() method uses JavaScript setInterval() to periodically check the network signal, and to accordingly update the component appearance.
  • The stopService() method can be used to stop the network signal updating, when it is no more needed.
* @param {Node} parentElement

NetworkSignal.prototype.appendTo = function(parentElement)
/* WRT 1.0 service */
NetworkSignal.prototype.startService = function()
var sysinfo = document.embeds[0];
var self = this;
var signalHandler = function()
var bars = sysinfo.signalbars;
if(bars == undefined || bars < 0)
bars = 0;
var value = bars * 100 / 7;
this.interval = setInterval(signalHandler, 2000);
NetworkSignal.prototype.stopService = function()
if(this.interval != null)
this.interval = null;

In the startService() method, an anonymous function is defined and referenced by the signalHandler variable. This function is periodically called to check the network signal, and then it calls the updateSignalValue() method to update the component UI with the latest retrieved value:

NetworkSignal.prototype.updateSignalValue = function(value)
{ = Math.round(((100 - value) * this.signalHeight) / 100) + 'px';


Here's a list of related downloads:

This page was last modified on 5 July 2012, at 01:51.
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