typescript – 如何在Angular2/4/5中实现自定义异步验证器

2.如果是，那么下面的代码有什么问题

export class someClass{

    myForm:ControlGroup;

    constructor(public http:Http,public formBuilder:FormBuilder)
       this.myForm = formBuilder.group({
            ImageId: ["",Validators.required,this.asynValidator]
    });

    asyncValidator(control: Control): {[key: string]: any} {

        return new Promise (resolve => {

          let headers = new Headers();
          headers.append('Content-Type','application/json');

          this.http.get('http://localhost/ImageIdValidate?id='+ control.value,{headers:headers})
                .map(res => res.json())
                .subscribe(data => {
                    console.log(data);
                    if(data != null) {
                        resolve({"duplicate": true})
                    }
                    else resolve(null);      
                })
            });
        });
      }
    }

它甚至没有提出服务器请求。