The source code

    /**
     * @class Ext.data.HttpProxy
     * @extends Ext.data.DataProxy
     * <p>An implementation of {@link Ext.data.DataProxy} that processes data requests within the same
     * domain of the originating page.</p>
     * <p><b>Note</b>: this class cannot be used to retrieve data from a domain other
     * than the domain from which the running page was served. For cross-domain requests, use a
     * {@link Ext.data.ScriptTagProxy ScriptTagProxy}.</p>
     * <p>Be aware that to enable the browser to parse an XML document, the server must set
     * the Content-Type header in the HTTP response to "<tt>text/xml</tt>".</p>
     * @constructor
     * @param {Object} conn
     * An {@link Ext.data.Connection} object, or options parameter to {@link Ext.Ajax#request}.
     * <p>Note that if this HttpProxy is being used by a {@link Ext.data.Store Store}, then the
     * Store's call to {@link #load} will override any specified <tt>callback</tt> and <tt>params</tt>
     * options. In this case, use the Store's {@link Ext.data.Store#events events} to modify parameters,
     * or react to loading events. The Store's {@link Ext.data.Store#baseParams baseParams} may also be
     * used to pass parameters known at instantiation time.</p>
     * <p>If an options parameter is passed, the singleton {@link Ext.Ajax} object will be used to make
     * the request.</p>
     */
    Ext.data.HttpProxy = function(conn){
        Ext.data.HttpProxy.superclass.constructor.call(this, conn);

        /**
         * The Connection object (Or options parameter to {@link Ext.Ajax#request}) which this HttpProxy
         * uses to make requests to the server. Properties of this object may be changed dynamically to
         * change the way data is requested.
         * @property
         */
        this.conn = conn;

        // nullify the connection url.  The url param has been copied to 'this' above.  The connection
        // url will be set during each execution of doRequest when buildUrl is called.  This makes it easier for users to override the
        // connection url during beforeaction events (ie: beforeload, beforewrite, etc).
        // Url is always re-defined during doRequest.
        this.conn.url = null;

        this.useAjax = !conn || !conn.events;

        // A hash containing active requests, keyed on action [Ext.data.Api.actions.create|read|update|destroy]
        var actions = Ext.data.Api.actions;
        this.activeRequest = {};
        for (var verb in actions) {
            this.activeRequest[actions[verb]] = undefined;
        }
    };

    Ext.extend(Ext.data.HttpProxy, Ext.data.DataProxy, {
        /**
         * Return the {@link Ext.data.Connection} object being used by this Proxy.
         * @return {Connection} The Connection object. This object may be used to subscribe to events on
         * a finer-grained basis than the DataProxy events.
         */
        getConnection : function() {
            return this.useAjax ? Ext.Ajax : this.conn;
        },

        /**
         * Used for overriding the url used for a single request.  Designed to be called during a beforeaction event.  Calling setUrl
         * will override any urls set via the api configuration parameter.  Set the optional parameter makePermanent to set the url for
         * all subsequent requests.  If not set to makePermanent, the next request will use the same url or api configuration defined
         * in the initial proxy configuration.
         * @param {String} url
         * @param {Boolean} makePermanent (Optional) [false]
         *
         * (e.g.: beforeload, beforesave, etc).
         */
        setUrl : function(url, makePermanent) {
            this.conn.url = url;
            if (makePermanent === true) {
                this.url = url;
                this.api = null;
                Ext.data.Api.prepare(this);
            }
        },

        /**
         * HttpProxy implementation of DataProxy#doRequest
         * @param {String} action The crud action type (create, read, update, destroy)
         * @param {Ext.data.Record/Ext.data.Record[]} rs If action is load, rs will be null
         * @param {Object} params An object containing properties which are to be used as HTTP parameters
         * for the request to the remote server.
         * @param {Ext.data.DataReader} reader The Reader object which converts the data
         * object into a block of Ext.data.Records.
         * @param {Function} callback
         * <div class="sub-desc"><p>A function to be called after the request.
         * The <tt>callback</tt> is passed the following arguments:<ul>
         * <li><tt>r</tt> : Ext.data.Record[] The block of Ext.data.Records.</li>
         * <li><tt>options</tt>: Options object from the action request</li>
         * <li><tt>success</tt>: Boolean success indicator</li></ul></p></div>
         * @param {Object} scope The scope (<code>this</code> reference) in which the callback function is executed. Defaults to the browser window.
         * @param {Object} arg An optional argument which is passed to the callback as its second parameter.
         * @protected
         */
        doRequest : function(action, rs, params, reader, cb, scope, arg) {
            var  o = {
                method: (this.api[action]) ? this.api[action]['method'] : undefined,
                request: {
                    callback : cb,
                    scope : scope,
                    arg : arg
                },
                reader: reader,
                callback : this.createCallback(action, rs),
                scope: this
            };

