<input [(ngModel)]="user.mobile" #mobile="ngModel" autocomplete="off" type="tel" class="form-control" name="mobile" required maxlength="11">
<div *ngIf="mobile.errors">
  <p *ngIf="mobile.errors.required">手机号必填</p>
  <p *ngIf="mobile.errors.pattern">手机号格式不正确</p>
</div>

@Directive({
 providers: [{
   provide: NG_VALIDATORS,
   useExisting: forwardRef(() => RequiredValidator),
   multi: true
  }]
})
export class RequiredValidator {}

export class NgModel extends NgControl {
  constructor(@Inject(NG_VALIDATORS) validators: Array<Validator|ValidatorFn>) {}
  
  get validator(): ValidatorFn|null {
    // 各种校验并返回结果
  }
}

@Directive({
  selector: '[user-mobile]',
  exportAs: 'userMobile',
  providers: [{
    provide: NG_VALIDATORS,
    useExisting: forwardRef(() => UserMobileDirective),
    multi: true
  }]
})
export class UserMobileDirective {}

export class UserMobileDirective implements Validator {
  validate(c: AbstractControl): { [key: string]: any; } {
    let value: string = c.value || '';
    if (!value.startsWith('159')) {
      return {
        mobile: {
          msg: '手机号必须是159开头',
          actualValue: value
        }
      };
    }
    return null;
  }
}

<input user-mobile [(ngModel)]="user.mobile" #mobile="ngModel" autocomplete="off" type="tel" class="form-control" name="mobile" id="mobile" required maxlength="11">
<div *ngIf="mobile.errors">
  <p *ngIf="mobile.errors.required">手机号必填</p>
  <p *ngIf="mobile.errors.mobile">{{mobile.errors.mobile.msg}}</p>
</div>

@Directive({
  selector: '[user-async]',
  exportAs: 'userAsync',
  providers: [{
    provide: NG_ASYNC_VALIDATORS,
    useExisting: forwardRef(() => UserAsyncDirective),
    multi: true
  }]
})
export class UserAsyncDirective implements Validator {
  validate(c: AbstractControl): Observable<any> {
    return c.valueChanges
        // 去抖
        .debounceTime(300)
        // 抑制重复值
        .distinctUntilChanged()
        // 1、可以使用flatMap进行远程校验
        // .flatMap(value => value)
        // 2、本地模拟判断
        .map((value: string) => {
          if ([ '15900000001', '15900000002' ].includes(value)) {
            return {
              mobile: {
                msg: '手机号为黑名',
                actualValue: value
              }
            }
          }
          return null;
        })
        .first();    
  }
}

export class MyValidators {
  static checkMobile(value: string): ValidationErrors|null {
    return !value.startsWith('159') ? { mobile: { msg: '手机号必须是159开头' } } : null;
  }
}
// 校验器类
export class UserMobileDirective implements Validator {
  validate(c: AbstractControl): { [key: string]: any; } {
    let value: string = c.value || '';
    return MyValidators.checkMobile(value);
  }
}