ngFor 迭代对象数组并计算数组的长度 属性

Question

我正在 Angular 中做一个 Web 应用程序 6. 我从 Firebase 接收到一组对象，我正在尝试使用 ngFor 向用户显示详细信息。我的对象数组是这样的：

export interface Competition {
  createdAt: Date;
  sessions: Session[];
}

export interface Session {
  date: Date;
  program: Program[];
  events: Event[];
}

export interface Program {
  name: string;
}

export interface Event {
  name: string;
}

我正在做的模板里面：

<ng-container *ngFor="let competition of competitions; index as i">
   <h3>{{competition.name}}</h3>
   {{competition.sessions[i].program.length}}
   {{competition.sessions[i].events.length}}
</ng-container>

read property 'program' of undefined, read property 'events' of undefined

我试过：

{{competition[i].sessions[i].program.length}}
{{competition.sessions[i].program.length}}
{{competition[i].sessions[i].program[i].length}}

我的目标是显示 program 和 events 的长度。

Answer 1

更改您的定义以直接定义字符串数组：

export interface Session {
  date: Date;
  program: string[];
  events: string[];
}

Answer 2

您遍历 competitions 数组但尝试获取 competition.sessions[i]。 你需要这样的东西：

<ng-container *ngFor="let competition of competitions; index as i">
   <h3>{{competition.name}}</h3>
   <div *ngFor="let session of competition.sessions">
      {{session.program.length}}
      {{session.events.length}}
   </div>
</ng-container>

如果你想得到一个比赛的会话和事件的总数，你应该在你的ts文件中计算它们

this.competitionResults = this.competitions
   .map(competition => competition.sessions
       .reduce((res, session) => 
           ({
                programLength: res.programLength + session.program.length, 
                eventLength: res.eventLength + session.events.length,
           }), {
                programLength: 0, 
                eventLength: 0,
           }))
   )

和HTML：

<ng-container *ngFor="let competitionResult of competitionResults">
   {{ competitionResult.programLength }}
   {{ competitionResult.eventLength}}
</ng-container>