ng g directive directives/mobileValidator　

<form #myForm="ngForm" (ngSubmit)="onSubmit2(myForm.value, myForm.valid)">
 <div>
 <h3>登錄</h3>
 </div>
 <div>用戶名：<input ngModel required name="username" type="text" (input)="onMobileInput(myForm)"></div>
 <div [hidden]="mobileValid || moblieUntouched">
 <div [hidden]="!myForm.form.hasError('required','username')">
 用戶名是必填項
 </div>
 </div>
 
 <div>電話： <input ngModel mobile name="mobile" type="text"></div>
 <button type="submit">登錄</button>
</form>

mobileValid: boolean = true;
moblieUntouched: boolean = true;
 
onMobileInput(form: NgForm) {
 if (form) {
 this.mobileValid = form.form.get('mobile').valid;
 this.moblieUntouched = form.form.get('mobile').untouched;
 }
}