The source code

    /**
     * @class Ext.tree.TreeLoader
     * @extends Ext.util.Observable
     * A TreeLoader provides for lazy loading of an {@link Ext.tree.TreeNode}'s child
     * nodes from a specified URL. The response must be a JavaScript Array definition
     * whose elements are node definition objects. e.g.:
     * <pre><code>
        [{
            id: 1,
            text: 'A leaf Node',
            leaf: true
        },{
            id: 2,
            text: 'A folder Node',
            children: [{
                id: 3,
                text: 'A child Node',
                leaf: true
            }]
       }]
    </code></pre>
     * <br><br>
     * A server request is sent, and child nodes are loaded only when a node is expanded.
     * The loading node's id is passed to the server under the parameter name "node" to
     * enable the server to produce the correct child nodes.
     * <br><br>
     * To pass extra parameters, an event handler may be attached to the "beforeload"
     * event, and the parameters specified in the TreeLoader's baseParams property:
     * <pre><code>
        myTreeLoader.on("beforeload", function(treeLoader, node) {
            this.baseParams.category = node.attributes.category;
        }, this);
    </code></pre>
     * This would pass an HTTP parameter called "category" to the server containing
     * the value of the Node's "category" attribute.
     * @constructor
     * Creates a new Treeloader.
     * @param {Object} config A config object containing config properties.
     */
    Ext.tree.TreeLoader = function(config){
        this.baseParams = {};
        Ext.apply(this, config);

        this.addEvents(
            /**
             * @event beforeload
             * Fires before a network request is made to retrieve the Json text which specifies a node's children.
             * @param {Object} This TreeLoader object.
             * @param {Object} node The {@link Ext.tree.TreeNode} object being loaded.
             * @param {Object} callback The callback function specified in the {@link #load} call.
             */
            "beforeload",
            /**
             * @event load
             * Fires when the node has been successfuly loaded.
             * @param {Object} This TreeLoader object.
             * @param {Object} node The {@link Ext.tree.TreeNode} object being loaded.
             * @param {Object} response The response object containing the data from the server.
             */
            "load",
            /**
             * @event loadexception
             * Fires if the network request failed.
             * @param {Object} This TreeLoader object.
             * @param {Object} node The {@link Ext.tree.TreeNode} object being loaded.
             * @param {Object} response The response object containing the data from the server.
             */
            "loadexception"
        );
        Ext.tree.TreeLoader.superclass.constructor.call(this);
        if(Ext.isString(this.paramOrder)){
            this.paramOrder = this.paramOrder.split(/[\s,|]/);
        }
    };

    Ext.extend(Ext.tree.TreeLoader, Ext.util.Observable, {
        /**
        * @cfg {String} dataUrl The URL from which to request a Json string which
        * specifies an array of node definition objects representing the child nodes
        * to be loaded.
        */
        /**
         * @cfg {String} requestMethod The HTTP request method for loading data (defaults to the value of {@link Ext.Ajax#method}).
         */
        /**
         * @cfg {String} url Equivalent to {@link #dataUrl}.
         */
        /**
         * @cfg {Boolean} preloadChildren If set to true, the loader recursively loads "children" attributes when doing the first load on nodes.
         */
        /**
        * @cfg {Object} baseParams (optional) An object containing properties which
        * specify HTTP parameters to be passed to each request for child nodes.
        */
        /**
        * @cfg {Object} baseAttrs (optional) An object containing attributes to be added to all nodes
        * created by this loader. If the attributes sent by the server have an attribute in this object,
        * they take priority.
        */
        /**
        * @cfg {Object} uiProviders (optional) An object containing properties which
        * specify custom {@link Ext.tree.TreeNodeUI} implementations. If the optional
        * <i>uiProvider</i> attribute of a returned child node is a string rather
        * than a reference to a TreeNodeUI implementation, then that string value
        * is used as a property name in the uiProviders object.
        */
        uiProviders : {},

        /**
        * @cfg {Boolean} clearOnLoad (optional) Default to true. Remove previously existing
        * child nodes before loading.
        */
        clearOnLoad : true,

        /**
         * @cfg {Array/String} paramOrder Defaults to <tt>undefined</tt>. Only used when using directFn.
         * Specifies the params in the order in which they must be passed to the server-side Direct method
         * as either (1) an Array of String values, or (2) a String of params delimited by either whitespace,
         * comma, or pipe. For example,
         * any of the following would be acceptable:<pre><code>
    nodeParameter: 'node',
    paramOrder: ['param1','param2','param3']
    paramOrder: 'node param1 param2 param3'
    paramOrder: 'param1,node,param2,param3'
    paramOrder: 'param1|param2|param|node'
         </code></pre>
         */
        paramOrder: undefined,

        /**
         * @cfg {Boolean} paramsAsHash Only used when using directFn.
         * Send parameters as a collection of named arguments (defaults to <tt>false</tt>). Providing a
         * <tt>{@link #paramOrder}</tt> nullifies this configuration.
         */
        paramsAsHash: false,

        /**
         * @cfg {String} nodeParameter The name of the parameter sent to the server which contains
         * the identifier of the node. Defaults to <tt>'node'</tt>.
         */
        nodeParameter: 'node',

        /**
         * @cfg {Function} directFn
         * Function to call when executing a request.
         */
        directFn : undefined,

