如果 HTML 绑定中的对象的 *ngFor Let 对象返回空数组，如何显示占位符

Question

我显示在另一个组件 (team-announcements.component) 中创建的模板中的数据...在主 team.component.ts 中，我使用 team-announcements 选择器，并添加 [announcement]="announcements" 和ngFor="让公告公告"加载数据。如果数组没有返回数据 IE 没有公告，如何显示“无公告”之类的占位符？

这是我在 team.component.html 中加载公告的位置。数据通过 API 服务提供，并在“team.component.ts”中检索，相关对象的 HTML 如下。

team.component.ts（获取公告函数）：

  getAnnouncements() {
    this.teamsService.getTeamAnnouncements(this.team.slug)
      .subscribe(announcements => this.announcements = announcements);
      console.log("announcements", this.announcements);
  }

team.component.html

<div class="team-announcement">
     <div class="announcement-title">Message of the Day</div>
     <app-team-announcements
        [announcement]="announcement"
         *ngFor="let announcement of announcements">
      </app-team-announcements>
   </div>

这就是上面的“app-team-announcements”如何在单独的文件“team-announcement.component.html”中模板化并导出，然后在上面的代码中使用......

团队公告.component.ts

import { Component, EventEmitter, Input, Output, OnInit, OnDestroy } from '@angular/core';

import { Team, Announcement, User, UserService } from '../core';
import { Subscription } from 'rxjs';

@Component({
  selector: 'app-team-announcements',
  templateUrl: './team-announcement.component.html'
})
export class TeamAnnouncementComponent implements OnInit, OnDestroy {
  constructor(
    private userService: UserService
  ) {}

  private subscription: Subscription;

  @Input() announcement: Announcement;
  @Output() deleteAnnouncement = new EventEmitter<boolean>();

  canModify: boolean;

  ngOnInit() {
    // Load the current user's data
    this.subscription = this.userService.currentUser.subscribe(
        (userData: User) => {
          this.canModify = (userData.username === this.announcement.author.username);
        }
      );
  }

  ngOnDestroy() {
    this.subscription.unsubscribe();
  }

}

team-announcements.component.html

<div class="announcement-text">
    {{announcement.body}}
</div>

我不确定如何或在哪里“如果”检查数组长度以显示占位符。有人可以帮忙吗？

Answer 1

如果您想隐藏它并显示其他内容，您可以使用以下else属性*ngIf：

<div class="team-announcement">
  <div class="announcement-title">Message of the Day</div>
  <ng-container *ngIf="announcements.length != 0; else emptyArray">
    <app-team-announcements
      [announcement]="announcement"
      *ngFor="let announcement of announcements">
    </app-team-announcements>
  </ng-container>
</div>
<ng-template #emptyArray>No announcements...</ng-template>

当您希望元素 with*ngFor依赖于条件 ( *ngIf) 时，一个不错的选择是将带有 an 的元素嵌套在带有an 的*ngFora 中。一个好处是它实际上不会成为 DOM 的一部分，但会遵守.<ng-container>*ngIf<ng-container>*ngIf