fix(ngModel): minimize jank when toggling CSS classes during validation

Previously, class toggling would always occur immediately. This causes problems
in cases where class changes happen super frequently, and can result in flickering
in some browsers which do not handle this jank well.

Closes angular#8234

src/ng/directive/input.js

@@ -2053,9 +2053,11 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
   };
 
   this.$$parseAndValidate = function() {
+    var pendingClassChanges = null;
     var viewValue = ctrl.$$lastCommittedViewValue;
     var modelValue = viewValue;
     var parserValid = isUndefined(modelValue) ? undefined : true;
+    var flushPendingClassChanges = schedulePendingClassChanges($scope, ctrl, $element, $animate);
 
     if (parserValid) {
       for(var i = 0; i < ctrl.$parsers.length; i++) {
@@ -2092,6 +2094,8 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
         ctrl.$$writeModelToScope();
       }
     }
+
+    flushPendingClassChanges();
   };
 
   this.$$writeModelToScope = function() {
@@ -2197,7 +2201,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     // TODO(perf): why not move this to the action fn?
     if (modelValue !== ctrl.$modelValue) {
       ctrl.$modelValue = modelValue;
-
+      var flushPendingClassChanges = schedulePendingClassChanges($scope, ctrl, $element, $animate);
       var formatters = ctrl.$formatters,
           idx = formatters.length;
 
@@ -2211,13 +2215,45 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
 
         ctrl.$$runValidators(undefined, modelValue, viewValue, noop);
       }
+      flushPendingClassChanges();
     }
 
     return modelValue;
   });
 }];
 
 
+function schedulePendingClassChanges(scope, ctrl, element, animate) {
+  if (!ctrl.$$pendingClassChanges) {
+    ctrl.$$pendingClassChanges = {};
+
+    if (scope.$$phase || scope.$root.$$phase) {
+      scope.$$postDigest(flushPendingClassChangesImmediately);
+    } else {
+      return flushPendingClassChangesImmediately;
+    }
+  }
+  return noop;
+
+  function flushPendingClassChangesImmediately() {
+    flushPendingClassChanges(animate, element, ctrl.$$pendingClassChanges);
+    ctrl.$$pendingClassChanges = null;
+  }
+}
+
+
+function flushPendingClassChanges($animate, element, pendingChanges) {
+  var keys = Object.keys(pendingChanges);
+
+  for (var i=0, ii = keys.length; i < ii; ++i) {
+    var key = keys[i];
+    var value = pendingChanges[key];
+    if (value < 0) $animate.removeClass(element, key);
+    else if (value > 0) $animate.addClass(element, key);
+  }
+}
+
+
 /**
  * @ngdoc directive
  * @name ngModel
@@ -3037,6 +3073,18 @@ function addSetValidityMethod(context) {
   }
 
   function cachedToggleClass(className, switchValue) {
+    var pendingChanges = ctrl.$$pendingClassChanges;
+    if (pendingChanges) {
+      if (switchValue) {
+        pendingChanges[className] = 1;
+        classCache[className] = true;
+      } else {
+        pendingChanges[className] = -1;
+        classCache[className] = false;
+      }
+      return;
+    }
+
     if (switchValue && !classCache[className]) {
       $animate.addClass($element, className);
       classCache[className] = true;

test/ng/directive/inputSpec.js

@@ -892,6 +892,51 @@ describe('NgModelController', function() {
       dealoc(element);
     }));
 
+
+    it('should minimize janky setting of classes during $validate() and ngModelWatch', inject(function($animate, $compile, $rootScope) {
+      var addClass = $animate.addClass;
+      var removeClass = $animate.removeClass;
+      var addClassCallCount = 0;
+      var removeClassCallCount = 0;
+      var input;
+      $animate.addClass = function(element, className) {
+        // Don't worry about classes that the input already has.
+        if (input && element[0] === input[0] && (' ' + element.attr('class') + ' ').indexOf(' ' + className + ' ') < 0) {
+          ++addClassCallCount;
+        }
+        return addClass.call($animate, element, className);
+      };
+
+      $animate.removeClass = function(element, className) {
+        // Don't worry about classes that the input doesn't have.
+        if (input && element[0] === input[0] && (' ' + element.attr('class') + ' ').indexOf(' ' + className + ' ') !== -1) {
+          ++removeClassCallCount;
+        }
+        return removeClass.call($animate, element, className);
+      };
+
+      dealoc(element);
+
+      $rootScope.value = "123456789";
+      element = $compile(
+        '<form name="form">' +
+            '<input type="text" ng-model="value" name="alias" ng-maxlength="10">' +
+        '</form>'
+      )($rootScope);
+
+      var form = $rootScope.form;
+      input = element.children().eq(0);
+
+      $rootScope.$digest();
+
+      expect(input).toBeValid();
+      expect(input).not.toHaveClass('ng-invalid-maxlength');
+      expect(input).toHaveClass('ng-valid-maxlength');
+      expect(addClassCallCount).toBe(2);
+      expect(removeClassCallCount).toBe(0);
+
+      dealoc(element);
+    }));
   });
 });