typescript - Angular subscribe elegant

Question

I'd like to ask about my subscribe method. I want to get my objects(employees) from firebase and do a loot of operations on my objects, like avg salaries, avg missed days... In my opinion it looks so horrible. My service void:

 getEmployees(): Observable<Employee[]> {
    return this.db.list<Employee>(this.Api_url).snapshotChanges()
      .pipe(map(response => response.map(car => this.assignKey(car))));
  }

And my ngOnInit:

  ngOnInit() {

    this.subscribtion = this.mainservice.getEmployees().subscribe((emps)=> {
      this.employees = emps;
      this.workersdatas = [];
      this.educationdatas = [];
      this.checkavaragesalaries(emps);
      this.countMonthAbsences(emps, 1);
      this.countMonthSalaries(emps, 1);
      this.departments.forEach((one)=> {
        const dep = emps.filter(on => on.department.includes(one) && this.FilterEmployees(on)).length;
        this.workersdatas.push({name: one, value: dep});
      });

      this.educations.forEach((one)=> {
        const edu = emps.filter(on => on.education.includes(one)).length;
        this.educationdatas.push({name: one, value: edu});
      });

      const mynumb =this.educationdatas.map(on => on.value);
      this.mosteducation = this.educationdatas.filter(one => one.value === Math.max(...mynumb))[0].name;
    }); }

Should I unsubscribe() it on ngOnDestroy or that's not necessary? Is that allowed to write it like that?

Answer 1

My favourite unsubscribe pattern is to use takeUntil. takeUntil() accepts an observable, and will stop the subscription when that observable emits a value.

The long-winded way to do this is emit a value in ngOnDestroy().

destroyed = new Subject<void>();

ngOnInit(): void {
  myService.getData().pipe(
    takeUntil(this.destroyed)
  ).subscribe(data => {
    // process data
  });
}

ngOnDestroy(): void {
  this.destroyed.next();
  this.destroyed.complete();
}

This gets quite tedious once you've done this a few times. The best solution I've seen for this (and currently use) is to set up a shared function that emits the destroyed value for you. Source Credit

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

import { ReplaySubject, Subject, Observable } from 'rxjs';

export function componentDestroyed(component: OnDestroy): Observable<void> {
  const oldNgOnDestroy: () => void = component.ngOnDestroy;
  const destroyed: Subject<void> = new ReplaySubject<void>(1);
  component.ngOnDestroy = () => {
    oldNgOnDestroy.apply(component);
    destroyed.next(undefined);
    destroyed.complete();
  };
  return destroyed.asObservable();
}

It is then almost trivial to use in your component:

ngOnInit(): void {
  myService.getData().pipe(
    takeUntil(componentDestroyed(this))
  ).subscribe(data => {
    // process data
  });
}

ngOnDestroy(): void {}

All you have to do is implement ngOnDestroy in your component and add takeUntil(componentDestroyed(this)) to your pipe.