fix(ng:view): ignore stale xhr callbacks

A lot of badness happens when we don't ignore stale xhrs. These
raceconditions are only apparent when user clicks through the app very
quckly without waiting for routes to fully load.

Closes angular#619

Author: IgorMinar

src/widgets.js

@@ -551,13 +551,16 @@ angularWidget('ng:view', function(element) {
         changeCounter++;
       });
 
-      this.$watch(function() {return changeCounter;}, function() {
+      this.$watch(function() {return changeCounter;}, function(scope, newChangeCounter) {
         var template = $route.current && $route.current.template;
         if (template) {
           //xhr's callback must be async, see commit history for more info
           $xhr('GET', template, function(code, response) {
-            element.html(response);
-            compiler.compile(element)($route.current.scope);
+            // ignore callback if another route change occured since
+            if (newChangeCounter == changeCounter) {
+              element.html(response);
+              compiler.compile(element)($route.current.scope);
+            }
           });
         } else {
           element.html('');

test/widgetsSpec.js

@@ -539,6 +539,26 @@ describe("widget", function() {
 
       expect(rootScope.log).toEqual(['parent', 'init', 'child']);
     });
+
+    it('should discard pending xhr callbacks if a new route is requested before the current ' +
+        'finished loading', function() {
+      // this is a test for a bad race condition that affected feedback
+
+      $route.when('/foo', {template: 'myUrl1'});
+      $route.when('/bar', {template: 'myUrl2'});
+
+      expect(rootScope.$element.text()).toEqual('');
+
+      $location.path('/foo');
+      $browser.xhr.expectGET('myUrl1').respond('<div>{{1+3}}</div>');
+      rootScope.$digest();
+      $location.path('/bar');
+      $browser.xhr.expectGET('myUrl2').respond('<div>{{1+1}}</div>');
+      rootScope.$digest();
+      $browser.xhr.flush(); // no that we have to requests pending, flush!
+
+      expect(rootScope.$element.text()).toEqual('2');
+    });
   });