Fix race condition with response attribute. Fixes #12.

There are two use cases as I see it. One is to fire off a series of requests
and react to all of them. The other is to treat all responses but the response to the most recent request as stale (e.g. in a search app you only want to show results for the current search and ignore out of order responses)

This change accomodates both use cases by firing events for every response (
which is unchanged), but setting up the response attribute to be either the
response for the most resent request, or null. Likewise it exposes an error
attribute that behaves the same way.

core-ajax.html

@@ -18,7 +18,7 @@
         params='{"alt":"json", "q":"chrome"}'
         handleAs="json"
         on-core-response="{{handleResponse}}"></core-ajax>
- 
+
 With `auto` set to `true`, the element performs a request whenever
 its `url` or `params` properties are changed.
 
@@ -32,19 +32,19 @@
 @homepage github.io
 -->
 <link rel="import" href="core-xhr.html">
-<polymer-element name="core-ajax" hidden attributes="url handleAs auto params response method headers body contentType withCredentials">
+<polymer-element name="core-ajax" hidden attributes="url handleAs auto params response error method headers body contentType withCredentials">
 <script>
 
   Polymer('core-ajax', {
     /**
      * Fired when a response is received.
-     * 
+     *
      * @event core-response
      */
 
     /**
      * Fired when an error is received.
-     * 
+     *
      * @event core-error
      */
 
@@ -56,7 +56,7 @@
 
     /**
      * The URL target of the request.
-     * 
+     *
      * @attribute url
      * @type string
      * @default ''
@@ -66,21 +66,21 @@
     /**
      * Specifies what data to store in the `response` property, and
      * to deliver as `event.response` in `response` events.
-     * 
+     *
      * One of:
-     * 
+     *
      *    `text`: uses `XHR.responseText`.
-     *    
+     *
      *    `xml`: uses `XHR.responseXML`.
-     *    
+     *
      *    `json`: uses `XHR.responseText` parsed as JSON.
      *
      *    `arraybuffer`: uses `XHR.response`.
      *
      *    `blob`: uses `XHR.response`.
      *
      *    `document`: uses `XHR.response`.
-     *  
+     *
      * @attribute handleAs
      * @type string
      * @default 'text'
@@ -98,22 +98,33 @@
 
     /**
      * Parameters to send to the specified URL, as JSON.
-     *  
+     *
      * @attribute params
      * @type string (JSON)
      * @default ''
      */
     params: '',
 
     /**
-     * Returns the response object.
+     * The response for the most recently made request, or null if it hasn't
+     * completed yet or the request resulted in error.
      *
      * @attribute response
      * @type Object
      * @default null
      */
     response: null,
 
+    /**
+     * The error for the most recently made request, or null if it hasn't
+     * completed yet or the request resulted in success.
+     *
+     * @attribute error
+     * @type Object
+     * @default null
+     */
+    error: null,
+
     /**
      * The HTTP method to use such as 'GET', 'POST', 'PUT', or 'DELETE'.
      * Default is 'GET'.
@@ -129,13 +140,13 @@
      *
      * Example:
      *
-     *     <core-ajax 
+     *     <core-ajax
      *         auto
      *         url="http://somesite.com"
      *         headers='{"X-Requested-With": "XMLHttpRequest"}'
      *         handleAs="json"
      *         on-core-response="{{handleResponse}}"></core-ajax>
-     *  
+     *
      * @attribute headers
      * @type Object
      * @default null
@@ -150,7 +161,7 @@
      *     <core-ajax method="POST" auto url="http://somesite.com"
      *         body='{"foo":1, "bar":2}'>
      *     </core-ajax>
-     *  
+     *
      * @attribute body
      * @type Object
      * @default null
@@ -168,26 +179,26 @@
 
     /**
      * Set the withCredentials flag on the request.
-     * 
+     *
      * @attribute withCredentials
      * @type boolean
      * @default false
      */
     withCredentials: false,
-    
+
     /**
      * Additional properties to send to core-xhr.
      *
      * Can be set to an object containing default properties
      * to send as arguments to the `core-xhr.request()` method
      * which implements the low-level communication.
-     * 
+     *
      * @property xhrArgs
      * @type Object
      * @default null
      */
     xhrArgs: null,
-     
+
     ready: function() {
       this.xhr = document.createElement('core-xhr');
     },
@@ -196,7 +207,7 @@
       if (this.isSuccess(xhr)) {
         this.processResponse(xhr);
       } else {
-        this.error(xhr);
+        this.processError(xhr);
       }
       this.complete(xhr);
     },
@@ -208,12 +219,17 @@
 
     processResponse: function(xhr) {
       var response = this.evalResponse(xhr);
-      this.response = response;
+      if (xhr === this.activeRequest) {
+        this.response = response;
+      }
       this.fire('core-response', {response: response, xhr: xhr});
     },
 
-    error: function(xhr) {
+    processError: function(xhr) {
       var response = xhr.status + ': ' + xhr.responseText;
+      if (xhr === this.activeRequest) {
+        this.error = response;
+      }
       this.fire('core-error', {response: response, xhr: xhr});
     },
 
@@ -277,8 +293,8 @@
       this.autoGo();
     },
 
-    // TODO(sorvell): multiple side-effects could call autoGo 
-    // during one micro-task, use a job to have only one action 
+    // TODO(sorvell): multiple side-effects could call autoGo
+    // during one micro-task, use a job to have only one action
     // occur
     autoGo: function() {
       if (this.auto) {
@@ -314,7 +330,10 @@
       args.callback = this.receive.bind(this);
       args.url = this.url;
       args.method = this.method;
-      return args.url && this.xhr.request(args);
+
+      this.response = this.error = null;
+      this.activeRequest = args.url && this.xhr.request(args);
+      return this.activeRequest;
     }
 
   });