来自 Promise 的 Angular 4 显示元素

Question

我有以下 Typescript 服务 (app.component.ts)：

import { Component, OnInit } from '@angular/core';
import { ApiService } from './shared/api.service';
import {PowerPlant} from './shared/models/powerplant.model';
import 'rxjs/add/operator/toPromise';

@Component({
  selector: 'app-root',
  providers: [ApiService],
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {
  // represents the URL's
  allPowerPlantsURL = 'powerPlants';
  // represents the data
  powerPlants: PowerPlant[];

  ngOnInit(): void {
    this.allPowerPlants();
  }

  constructor(private apiService: ApiService) {
  }

  allPowerPlants(onlyActive: boolean = false, page: number = 1): void {
    const params: string = [
      `onlyActive=${onlyActive}`,
      `page=${page}`
    ].join('&');
    const path = `${this.allPowerPlantsURL}?${params}`;
    this.apiService.get(path)
    .toPromise()
    .then(elem => {
    console.log('In the allPowerPlants');
    console.log(elem); **// prints undefined here**
    this.powerPlants = <PowerPlant[]> elem; }
  )
      .catch(this.handleError);
  }

  private handleError(error: any): Promise<any> {
    console.error('An error occurred', error);
    return Promise.reject(error.message || error);
  }
}

这是我的 app.component.html（只是其中的一个 sn-p）：

<div class="ui grid posts">
  <app-powerplant
    *ngFor="let powerPlant of powerPlants"
    [powerPlant]="powerPlant">
  </app-powerplant>
</div>

现在，在我的 powerplant.component.html 中，我有这个：

从 '@angular/core' 导入 {Component, Input, OnInit}; 从'../shared/models/powerplant.model'导入{PowerPlant}；

@Component({
  selector: 'app-powerplant',
  templateUrl: './powerplant.component.html',
  styleUrls: ['./powerplant.component.css']
})
export class PowerplantComponent implements OnInit {

  @Input() powerPlant: PowerPlant;

  constructor() { }

  ngOnInit() {
  }
}

最后，应该显示 PowerPlant 项目的项目是这样的：

<div class="four wide column center aligned votes">
  <div class="ui statistic">
    <div class="value">
      {{ powerPlant.powerPlantId }}
    </div>
    <div class="label">
      Points
    </div>
  </div>
</div>
<div class="twelve wide column">
  <div class="value">
    MaxPower: {{ powerPlant.maxPower }} MinPower: {{ powerPlant.minPower }}
  </div>
  <div class="value">
    MaxPower: {{ powerPlant.maxPower }} MinPower: {{ powerPlant.minPower }}
  </div>
  <div class="value">
    PowerPlantType: {{ powerPlant.powerPlantType }} Organization: {{ powerPlant.powerPlantName }}
  </div>
</div>

我可以看到服务器正在向我发送数组，因为 get 方法上的以下控制台日志显示：

  get(path: string, params: URLSearchParams = new URLSearchParams()): Observable<any> {
    console.log('sending request to ' + `${environment.api_url}${path}`);
    return this.http.get(`${environment.api_url}${path}`, { search: params })
      .catch(this.formatErrors)
      .map((res: Response) => {
        console.log(res.json());
        res.json();
      });
  }

console.log 行在屏幕截图中显示以下内容：

那么为什么 toPromise() 会失败？仅供参考，我的 PowerPlant 模型如下所示：

export interface PowerPlant {
  powerPlantId: number;
  powerPlantName: string;
  minPower: number;
  maxPower: number;
  powerPlantType: string;
  rampRateInSeconds?: number;
  rampPowerRate?: number;
}

Answer 1

使用toPromise() 方法是否有特定原因？正常订阅时有效吗？

尝试更改此设置

this.apiService.get(path)
  .toPromise()
  .then(elem => {
  console.log('In the allPowerPlants');
  console.log(elem); **// prints undefined here**
  this.powerPlants = <PowerPlant[]> elem; }
)

到这里：

this.apiService.get(path).subscribe(result => {
    console.log('Im the result => ', result);
    this.powerPlants = <PowerPlant[]> result;
  });

---

那可能是因为您没有在 .map() 方法中返回解析结果，因此您无法在您的承诺/订阅中获得响应。

.map((res: Response) => res.json()); // return is inferred in this syntax


.map((res: Response) => { 
  return res.json(); // here it's not
});

Answer 2

和你的ApiService有关，你的.map忘记回res.json

  get(path: string, params: URLSearchParams = new URLSearchParams()): Observable<any> {
console.log('sending request to ' + `${environment.api_url}${path}`);
return this.http.get(`${environment.api_url}${path}`, { search: params })
  .catch(this.formatErrors)
  .map((res: Response) => {
    console.log(res.json());
    return res.json();
  });
}