Subject turlari

Subject — ham Observable ham Observer. Hot stream yaratish, component'lar orasida event uzatish va service'da state saqlash uchun Angular'da keng qo'llaniladi.

Subject — sodda, hot observable

Middle

Nima bu?

Subject — multicast hot Observable. .next(), .error(), .complete() bilan qiymat yuborish mumkin. Subscribe qilgandan keyin kelgan qiymatlarni oladi — oldingi qiymatlar yo'q. Event bus, component'dan service'ga signal va oddiy pub/sub pattern uchun.

Kod misoli

import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class NotificationService {
  private notificationSubject = new Subject<string>();

  // Faqat o'qish — tashqaridan next qilib bo'lmaydi
  notifications$ = this.notificationSubject.asObservable();

  push(message: string) {
    this.notificationSubject.next(message);
  }
}

// Component
this.notificationService.notifications$.subscribe((msg) => {
  this.toast.show(msg);
});

// Subscribe qilishdan OLDIN
service.push('Birinchi'); // yangi subscriber ko'rmaydi

const sub = service.notifications$.subscribe(console.log);
service.push('Ikkinchi'); // ✅ ko'radi

Imtihonda

Subject cold yoki hot?
asObservable() nima uchun kerak?

Yodlash uchun

Subject = hot, oxirgi qiymat saqlanmaydi. asObservable() = faqat subscribe, next yashirin.

BehaviorSubject — oxirgi qiymat saqlanadi

Middle

Nima bu?

BehaviorSubject — dastlabki qiymat (seed) bilan yaratiladi va oxirgi qiymatni saqlaydi. Yangi subscriber darhol oxirgi qiymatni oladi. Angular service'larda current user, theme, cart state va value/getValue() orqali sinxron o'qish uchun eng ko'p ishlatiladi.

Kod misoli

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';
import { map } from 'rxjs/operators';

interface CartItem {
  id: number;
  qty: number;
}

@Injectable({ providedIn: 'root' })
export class CartService {
  private cartSubject = new BehaviorSubject<CartItem[]>([]);

  cart$ = this.cartSubject.asObservable();
  totalItems$ = this.cart$.pipe(
    map((items) => items.reduce((sum, i) => sum + i.qty, 0))
  );

  addItem(item: CartItem) {
    const current = this.cartSubject.getValue();
    this.cartSubject.next([...current, item]);
  }

  // Sinxron o'qish
  get snapshot(): CartItem[] {
    return this.cartSubject.getValue();
  }
}

// Yangi subscriber darhol [] yoki oxirgi cart oladi
cartService.cart$.subscribe(console.log);

Imtihonda

BehaviorSubject va Subject farqi?
Angular'da state uchun nima uchun BehaviorSubject, oddiy Subject emas?

Yodlash uchun

BehaviorSubject = seed + oxirgi qiymat + yangi subscriber darhol oladi. State store klassikasi.

Angular 17+ da yangi state uchun `signal()` afzal, lekin BehaviorSubject hali ko'p legacy kodda.

ReplaySubject(n) — oxirgi n ta saqlanadi

Middle

Nima bu?

ReplaySubject(bufferSize) — oxirgi n ta qiymatni buffer'da saqlaydi. Kech subscribe qilganlar ham so'nggi n ta qiymatni oladi. Chat history, so'nggi N ta log, yoki HTTP cache (shareReplay ichida) uchun. refCount: true bilan barcha subscriber ketganda tozalanadi.

Gotcha: bufferSize bermasdan new ReplaySubject() yaratilsa, u cheksiz qiymatni xotirada saqlaydi — uzoq ishlaydigan Subject'da (masalan, global event bus) bu asta-sekin xotira sızıntısiga aylanadi. Ikkinchi argument windowTime (ms) bilan vaqt bo'yicha ham cheklash mumkin — new ReplaySubject(10, 5000) faqat oxirgi 5s ichidagi (max 10 ta) qiymatni saqlaydi.

Kod misoli

import { ReplaySubject } from 'rxjs';
import { shareReplay } from 'rxjs/operators';

// Oxirgi 3 ta xabar
const chat$ = new ReplaySubject<string>(3);

chat$.next('Salom');
chat$.next('Qalaysiz?');
chat$.next('Yaxshi');
chat$.next('Rahmat');

chat$.subscribe((msg) => console.log('User A:', msg)); // Qalaysiz?, Yaxshi, Rahmat

// shareReplay — HTTP uchun
@Injectable({ providedIn: 'root' })
export class ConfigService {
  config$ = this.http.get<AppConfig>('/api/config').pipe(
    shareReplay({ bufferSize: 1, refCount: true })
  );
}

Imtihonda

ReplaySubject(1) va BehaviorSubject farqi?
shareReplay(1) qachon ishlatiladi?

Yodlash uchun

ReplaySubject(n) = oxirgi n ta replay. shareReplay(1) = HTTP cache pattern.

AsyncSubject — faqat complete'dagi

Senior

Nima bu?

AsyncSubject — faqat complete bo'lganda oxirgi bitta qiymatni subscriber'larga beradi. .next() lar saqlanadi, complete'da faqat oxirgisi emit qilinadi. Error bo'lsa hech narsa bermaydi. Kam ishlatiladi — odatda Promise-like pattern yoki test'larda uchraydi. Amaliy Angular kodida BehaviorSubject yoki firstValueFrom ko'proq.

Kod misoli

import { AsyncSubject } from 'rxjs';

const subject = new AsyncSubject<number>();

subject.subscribe((v) => console.log('A:', v));
subject.next(1);
subject.next(2);
subject.next(3);
subject.complete(); // A: 3 (faqat oxirgi)

// Error — hech kim olmaydi
const errSubject = new AsyncSubject<string>();
errSubject.subscribe({ next: console.log, error: console.error });
errSubject.next('data');
errSubject.error(new Error('fail'));

Imtihonda

AsyncSubject qachon qiymat emit qiladi?
To'rtta Subject turini taqqoslang.

Yodlash uchun

AsyncSubject = complete → oxirgi 1 ta. Subject, BehaviorSubject, ReplaySubject kundalik ishda.