/*
 * Ext JS Library 2.2
 * Copyright(c) 2006-2008, Ext JS, LLC.
 * licensing@extjs.com
 * 
 * http://extjs.com/license
 */

/**
 * @class Ext.data.ScriptTagProxy
 * @extends Ext.data.DataProxy
 * An implementation of Ext.data.DataProxy that reads a data object from a URL which may be in a domain
 * other than the originating domain of the running page.<br>
 * <p>
 * <b>Note that if you are retrieving data from a page that is in a domain that is NOT the same as the originating domain
 * of the running page, you must use this class, rather than HttpProxy.</b><br>
 * <p>
 * The content passed back from a server resource requested by a ScriptTagProxy <b>must</b> be executable JavaScript
 * source code because it is used as the source inside a &lt;script> tag.<br>
 * <p>
 * In order for the browser to process the returned data, the server must wrap the data object
 * with a call to a callback function, the name of which is passed as a parameter by the ScriptTagProxy.
 * Below is a Java example for a servlet which returns data for either a ScriptTagProxy, or an HttpProxy
 * depending on whether the callback name was passed:
 * <p>
 * <pre><code>
boolean scriptTag = false;
String cb = request.getParameter("callback");
if (cb != null) {
    scriptTag = true;
    response.setContentType("text/javascript");
} else {
    response.setContentType("application/x-json");
}
Writer out = response.getWriter();
if (scriptTag) {
    out.write(cb + "(");
}
out.print(dataBlock.toJsonString());
if (scriptTag) {
    out.write(");");
}
</code></pre>
 *
 * @constructor
 * @param {Object} config A configuration object.
 */
Ext.data.ScriptTagProxy = function(config){
    Ext.data.ScriptTagProxy.superclass.constructor.call(this);
    Ext.apply(this, config);
    this.head = document.getElementsByTagName("head")[0];
    
    /**
     * @event loadexception
     * Fires if an exception occurs in the Proxy during data loading.  This event can be fired for one of two reasons:
     * <ul><li><b>The load call timed out.</b>  This means the load callback did not execute within the time limit
     * specified by {@link #timeout}.  In this case, this event will be raised and the
     * fourth parameter (read error) will be null.</li>
     * <li><b>The load succeeded but the reader could not read the response.</b>  This means the server returned
     * data, but the configured Reader threw an error while reading the data.  In this case, this event will be 
     * raised and the caught error will be passed along as the fourth parameter of this event.</li></ul>
     * Note that this event is also relayed through {@link Ext.data.Store}, so you can listen for it directly
     * on any Store instance.
     * @param {Object} this
     * @param {Object} options The loading options that were specified (see {@link #load} for details).  If the load
     * call timed out, this parameter will be null.
     * @param {Object} arg The callback's arg object passed to the {@link #load} function
     * @param {Error} e The JavaScript Error object caught if the configured Reader could not read the data.
     * If the load call returned success: false, this parameter will be null.
     */
};

Ext.data.ScriptTagProxy.TRANS_ID = 1000;

Ext.extend(Ext.data.ScriptTagProxy, Ext.data.DataProxy, {
    /**
     * @cfg {String} url The URL from which to request the data object.
     */
    /**
     * @cfg {Number} timeout (optional) The number of milliseconds to wait for a response. Defaults to 30 seconds.
     */
    timeout : 30000,
    /**
     * @cfg {String} callbackParam (Optional) The name of the parameter to pass to the server which tells
     * the server the name of the callback function set up by the load call to process the returned data object.
     * Defaults to "callback".<p>The server-side processing must read this parameter value, and generate
     * javascript output which calls this named function passing the data object as its only parameter.
     */
    callbackParam : "callback",
    /**
     *  @cfg {Boolean} nocache (optional) Defaults to true. Disable caching by adding a unique parameter
     * name to the request.
     */
    nocache : true,

    /**
     * Load data from the configured URL, read the data object into
     * a block of Ext.data.Records using the passed Ext.data.DataReader implementation, and
     * process that block using the passed callback.
     * @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 The function into which to pass the block of Ext.data.Records.
     * The function must be passed <ul>
     * <li>The Record block object</li>
     * <li>The "arg" argument from the load function</li>
     * <li>A boolean success indicator</li>
     * </ul>
     * @param {Object} scope The scope in which to call the callback
     * @param {Object} arg An optional argument which is passed to the callback as its second parameter.
     */
    load : function(params, reader, callback, scope, arg){
        if(this.fireEvent("beforeload", this, params) !== false){

            var p = Ext.urlEncode(Ext.apply(params, this.extraParams));

            var url = this.url;
            url += (url.indexOf("?") != -1 ? "&" : "?") + p;
            if(this.nocache){
                url += "&_dc=" + (new Date().getTime());
            }
            var transId = ++Ext.data.ScriptTagProxy.TRANS_ID;
            var trans = {
                id : transId,
                cb : "stcCallback"+transId,
                scriptId : "stcScript"+transId,
                params : params,
                arg : arg,
                url : url,
                callback : callback,
                scope : scope,
                reader : reader
            };
            var conn = this;

            window[trans.cb] = function(o){
                conn.handleResponse(o, trans);
            };

            url += String.format("&{0}={1}", this.callbackParam, trans.cb);

            if(this.autoAbort !== false){
                this.abort();
            }

            trans.timeoutId = this.handleFailure.defer(this.timeout, this, [trans]);

            var script = document.createElement("script");
            script.setAttribute("src", url);
            script.setAttribute("type", "text/javascript");
            script.setAttribute("id", trans.scriptId);
            this.head.appendChild(script);

            this.trans = trans;
        }else{
            callback.call(scope||this, null, arg, false);
        }
    },

    // private
    isLoading : function(){
        return this.trans ? true : false;
    },

    /**
     * Abort the current server request.
     */
    abort : function(){
        if(this.isLoading()){
            this.destroyTrans(this.trans);
        }
    },

    // private
    destroyTrans : function(trans, isLoaded){
        this.head.removeChild(document.getElementById(trans.scriptId));
        clearTimeout(trans.timeoutId);
        if(isLoaded){
            window[trans.cb] = undefined;
            try{
                delete window[trans.cb];
            }catch(e){}
        }else{
            // if hasn't been loaded, wait for load to remove it to prevent script error
            window[trans.cb] = function(){
                window[trans.cb] = undefined;
                try{
                    delete window[trans.cb];
                }catch(e){}
            };
        }
    },

    // private
    handleResponse : function(o, trans){
        this.trans = false;
        this.destroyTrans(trans, true);
        var result;
        try {
            result = trans.reader.readRecords(o);
        }catch(e){
            this.fireEvent("loadexception", this, o, trans.arg, e);
            trans.callback.call(trans.scope||window, null, trans.arg, false);
            return;
        }
        this.fireEvent("load", this, o, trans.arg);
        trans.callback.call(trans.scope||window, result, trans.arg, true);
    },

    // private
    handleFailure : function(trans){
        this.trans = false;
        this.destroyTrans(trans, false);
        this.fireEvent("loadexception", this, null, trans.arg);
        trans.callback.call(trans.scope||window, null, trans.arg, false);
    }
});