fix/perf: combine value and metaVar listener

checking metaVars and scopeValue separately causes the listener function
to be called twice during a $digest cycle if both the metaVars and value
change. This definitely causes a performance hit, and might be leading to
subtle bugs

Author: jamestalmage

src/FirebaseObject.js

@@ -354,20 +354,16 @@
 
             // $watch will not check any vars prefixed with $, so we
             // manually check $priority and $value using this method
-            function checkMetaVars() {
-              var dat = parsed(scope);
-              if( dat.$value !== rec.$value || dat.$priority !== rec.$priority ) {
-                scopeUpdated();
-              }
+            function watchExp(){
+              var obj = parsed(scope);
+              return [obj, obj.$priority, obj.$value];
             }
 
-            self.subs.push(scope.$watch(checkMetaVars));
-
             setScope(rec);
             self.subs.push(scope.$on('$destroy', self.unbind.bind(self)));
 
             // monitor scope for any changes
-            self.subs.push(scope.$watch(varName, scopeUpdated, true));
+            self.subs.push(scope.$watch(watchExp, scopeUpdated, true));
 
             // monitor the object for changes
             self.subs.push(rec.$watch(recUpdated));

tests/unit/FirebaseObject.spec.js

@@ -331,8 +331,7 @@ describe('$FirebaseObject', function() {
         $scope.test.$priority = 999;
       });
       $interval.flush(500);
-      $timeout.flush(); // for $interval
-      $timeout.flush(); // for $watch
+      $timeout.flush();
       expect(spy).toHaveBeenCalledWith(jasmine.objectContaining({'.priority': 999}));
     });
 
@@ -348,11 +347,39 @@ describe('$FirebaseObject', function() {
         $scope.test.$value = 'bar';
       });
       $interval.flush(500);
-      $timeout.flush(); // for $interval
-      $timeout.flush(); // for $watch
+      $timeout.flush();
       expect(spy).toHaveBeenCalledWith(jasmine.objectContaining({'.value': 'bar'}));
     });
 
+    it('should only call $$scopeUpdated once if both metaVars and properties change in the same $digest',function(){
+      var $scope = $rootScope.$new();
+      var fb = new MockFirebase();
+      fb.autoFlush(true);
+      fb.setWithPriority({text:'hello'},3);
+      var $fb = new $firebase(fb);
+      var obj = $fb.$asObject();
+      flushAll();
+      flushAll();
+      obj.$bindTo($scope, 'test');
+      $scope.$apply();
+      expect($scope.test).toEqual({text:'hello', $id: obj.$id, $priority: 3});
+      var callCount = 0;
+      var old$scopeUpdated = obj.$$scopeUpdated;
+      obj.$$scopeUpdated = function(){
+        callCount++;
+        return old$scopeUpdated.apply(this,arguments);
+      };
+      $scope.$apply(function(){
+        $scope.test.text='goodbye';
+        $scope.test.$priority=4;
+      });
+      flushAll();
+      flushAll();
+      flushAll();
+      flushAll();
+      expect(callCount).toEqual(1);
+    });
+
     it('should throw error if double bound', function() {
       var $scope = $rootScope.$new();
       var aSpy = jasmine.createSpy('firstBind');