AngularJS’控制器为’和形式.$有效

当我尝试访问表单时.$valid我收到错误消息“无法读取属性’$valid’of undefined”.

plunkr：http://plnkr.co/edit/w54i1bZVD8UMhxB4L2JX?p=preview

HTML

<div ng-app="demoControllerAs" ng-controller="MainController as main">
  <form name="contactForm" novalidate>
    <p>
      <label>Email</label>
      <input type="email" name="email" ng-model="main.email" required />
    </p>
    <p>
      <label>Message</label>
      <textarea name="message" ng-model="main.message" required></textarea>
    </p>
    <input type="submit" value="submit" ng-click="main.submit()" />
  </form>
</div>

JS

var app = angular.module('demoControllerAs',[]);

app.controller('MainController',[function () {
    var main = this;

    main.submit = function () {
        var isValid = main.contactForm.$valid;
        console.log(isValid);
    };
}]);