如何验证和显示错误消息 - Angular2 Material Design？

我有这个输入：

<form #f="ngForm" name="productForm">
     <md-input [(ngModel)]="product.price" name="price" required="true" placeholder="Price (USD)"></md-input>
     <div ng-messages="productForm.price.$error" role="alert">
         <div ng-message-exp="['required']">
              Price is required
          </div>
     </div>
</form>

但未显示“需要价格”消息。

我应该如何正确格式化错误消息？

当价格输入为空时，会出现 ng-invalid 类：

When I fill in something:

Angular injects ng-valid class in it.

我想要的是类似的风格Angular1MD设计 https://material.angularjs.org/latest/demo/input看起来像这样：

希望这将随着 angular2-material 的发展而添加，但目前模仿此的方法是设置 diverColor 并使用 MD-HINT 指令。例子：

<md-input placeholder="Email address"
    #email="ngModel"
    name="email"
    type="text"
    fullWidth={true}
    [(ngModel)]="model.email"
    required
    email
    dividerColor="{{ !email.valid ? 'warn' : 'primary' }}">
    <md-hint [hidden]="email.pristine || email.valid">
        <span [hidden]="email.errors?.required || !email.errors?.email">
            This doesn't appear to be a valid email address.
        </span>
        <span [hidden]="!email.errors?.required">Email address is required.</span>
    </md-hint>
</md-input>