groupBy 成键和值

Question

下面粗体中的问题。鉴于这是我的 HTML UI 组件：

<mat-tab label="Beginners">
  <courses-card-list [courses]="coursesBeginners$ | async">
  </courses-card-list>
</mat-tab>

<mat-tab label="Advanced">
  <courses-card-list [courses]="coursesAdvanced$ | async">
  </courses-card-list>
</mat-tab>

我有一个 courses$ observable，我需要根据每门课程的类别过滤成多个 observable。为简洁起见，我将示例限制为两类。我在 Angular 7 应用程序的 ts 组件中从这个片段开始：

this.beginnersCourses$ = this.courses$.pipe(
  map(courses => courses.filter(
    course => course.categories.includes("BEGINNER"))));

this.advancedCourses$ = this.courses$.pipe(
  map(courses => courses.filter(
    course => course.categories.includes("ADVANCED"))));

给这个：

let courses0: Course[] = [];
let courses1: Course[] = [];
courses$
  .pipe(
    map(courses => {
      courses.forEach(course => {
        if (course.categories.includes('BEGINNER')) {
          courses0.push(course);
        } else if (course.categories.includes('ADVANCED')) {
          courses1.push(course);
        }
      });
    }),
  )
  .subscribe(() => {
    this.coursesBeginners$ = of(courses0);
    this.coursesAdvanced$ = of(courses1);
  });

虽然第二个版本的代码更多，但效率更高，因为我们只遍历课程集合一次，而不是有类别那么多次。 对了，能再短一点吗？

现在，我想做的是使用 groupBy 编写更高效的代码，如 rxjs 文档中 here in the second example in the learnrxjs site, but I am having trouble mapping my problem to the simple one shown in the example - my value is an entire course object, not just a property. I also tried the approach shown here 所示，但我我没有在没有编译错误的情况下通过 reduce 函数。

我也意识到按照includes的方式划分可能比较困难，这种情况下如果没有别的办法，即使选择值course.categories[0]也能满足。

谢谢！

Answer 1

您可以静态定义可用的难度，然后使用它们遍历课程以按难度类型对它们进行分组，然后您还可以在模板中使用它们为每个难度创建 UI 通过使用ngForOf.

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

import { of } from 'rxjs';
import { map, reduce, concatAll } from 'rxjs/operators';

enum CourseDifficulties {
  BEGINNER = 'BEGINNER',
  ADVANCED = 'ADVANCED',
  EXPERT = 'EXPERT',
};

const COURSES_DATA = [
  { name: 'Course 1', categories: [CourseDifficulties.BEGINNER, CourseDifficulties.ADVANCED] },
  { name: 'Course 2', categories: [CourseDifficulties.ADVANCED] },
  { name: 'Course 3', categories: [CourseDifficulties.EXPERT] },
  { name: 'Course 4', categories: [CourseDifficulties.EXPERT] },
  { name: 'Course 5', categories: [CourseDifficulties.ADVANCED] },
];

const AVAILABLE_DIFFICULTIES = [
  { name: 'Beginner', type: CourseDifficulties.BEGINNER },
  { name: 'Advanced', type: CourseDifficulties.ADVANCED },
  { name: 'Expert', type: CourseDifficulties.EXPERT },
];

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  public readonly difficulties = AVAILABLE_DIFFICULTIES;
  public readonly coursesByDifficulty$ = this.getCourses();



  public getCourses()
  {
    return of(COURSES_DATA).pipe(
      map(courses => courses.reduce((result, course) =>
      {
        this.difficulties.forEach(({ type }) =>
        {
          if(course.categories.indexOf(type) > -1)
          {
            result[type] = (result[type] || []).concat(course);
          }
        });
        return result;
      }, {}))
    );
  }
}

DEMO

groupBy 成键和值

groupBy into key and values

grouping

partitioning

observable

rxjs

angular