Question

我在从方法中返回值时遇到问题，该值始终是未定义的，并且Promise仅在方法返回后才产生该值。

我已尝试观察到但未获得预期的结果。

import { Router } from "@angular/router";
import { JwtHelperService } from "@auth0/angular-jwt";
import { HttpClient } from "@angular/common/http";
import { map } from "rxjs/operators";
import { BehaviorSubject, Observable, of, Subject } from "rxjs";



interface ServerStatus {
  status: string;
}

interface BaseApi {
  baseURL: string;
  apiURL: string;
}

const helper = new JwtHelperService();

@Injectable({
  providedIn: "root"
})
export class AuthService {
  public token: string = "";

  public api: string;

  constructor(
    private httpService: HttpClient,
    private router: Router
  ) {

  }

  public decodedToken: JSON;
  public role: string;

  public timersubscribe: any;
  public myDate: Date;

  public stage: number;

  public tmp: any;



  getStage() {
    let url ;

    this.getApi().then(ret => url = ret))

    //this will appear only after the return is done
    console.log(url);

    //the url is always undefined
    return this.httpService.get<ServerStatus>(url);
  }



  getApi() {
    return this.httpService.get<BaseApi>("../../assets/api.json").toPromise().then(result => {
      return result.apiURL;
    })
  }

}

我希望在方法（getStage（））返回之前将承诺解析为字符串。

Answer 1

您可以使用async await

async getStage() {
    let url = await this.getApi();

    //this will appear only after the return is done
    console.log(url);

    //the url is always undefined
    return this.httpService.get<ServerStatus>(url);
}

Answer 2

您好，您的问题是如何解决并兑现诺言。尝试此操作：

 getStage() {

return new Promise<string>((resolve,reject)=>{

 let url ;

        this.getApi().then(ret => {

        url = ret
        //this will appear only after the return is done
        console.log(url);

        //the url is always undefined
        this.httpService.get<ServerStatus>(url).then((res)=>{
resolve(res)

});


        })


      }



      getApi() {
        return new Promise<string>((resolve,reject)=>{
     this.httpService.get<BaseApi>("../../assets/api.json").toPromise().then(result => {
          resolve( result.apiURL);
        })
        })
}

Answer 3

您无法从Promise中收到纯字符串-Promise是异步的，因此您必须异步处理它。在js中，没有像“等待诺言”这样的方法

我会说做类似的事情：

getStage(): Promise<any> {
    return this.getApi().then(url => {
        return this.httpService.get<ServerStatus >(url).toPromise();
    });
}

在这里，您从getStage返回了Promise，在这里您还可以返回可观察到的（如：）。

getStage(): Observable<any> {
    return this.getApi().then(url => {
        return this.httpService.get<ServerStatus >(url);
    });
}

但是，别忘了订阅;）

getStage().subscribe(...);

Answer 4

使用

之类的异步等待功能

async getStage（）{ 让url =等待this.getApi（）;

//url won't be undefined here
return this.httpService.get<ServerStatus>(url);

}

Angular 7从promise <string>返回字符串

4 个答案: