perf(ngOptions): only perform deep equality check on ngModel if using track by

Closes #11448
Closes #11447

Author: booleanbetrayal

src/ng/directive/ngOptions.js

@@ -32,8 +32,9 @@ var ngOptionsMinErr = minErr('ngOptions');
  * option. See example below for demonstration.
  *
  * <div class="alert alert-warning">
- * **Note:** `ngModel` compares by reference, not value. This is important when binding to an
- * array of objects. See an example [in this jsfiddle](http://jsfiddle.net/qWzTb/).
+ * **Note:** By default, `ngModel` compares by reference, not value. This is important when binding to an
+ * array of objects. See an example [in this jsfiddle](http://jsfiddle.net/qWzTb/). When using `track by`
+ * in an `ngOptions` expression, however, deep equality checks will be performed.
  * </div>
  *
  * ## `select` **`as`**
@@ -275,6 +276,7 @@ var ngOptionsDirective = ['$compile', '$parse', function($compile, $parse) {
     }
 
     return {
+      trackBy: trackBy,
       getWatchables: $parse(valuesFn, function(values) {
         // Create a collection of things that we would like to watch (watchedArray)
         // so that they can all be watched using a single $watchCollection
@@ -500,8 +502,9 @@ var ngOptionsDirective = ['$compile', '$parse', function($compile, $parse) {
 
       // We also need to watch to see if the internals of the model changes, since
       // ngModel only watches for object identity change
-      scope.$watch(attr.ngModel, function() { ngModelCtrl.$render(); }, true);
-
+      if (ngOptions.trackBy) {
+        scope.$watch(attr.ngModel, function() { ngModelCtrl.$render(); }, true);
+      }
       // ------------------------------------------------------------------ //
 
 
@@ -643,10 +646,13 @@ var ngOptionsDirective = ['$compile', '$parse', function($compile, $parse) {
         // Check to see if the value has changed due to the update to the options
         if (!ngModelCtrl.$isEmpty(previousValue)) {
           var nextValue = selectCtrl.readValue();
-          if (!equals(previousValue, nextValue)) {
+          if (ngOptions.trackBy && !equals(previousValue, nextValue) ||
+                previousValue !== nextValue) {
             ngModelCtrl.$setViewValue(nextValue);
+            ngModelCtrl.$render();
           }
         }
+
       }
 
     }

src/ng/directive/select.js

@@ -135,8 +135,9 @@ var SelectController =
  * option. See example below for demonstration.
  *
  * <div class="alert alert-warning">
- * **Note:** `ngModel` compares by reference, not value. This is important when binding to an
- * array of objects. See an example [in this jsfiddle](http://jsfiddle.net/qWzTb/).
+ * **Note:** By default, `ngModel` compares by reference, not value. This is important when binding to an
+ * array of objects. See an example [in this jsfiddle](http://jsfiddle.net/qWzTb/). When using `track by`
+ * in an `ngOptions` expression, however, deep equality checks will be performed.
  * </div>
  *
  */

test/ng/directive/ngOptionsSpec.js

@@ -1041,6 +1041,50 @@ describe('ngOptions', function() {
         });
       }).not.toThrow();
     });
+
+    it('should setup equality watches on ngModel changes if using trackBy', function() {
+
+      createSelect({
+        'ng-model': 'selected',
+        'ng-options': 'item for item in arr track by item.id'
+      });
+
+      scope.$apply(function() {
+        scope.selected = scope.arr[0];
+      });
+
+      spyOn(element.controller('ngModel'), '$render');
+
+      scope.$apply(function() {
+        scope.selected.label = 'changed';
+      });
+
+      // update render due to equality watch
+      expect(element.controller('ngModel').$render).toHaveBeenCalled();
+
+    });
+
+    it('should not setup equality watches on ngModel changes if not using trackBy', function() {
+
+      createSelect({
+        'ng-model': 'selected',
+        'ng-options': 'item for item in arr'
+      });
+
+      scope.$apply(function() {
+        scope.selected = scope.arr[0];
+      });
+
+      spyOn(element.controller('ngModel'), '$render');
+
+      scope.$apply(function() {
+        scope.selected.label = 'changed';
+      });
+
+      // no render update as no equality watch
+      expect(element.controller('ngModel').$render).not.toHaveBeenCalled();
+    });
+
   });