Angular 6 - Behaviour Subject: get data to stay after refresh

Question

I'm trying to pass data between course-detail component and course-play component. I used shared service and BehaviorSubject. The data is passing correctly to course-detail through the service, and when go to course-play html page through course-detail html page it works just fine, but when I refresh the page, it's using the default id I gave courseId in the service. I need to find a way for the id to stay in the service all the time after getting it from course-play and just keep that update when I get another course id. Here's my code:

course.ts

export interface ICourse {
  course_id: number;
  title: string;
  autor: string;
  segments: ISegment[];
}

export interface ISegment {
  segment_id: number;
  unit_id: number;
  unit_title: string;
  name: string;
  type: string;
  data: string;
}

course.service

import { Injectable } from '@angular/core';
import { HttpClient, HttpErrorResponse } from '@angular/common/http';
import { BehaviorSubject } from 'rxjs';

import { Observable, throwError } from 'rxjs';
import { catchError, groupBy } from 'rxjs/operators';

import { ICourse } from './course';

// Inject Data from Rails app to Angular app
@Injectable()
export class CourseService{

  // JSON url to get data from
  private url = 'http://localhost:3000/courses';
  private courseUrl = 'http://localhost:3000/courses.json';

  // Subscribe data
  private courseId = new BehaviorSubject(1);
  public courseId$ = this.courseId.asObservable();

  // here we set/change value of the observable
  setId(courseId) {
    this.courseId.next(courseId)
  }

  constructor(private http: HttpClient) { }

  // Handle Any Kind of Errors
  private handleError(error: HttpErrorResponse) {

    // A client-side or network error occured. Handle it accordingly.
    if (error.error instanceof ErrorEvent) {
      console.error('An error occured:', error.error.message);
    }

    // The backend returned an unsuccessful response code.
    // The response body may contain clues as to what went wrong.
    else {
      console.error(
        'Backend returned code ${error.status}, ' +
        'body was ${error.error}');
    }

    // return an Observable with a user-facing error error message
    return throwError(
      'Something bad happend; please try again later.');
  }

  // Get All Courses from Rails API App
  getCourses(): Observable<ICourse[]> {
  const coursesUrl = `${this.url}` + '.json';

  return this.http.get<ICourse[]>(coursesUrl)
      .pipe(catchError(this.handleError));
  }

  // Get Single Course by id. will 404 if id not found
  getCourse(id: number): Observable<ICourse> {
    const detailUrl = `${this.url}/${id}` + '.json';
    return this.http.get<ICourse>(detailUrl)
        .pipe(catchError(this.handleError));
  }


}

course-detail.component

import { Component, OnInit, Pipe, PipeTransform } from '@angular/core';
import { ActivatedRoute, Router, Routes } from '@angular/router';

import { ICourse } from '../course';
import { CourseService } from '../course.service';


// Course-detail decorator
@Component({
  selector: 'lg-course-detail',
  templateUrl: './course-detail.component.html',
  styleUrls: ['./course-detail.component.sass']
})

export class CourseDetailComponent implements OnInit {
  course: ICourse;
  errorMessage: string;

  constructor(private courseService: CourseService,
        private route: ActivatedRoute,
        private router: Router) {
  }

  // On start of the life cycle
  ngOnInit() {
    // get the current course id to use it on the html file
    const id = +this.route.snapshot.paramMap.get('id');

    // set curretn course Id in the service to use it later
    this.courseService.setId(id);
    this.getCourse(id);
    }

    // Get course detail by id
    getCourse(id: number) {
        this.courseService.getCourse(id).subscribe(
          course => this.course = course,
          error  => this.errorMessage = <any>error;
         )
      }

   // When we click the back button in browser
   onBack(): void {
     this.router.navigate(['/courses']);
   }

}

course-play-component

import { Component, OnInit, Input} from '@angular/core';
import { ActivatedRoute, Router, Routes, NavigationEnd } from '@angular/router';
import { MatSidenavModule } from '@angular/material/sidenav';

import { ICourse } from '../course';
import { CourseService } from '../course.service';


// Couse-play decorator
@Component({
  selector: 'lg-course-play-course-play',
  templateUrl: './course-play.component.html',
  styleUrls: ['./course-play.component.sass']
})

export class CoursePlayComponent implements OnInit {
  errorMessage: string;
  course: ICourse;
  courseId: number;


  constructor(private courseService: CourseService,
      private route: ActivatedRoute,
      private router: Router) {
        courseService.courseId$.subscribe( courseId => {
          this.courseId = courseId;
        })

  }

    // On start of the life cycle
    ngOnInit() {
        // get the current segment id to use it on the html file
        const segmentId = +this.route.snapshot.paramMap.get('id');
        this.getCourse(this.courseId);
      }

      // Get course detail by id
      getCourse(id: number) {
          console.log(id);
          this.courseService.getCourse(id).subscribe(
            course => this.course = course,
            error  => this.errorMessage = <any>error;
           )
        }

    // When we click the back button in browser
     onBack(): void {
       this.router.navigate(['/courses/:id']);
     }

}

Answer 1

It is default behavior of BehaviorSubject. When ever you refresh page you will get default id from service. If you want to get your updated id, then store it in local storage/cookie and update your behavior subject from that value.

To store value in localStorage when you update your id:

localStorage.setItem(key, value);

To get item from localStorage:

localStorage.getItem(key);

And when page refresh, you need to read value from localStorage and emit value to subscriber using next();. Example:

this.your_service.your_behavior_subject.next(value_from_localstorage);

you need to emit in global component [app.component.ts]. So it is available through out your application

Answer 2

Need to set the initial state to read from local storage:

private myValue = JSON.parse(localStorage.getItem('myData')) as MyData;

and then seed the store:

public myData: BehaviorSubject<myData> = new BehaviorSubject(this.myValue);

To change or seed when there's no local storage, just use .next to add values to it.