How to implement a custom XUL query processor component

XUL supports templating to create a block of content from a datasource query. The XUL Template Guide has lots of detailed information on using XUL templates. XUL provides template query processors for RDF, XML and SQL (mozStorage). The templating system also supports building custom query processors. Custom query processors are XPCOM components, must implement the nsIXULTemplateQueryProcessor interface and follow some conventions for when registering the component.

In this example, we will create a simple JavaScript XPCOM component. The component will hold a small array of JavaScript objects as its datasource. In practice, you would use your own custom source of data.

Here is an example of what our XUL might look like when using a custom query processor:

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
<window xmlns="">
            <column flex="1"/>
            <column flex="3"/>
            <column flex="2"/>
            <column flex="1"/>

        <rows datasources="dummy" ref="." querytype="simpledata">
                <row uri="?">
                    <label value="?name"/>
                    <label value="?age"/>
                    <label value="?hair"/>
                    <label value="?eye"/>

A few things to note. We are not really using the datasources in our sample component, so we set it to a dummy value. An empty string would be ok too. The querytype is important. It is used to create an instance of our XPCOM object. The contract id of our XPCOM component should be of the form ";1?name=xxx", where the xxx is the querytype used in the XUL template block.

Here is our sample JavaScript XPCOM query processor:


// basic wrapper for nsIXULTemplateResult
function TemplateResult(aData) {
  this._data = aData;
  // just make a random number for the id
  this._id = Math.random(100000).toString();

TemplateResult.prototype = {
  QueryInterface: XPCOMUtils.generateQI([Components.interfaces.nsIXULTemplateResult]),

  // private storage
  _data: null,

  // right now our results are flat lists, so no containing/recursion take place
  isContainer: false,
  isEmpty: true,
  mayProcessChildren: false,
  resource: null,
  type: "simple-item",

  get id() {
    return this._id;

  // return the value of that bound variable such as ?name
  getBindingFor: function(aVar) {
    // strip off the ? from the beginning of the name
    var name = aVar.toString().slice(1);
    return this._data[name];

  // return an object instead of a string for convenient comparison purposes
  // or null to say just use string value
  getBindingObjectFor: function(aVar) {
    return null;

  // called when a rule matches this item.
  ruleMatched: function(aQuery, aRuleNode) { },

  // the output for a result has been removed and the result is no longer being used by the builder
  hasBeenRemoved: function() { }

// basic wrapper for nsISimpleEnumerator
function TemplateResultSet(aArrayOfData) {
  this._index = 0;
  this._array = aArrayOfData;

TemplateResultSet.prototype = {
  QueryInterface: XPCOMUtils.generateQI([Components.interfaces.nsISimpleEnumerator]),

  hasMoreElements: function() {
    return this._index < this._array.length;

  getNext: function() {
    return new TemplateResult(this._array[this._index++]);

// The query processor class - implements nsIXULTemplateQueryProcessor
function TemplateQueryProcessor() {
  // our basic list of data
  this._data = [
                {name: "mark", age: 36, hair: "brown", eye: "brown"},
                {name: "bill", age: 25, hair: "red", eye: "black"},
                {name: "joe", age: 15, hair: "blond", eye: "blue"},
                {name: "jimmy", age: 65, hair: "gray", eye: "dull"}

TemplateQueryProcessor.prototype = {
  QueryInterface: XPCOMUtils.generateQI([Components.interfaces.nsIXULTemplateQueryProcessor]),
  classDescription: "Sample XUL Template Query Processor",
  classID: Components.ID("{282cc4ea-a49c-44fc-81f4-1f03cbb7825f}"),
  contractID: ";1?name=simpledata",

  getDatasource: function(aDataSources, aRootNode, aIsTrusted, aBuilder, aShouldDelayBuilding) {
    // TODO: parse the aDataSources variable
    // for now, ignore everything and let's just signal that we have data
    return this._data;

  initializeForBuilding: function(aDatasource, aBuilder, aRootNode) {
    // perform any initialization that can be delayed until the content builder
    // is ready for us to start

  done: function() {
    // called when the builder is destroyed to clean up state

  compileQuery: function(aBuilder, aQuery, aRefVariable, aMemberVariable) {
    // outputs a query object.
    // eventually we should read the <query> to create filters
    return this._data;

  generateResults: function(aDatasource, aRef, aQuery) {
    // preform any query and pass the data to the result set
    return new TemplateResultSet(this._data);

  addBinding: function(aRuleNode, aVar, aRef, aExpr) {
    // add a variable binding for a particular rule, which we aren't using yet

  translateRef: function(aDatasource, aRefstring) {
    // if we return null, everything stops
    return new TemplateResult(null);

  compareResults: function(aLeft, aRight, aVar) {
    // -1 less, 0 ==, +1 greater
    var leftValue = aLeft.getBindingFor(aVar);
    var rightValue = aRight.getBindingFor(aVar);
    if (leftValue < rightValue) {
      return -1;
    else if (leftValue > rightValue) {
      return  1;
    else {
      return 0;

var components = [TemplateQueryProcessor];

function NSGetModule(compMgr, fileSpec) {
  return XPCOMUtils.generateModule(components);

Our sample query processor is very simple. A few explanatory notes:

  • We are using a the getBindingFor rather than getBindingObjectFor to simplify the code. The variable passed into getBindingFor still has the "?" prepended to it, so be sure to handle it appropriately.
  • We are not handling any datasources, but instead hard code the datasource in the component. You could easily extend this sample to handle multiple datasources by checking the datasources value in getDatasource and initializeForBuilding.
  • We do not make use of any <query/> or <rule/> elements in the XUL template block. These could be used to support filtering the datasource.