            // If possible, transmit data using jsonData || xmlData on Ext.Ajax.request (An installed DataWriter would have written it there.).
            // Use std HTTP params otherwise.
            if (params.jsonData) {
                o.jsonData = params.jsonData;
            } else if (params.xmlData) {
                o.xmlData = params.xmlData;
            } else {
                o.params = params || {};
            }
            // Set the connection url.  If this.conn.url is not null here,
            // the user must have overridden the url during a beforewrite/beforeload event-handler.
            // this.conn.url is nullified after each request.
            this.conn.url = this.buildUrl(action, rs);

            if(this.useAjax){

                Ext.applyIf(o, this.conn);

                // If a currently running read request is found, abort it
                if (action == Ext.data.Api.actions.read && this.activeRequest[action]) {
                    Ext.Ajax.abort(this.activeRequest[action]);
                }
                this.activeRequest[action] = Ext.Ajax.request(o);
            }else{
                this.conn.request(o);
            }
            // request is sent, nullify the connection url in preparation for the next request
            this.conn.url = null;
        },

        /**
         * Returns a callback function for a request.  Note a special case is made for the
         * read action vs all the others.
         * @param {String} action [create|update|delete|load]
         * @param {Ext.data.Record[]} rs The Store-recordset being acted upon
         * @private
         */
        createCallback : function(action, rs) {
            return function(o, success, response) {
                this.activeRequest[action] = undefined;
                if (!success) {
                    if (action === Ext.data.Api.actions.read) {
                        // @deprecated: fire loadexception for backwards compat.
                        // TODO remove
                        this.fireEvent('loadexception', this, o, response);
                    }
                    this.fireEvent('exception', this, 'response', action, o, response);
                    o.request.callback.call(o.request.scope, null, o.request.arg, false);
                    return;
                }
                if (action === Ext.data.Api.actions.read) {
                    this.onRead(action, o, response);
                } else {
                    this.onWrite(action, o, response, rs);
                }
            };
        },

        /**
         * Callback for read action
         * @param {String} action Action name as per {@link Ext.data.Api.actions#read}.
         * @param {Object} o The request transaction object
         * @param {Object} res The server response
         * @fires loadexception (deprecated)
         * @fires exception
         * @fires load
         * @protected
         */
        onRead : function(action, o, response) {
            var result;
            try {
                result = o.reader.read(response);
            }catch(e){
                // @deprecated: fire old loadexception for backwards-compat.
                // TODO remove
                this.fireEvent('loadexception', this, o, response, e);

                this.fireEvent('exception', this, 'response', action, o, response, e);
                o.request.callback.call(o.request.scope, null, o.request.arg, false);
                return;
            }
            if (result.success === false) {
                // @deprecated: fire old loadexception for backwards-compat.
                // TODO remove
                this.fireEvent('loadexception', this, o, response);

                // Get DataReader read-back a response-object to pass along to exception event
                var res = o.reader.readResponse(action, response);
                this.fireEvent('exception', this, 'remote', action, o, res, null);
            }
            else {
                this.fireEvent('load', this, o, o.request.arg);
            }
            // TODO refactor onRead, onWrite to be more generalized now that we're dealing with Ext.data.Response instance
            // the calls to request.callback(...) in each will have to be made identical.
            // NOTE reader.readResponse does not currently return Ext.data.Response
            o.request.callback.call(o.request.scope, result, o.request.arg, result.success);
        },
        /**
         * Callback for write actions
         * @param {String} action [Ext.data.Api.actions.create|read|update|destroy]
         * @param {Object} trans The request transaction object
         * @param {Object} res The server response
         * @fires exception
         * @fires write
         * @protected
         */
        onWrite : function(action, o, response, rs) {
            var reader = o.reader;
            var res;
            try {
                res = reader.readResponse(action, response);
            } catch (e) {
                this.fireEvent('exception', this, 'response', action, o, response, e);
                o.request.callback.call(o.request.scope, null, o.request.arg, false);
                return;
            }
            if (res.success === true) {
                this.fireEvent('write', this, action, res.data, res, rs, o.request.arg);
            } else {
                this.fireEvent('exception', this, 'remote', action, o, res, rs);
            }
            // TODO refactor onRead, onWrite to be more generalized now that we're dealing with Ext.data.Response instance
            // the calls to request.callback(...) in each will have to be made similar.
            // NOTE reader.readResponse does not currently return Ext.data.Response
            o.request.callback.call(o.request.scope, res.data, res, res.success);
        },

        // inherit docs
        destroy: function(){
            if(!this.useAjax){
                this.conn.abort();
            }else if(this.activeRequest){
                var actions = Ext.data.Api.actions;
                for (var verb in actions) {
                    if(this.activeRequest[actions[verb]]){
                        Ext.Ajax.abort(this.activeRequest[actions[verb]]);
                    }
                }
            }
            Ext.data.HttpProxy.superclass.destroy.call(this);
        }
    });