feat(ngModel): add ngModelContext for getter/setter bindings

Adds an optional attribute, `ng-model-context`, that will be evaluated to provide the calling
context used when invoking the ngModel getter/setter function. When not provided, falls back to the
existing behavior of invoking getter/setter functions from the global context.

Closes #9394

Author: NevilleS

src/ng/directive/input.js

@@ -1716,13 +1716,19 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
 
 
   var parsedNgModel = $parse($attr.ngModel),
+      parsedNgModelContext = null,
       pendingDebounce = null,
       ctrl = this;
 
+  if ($attr.ngModelContext) {
+    parsedNgModelContext = $parse($attr.ngModelContext);
+  }
+
   var ngModelGet = function ngModelGet() {
     var modelValue = parsedNgModel($scope);
     if (ctrl.$options && ctrl.$options.getterSetter && isFunction(modelValue)) {
-      modelValue = modelValue();
+      var context = parsedNgModelContext ? parsedNgModelContext($scope) : this;
+      modelValue = modelValue.call(context);
     }
     return modelValue;
   };
@@ -1732,7 +1738,8 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     if (ctrl.$options && ctrl.$options.getterSetter &&
         isFunction(getterSetter = parsedNgModel($scope))) {
 
-      getterSetter(ctrl.$modelValue);
+      var context = parsedNgModelContext ? parsedNgModelContext($scope) : this;
+      getterSetter.call(context, ctrl.$modelValue);
     } else {
       parsedNgModel.assign($scope, ctrl.$modelValue);
     }
@@ -1745,6 +1752,10 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
       throw $ngModelMinErr('nonassign', "Expression '{0}' is non-assignable. Element: {1}",
           $attr.ngModel, startingTag($element));
     }
+    if (parsedNgModelContext && !parsedNgModelContext.assign) {
+      throw $ngModelMinErr('nonassign', "Expression '{0}' is non-assignable. Element: {1}",
+          $attr.ngModelContext, startingTag($element));
+    }
   };
 
   /**

test/ng/directive/inputSpec.js

@@ -1975,6 +1975,84 @@ describe('input', function() {
           'ng-model-options="{ getterSetter: true }" />');
     });
 
+    it('should try to invoke a model with global context if getterSetter is true and ngModelContext is not provided', inject(function($window) {
+      $window.value = 'global';
+      compileInput(
+        '<input type="text" ng-model="someService.getterSetter" '+
+          'ng-model-options="{ getterSetter: true }" />');
+
+      scope.someService = {
+        value: 'b',
+        getterSetter: function(newValue) {
+          this.value = newValue || this.value;
+          return this.value;
+        }
+      };
+      spyOn(scope.someService, 'getterSetter').andCallThrough();
+      scope.$apply();
+      expect(inputElm.val()).toBe('global');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith();
+      expect(scope.someService.value).toBe('b'); // 'this' is not bound to the service w/o ngModelContext
+      expect($window.value).toBe('global');
+
+      changeInputValueTo('a');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith('a');
+      expect(scope.someService.value).toBe('b');
+      expect($window.value).toBe('a');
+
+      scope.someService.value = 'c';
+      scope.$apply();
+      expect(inputElm.val()).toBe('a');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith();
+      expect(scope.someService.value).toBe('c');
+      expect($window.value).toBe('a');
+
+      $window.value = 'd';
+      scope.$apply();
+      expect(inputElm.val()).toBe('d');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith();
+      expect(scope.someService.value).toBe('c');
+      expect($window.value).toBe('d');
+    }));
+
+    it('should try to invoke a model with the provided context if getterSetter is true and ngModelContext is provided', function() {
+      compileInput(
+        '<input type="text" ng-model="someService.getterSetter" '+
+          'ng-model-context="someService" '+
+          'ng-model-options="{ getterSetter: true }" />');
+
+      scope.someService = {
+        value: 'b',
+        getterSetter: function(newValue) {
+          this.value = newValue || this.value;
+          return this.value;
+        }
+      };
+      spyOn(scope.someService, 'getterSetter').andCallThrough();
+      scope.$apply();
+      expect(inputElm.val()).toBe('b');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith();
+      expect(scope.someService.value).toBe('b');
+
+      changeInputValueTo('a');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith('a');
+      expect(scope.someService.value).toBe('a');
+
+      scope.someService.value = 'c';
+      scope.$apply();
+      expect(inputElm.val()).toBe('c');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith();
+      expect(scope.someService.value).toBe('c');
+    });
+
+    it('should fail on non-assignable ngModelContext binding', function() {
+      expect(function() {
+        compileInput('<input type="text" ng-model="someService.getterSetter" '+
+          'ng-model-context="someService.getInstance()" ' +
+          'ng-model-options="{ getterSetter: true }" />');
+      }).toThrowMinErr('ngModel', 'nonassign', 'Expression \'someService.getInstance()\' is non-assignable.');
+    });
+
     it('should assign invalid values to the scope if allowInvalid is true', function() {
       compileInput('<input type="text" name="input" ng-model="value" maxlength="1" ' +
                    'ng-model-options="{allowInvalid: true}" />');