我有一个日期选择器控件，我在其中添加了一个必需的字段验证器。错误消息显示正确，如果我在字段中执行退格，从而删除日期，然后单击提交。但是，如果我选择日期，然后单击键盘上的删除，然后单击提交按钮，则不会显示错误消息。 以下是现有代码：
Index.cshtml
<!DOCTYPE html>
<html>
<head>
<!-- Compiled and minified CSS -->
<!-- Compiled and minified JavaScript -->
<!--Import Google Icon Font-->
<link href="http://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
<!--Import materialize.css-->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/materialize/0.97.8/css/materialize.min.css">
<!--Let browser know website is optimized for mobile-->
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
</head>
<body>
<!--Import jQuery before materialize.js-->
<div class="input-field col m6">
<i class="material-icons prefix">picture_in_picture</i>
<input id="hello_id" type="date"/>
<label for="hello_id">Hello, this text is a test</label>
</div>
<script type="text/javascript" src="https://code.jquery.com/jquery-2.1.1.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/materialize/0.97.8/js/materialize.min.js"></script>
</body>
</html>
在 controller.js 中，日期设置如下：
<div ng-controller="testController" ng-init="init()">
<form name="mainForm" id="createForm" ng-submit="mainForm.$valid && add()" novalidate="">
<div class="col-sm-6">
<label class="control-label">Start date</label>
<div class="form-group">
<div class="input-group date" id="startDatepicker">
<input type="text" required="" class="form-control" placeholder="MM/DD/YYYY" ng-model="defaultStartDate" name="startDate">
<span class="input-group-addon">
<span class="glyphicon glyphicon-calendar"></span>
</span>
</div>
</div>
<span style="color:red" ng-show="submitted == true && mainForm.startDate.$error.required">Start Date is required</span>
</div>
<input type="submit" value="Submit" ng-click="submitted=true" />
</form>
</div>
如何设置验证？