Angular，使用 ngIf、异步和 Observable 等到数据准备好显示内容时遇到问题

Question

我是 Angular 的新手，我被困住了。我似乎无法让它工作，我认为我只是在如何实现 Observable 方面犯了一些错误。目前我使用本地 json 文件作为我的数据源，但在我的主项目中，我将连接到外部 API。我已经把所有东西都去掉了，让它尽可能基本，但仍然没有运气。

这里是campaign.component.ts

    import { Component, OnInit } from '@angular/core';

    import { Observable } from 'rxjs';

    import { CampaignService } from '../campaign.service';

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

    $campaign: Observable<any>;

      constructor(
        private campaignService: CampaignService
      ) {}

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

      getCampaign(): void {
        this.campaignService.getCampaign().subscribe((data) => {
          this.$campaign = data;
          console.log(this.$campaign);
        });
      }
    }

这是模板 html，campaign.component.html

    <div *ngIf="($campaign | async) as campaign; else loading">
    <!--this never loads-->
      {{campaign.shortName}}
    </div>

    <ng-template #loading>
    <!--this is all I see-->
      Loading stuff in ngIf...
    </ng-template>

    <br>
    <br>
    <!--this works so I know the data loads and that my json file is formatted correctly-->
    <p>Outside of ngIf works: {{$campaign.shortName}}</p>

这是服务，campaign.service.ts

    import { Injectable } from '@angular/core';
    import { HttpClient, HttpHeaders, HttpErrorResponse } from '@angular/common/http';
    import { Observable, of } from 'rxjs';
    import { map} from 'rxjs/operators';

    const endpoint = 'assets/api.json';

    const httpOptions = {
      headers: new HttpHeaders({
        'Content-Type':  'application/json'
      })
    };

    @Injectable()
    export class CampaignService {

      constructor(private http: HttpClient) {}

      private extractData(res: Response) {
        let body = res;
        return body || { };
      }

      getCampaign(): Observable<any> {
        const url = endpoint;
        console.log(url);
        return this.http.get(url).pipe(
          map(this.extractData));
      }

    }

感谢您抽出宝贵时间提供帮助。

Answer 1

  getCampaign(): void {
    this.campaignService.getCampaign().subscribe((data) => {
      this.$campaign = data;
      console.log(this.$campaign);
    });
  }

上面将数据值分配给属性this.$campaign，但您已将该属性声明为可观察的。

<div *ngIf="($campaign | async) as campaign; else loading">
<!--this never loads-->
  {{campaign.shortName}}
</div>

$campaign 不是可观察的，因此异步管道解析为未定义。条件总是false。

<!--this works so I know the data loads and that my json file is formatted correctly-->
<p>Outside of ngIf works: {{$campaign.shortName}}</p>

上述方法有效，因为 $campaign 被分配了数据值。

<p>Outside of ngIf works: {{($campaign | async)?.shortName}}</p>

您应该始终在可观察对象的模板中使用async。

您可以通过在构造函数中分配 observable 来简化组件。

  constructor(private campaignService: CampaignService) {
      this.$campaign = campaignService.getCampaign(); 
  }

或者，如果您订阅和分配数据，则不必使用async。

<div *ngIf="campaign; else loading">
<!--this never loads-->
  {{campaign.shortName}}
</div>

<p>Outside of ngIf works: {{campaign?.shortName}}</p>