Angular 2 : Validate form without defining error messages at each input?

Question

我想要 angular2 中的验证表单。我学习了一些文档，它会这样定义:

<form [ngFormModel]="form">
        <input type="text" ngControl="username" />

        <p *ngIf="username.pending">Fetching data from the server...</p>

        <div *ngIf="username.dirty && !username.valid && !username.pending">
          <p *ngIf="username.errors.required">Username is required.</p>
          <p *ngIf="username.errors.startsWithNumber">Your username can't start with a number</p>
          <p *ngIf="username.errors.usernameTaken">This username is taken</p>
        </div>

        <button (click)="submitData()" [disabled]="!form.valid" class="btn btn-primary">Sumbit data</button>
</form>

constructor(private builder: FormBuilder) {

    this.username = new Control(
        "", 
        Validators.compose([Validators.required, UsernameValidator.startsWithNumber]),
        UsernameValidator.usernameTaken
    );

    this.form = builder.group({
        username:  this.username
    });
}

对于每个输入，我都需要定义许多错误消息。我觉得不好。
我想要像 jquery.validate，我只是定义 input 并且错误消息将自动呈现，就像这样

<input required name="username" maxlength='8' pattern="^(?!\s|.*\s$).*$" ...> 

Answer 1

要减少代码，一种方法是编写一个组件。

它将处理所有的条件检查和错误消息

This guy did a nice job here of implementing and explaining it.

---

带有组件的 HTML 看起来像这样

<input ngControl="email" id="email" />
<control-messages control="email"></control-messages>

所有的肮脏的工作都将进入control-messages组件。