如果 *ngFor Let Object of Objects from HTML Binding returns an empty array 如何显示占位符

Question

我显示来自我在另一个组件 (team-announcements.component) 中创建的模板的数据...在主 team.component.ts 我使用选择器进行团队公告，并添加 [ announcement]="announcements" 和 ngFor="let announcement of announcements" 加载数据。如果数组 returns 没有数据 IE 没有公告，我如何显示占位符 "No Announcements"?

这是我在 team.component.html 中加载公告的地方。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（获取公告功能）：

  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 "app-team-announcements" 在一个单独的文件中模板化，"team-announcement.component.html" 并导出，然后在上面的代码中使用的方式...

团队-announcements.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();
  }

}

团队-announcements.component.html

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

我不确定如何或在何处"If" 检查数组长度以显示占位符。有人可以帮忙吗？

Answer 1

您可以插入一个 div，它仅在您的数组为空时显示：

<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 *ngIf="announcements.length===0"> No announcements </div>
</div>

编辑：更正了错误

Answer 2

如果你想隐藏它和显示其他东西你可以使用 else 属性 from *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>

当您希望具有 *ngFor 的元素依赖于条件 (*ngIf) 时，一个好的替代方法是将具有 *ngFor 的元素嵌套在 [=16= 中] 与 *ngIf。 <ng-container> 的一个好处是它实际上不会成为 DOM 的一部分，但会遵守 *ngIf.

如果 *ngFor Let Object of Objects from HTML Binding returns an empty array 如何显示占位符

How to Show Placeholder if *ngFor Let Object of Objects from HTML Binding returns an empty array

html

angular

typescript

binding

ngfor