        /**
         * Load an {@link Ext.tree.TreeNode} from the URL specified in the constructor.
         * This is called automatically when a node is expanded, but may be used to reload
         * a node (or append new children if the {@link #clearOnLoad} option is false.)
         * @param {Ext.tree.TreeNode} node
         * @param {Function} callback Function to call after the node has been loaded. The
         * function is passed the TreeNode which was requested to be loaded.
         * @param {Object} scope The scope (<code>this</code> reference) in which the callback is executed.
         * defaults to the loaded TreeNode.
         */
        load : function(node, callback, scope){
            if(this.clearOnLoad){
                while(node.firstChild){
                    node.removeChild(node.firstChild);
                }
            }
            if(this.doPreload(node)){ // preloaded json children
                this.runCallback(callback, scope || node, [node]);
            }else if(this.directFn || this.dataUrl || this.url){
                this.requestData(node, callback, scope || node);
            }
        },

        doPreload : function(node){
            if(node.attributes.children){
                if(node.childNodes.length < 1){ // preloaded?
                    var cs = node.attributes.children;
                    node.beginUpdate();
                    for(var i = 0, len = cs.length; i < len; i++){
                        var cn = node.appendChild(this.createNode(cs[i]));
                        if(this.preloadChildren){
                            this.doPreload(cn);
                        }
                    }
                    node.endUpdate();
                }
                return true;
            }
            return false;
        },

        getParams: function(node){
            var bp = Ext.apply({}, this.baseParams),
                np = this.nodeParameter,
                po = this.paramOrder;

            np && (bp[ np ] = node.id);

            if(this.directFn){
                var buf = [node.id];
                if(po){
                    // reset 'buf' if the nodeParameter was included in paramOrder
                    if(np && po.indexOf(np) > -1){
                        buf = [];
                    }

                    for(var i = 0, len = po.length; i < len; i++){
                        buf.push(bp[ po[i] ]);
                    }
                }else if(this.paramsAsHash){
                    buf = [bp];
                }
                return buf;
            }else{
                return bp;
            }
        },

        requestData : function(node, callback, scope){
            if(this.fireEvent("beforeload", this, node, callback) !== false){
                if(this.directFn){
                    var args = this.getParams(node);
                    args.push(this.processDirectResponse.createDelegate(this, [{callback: callback, node: node, scope: scope}], true));
                    this.directFn.apply(window, args);
                }else{
                    this.transId = Ext.Ajax.request({
                        method:this.requestMethod,
                        url: this.dataUrl||this.url,
                        success: this.handleResponse,
                        failure: this.handleFailure,
                        scope: this,
                        argument: {callback: callback, node: node, scope: scope},
                        params: this.getParams(node)
                    });
                }
            }else{
                // if the load is cancelled, make sure we notify
                // the node that we are done
                this.runCallback(callback, scope || node, []);
            }
        },

        processDirectResponse: function(result, response, args){
            if(response.status){
                this.handleResponse({
                    responseData: Ext.isArray(result) ? result : null,
                    responseText: result,
                    argument: args
                });
            }else{
                this.handleFailure({
                    argument: args
                });
            }
        },

        // private
        runCallback: function(cb, scope, args){
            if(Ext.isFunction(cb)){
                cb.apply(scope, args);
            }
        },

        isLoading : function(){
            return !!this.transId;
        },

        abort : function(){
            if(this.isLoading()){
                Ext.Ajax.abort(this.transId);
            }
        },

        /**
        * <p>Override this function for custom TreeNode node implementation, or to
        * modify the attributes at creation time.</p>
        * Example:<pre><code>
    new Ext.tree.TreePanel({
        ...
        loader: new Ext.tree.TreeLoader({
            url: 'dataUrl',
            createNode: function(attr) {
    //          Allow consolidation consignments to have
    //          consignments dropped into them.
                if (attr.isConsolidation) {
                    attr.iconCls = 'x-consol',
                    attr.allowDrop = true;
                }
                return Ext.tree.TreeLoader.prototype.createNode.call(this, attr);
            }
        }),
        ...
    });
    </code></pre>
        * @param attr {Object} The attributes from which to create the new node.
        */
        createNode : function(attr){
            // apply baseAttrs, nice idea Corey!
            if(this.baseAttrs){
                Ext.applyIf(attr, this.baseAttrs);
            }
            if(this.applyLoader !== false && !attr.loader){
                attr.loader = this;
            }
            if(Ext.isString(attr.uiProvider)){
               attr.uiProvider = this.uiProviders[attr.uiProvider] || eval(attr.uiProvider);
            }
            if(attr.nodeType){
                return new Ext.tree.TreePanel.nodeTypes[attr.nodeType](attr);
            }else{
                return attr.leaf ?
                            new Ext.tree.TreeNode(attr) :
                            new Ext.tree.AsyncTreeNode(attr);
            }
        },

        processResponse : function(response, node, callback, scope){
            var json = response.responseText;
            try {
                var o = response.responseData || Ext.decode(json);
                node.beginUpdate();
                for(var i = 0, len = o.length; i < len; i++){
                    var n = this.createNode(o[i]);
                    if(n){
                        node.appendChild(n);
                    }
                }
                node.endUpdate();
                this.runCallback(callback, scope || node, [node]);
            }catch(e){
                this.handleFailure(response);
            }
        },

        handleResponse : function(response){
            this.transId = false;
            var a = response.argument;
            this.processResponse(response, a.node, a.callback, a.scope);
            this.fireEvent("load", this, a.node, response);
        },

        handleFailure : function(response){
            this.transId = false;
            var a = response.argument;
            this.fireEvent("loadexception", this, a.node, response);
            this.runCallback(a.callback, a.scope || a.node, [a.node]);
        },

        destroy : function(){
            this.abort();
            this.purgeListeners();
        }
    });