Kateri so ključni RxJS operatorji in operatorji za sploščanje?

Question

Accepted Answer

**RxJS operatorji** transformirajo, filtrirajo in kombinirajo tokove Observable. Najpomembnejši — in najbolj zmedeni — so štirje **operatorji za sploščanje**, ki obdelujejo Observable-Observablev (npr. HTTP klic sprožen z vsako emisijo drugega toka).

## Pogosti operatorji za transformacijo/filtriranje

```ts
import { map, filter, tap, debounceTime, distinctUntilChanged, catchError } from "rxjs/operators";

source$.pipe(
  map(x => x * 2),                 // transform each value
  filter(x => x > 10),             // keep values passing a test
  tap(x => console.log(x)),        // side effect, doesn't change the stream
  debounceTime(300),               // wait for a pause before emitting (search inputs)
  distinctUntilChanged(),          // ignore consecutive duplicates
);
```

## Štirje operatorji za sploščanje (težaven del)

Ko mora vsaka vrednost iz vira sprožiti **drug** Observable (kot je HTTP zahtevek), morate "sploščiti" notranji Observable. Štirje se razlikujejo po **načinu obravnavanja prekrivanja**:

```text
switchMap   → cancels the previous inner Observable when a new value arrives
mergeMap    → runs all inner Observables concurrently (no cancellation)
concatMap   → queues them, runs one at a time in order
exhaustMap  → ignores new values while an inner Observable is still running
```

## Kdaj uporabiti vsakega

```ts
// switchMap — type-ahead search: cancel the stale request when the user types again
this.searchInput.valueChanges.pipe(
  debounceTime(300),
  switchMap(query => this.api.search(query)) // only the LATEST query's result matters
).subscribe(results => this.results = results);

// mergeMap — fire independent requests in parallel (order doesn't matter)
from(ids).pipe(mergeMap(id => this.api.get(id)));

// concatMap — operations that MUST run in order (e.g. sequential saves)
actions$.pipe(concatMap(action => this.api.save(action)));

// exhaustMap — ignore repeated clicks while a login is in flight
this.loginClick$.pipe(exhaustMap(() => this.auth.login())); // ignore spam clicks
```

## Pomnjenjski stavek

```text
switchMap  = "switch to the new one" (cancel old) → search
mergeMap   = "merge all" (parallel)              → independent requests
concatMap  = "concatenate" (sequential)          → ordered operations
exhaustMap = "exhaust current first" (ignore new) → prevent double-submit
```

## Zakaj je to pomembno

Operatorji za sploščanje so med najbolj pomembnimi — in najbolj pogosto napačno uporabljenimi — RxJS orodji v Angularju.

Napačna izbira povzroči prave napake: uporaba `mergeMap` za iskanje prikaže zastarele rezultate (brez preklicev); uporaba `switchMap` za zaporedne shranitve jih prekine v letu.

Znati, da `switchMap` prekliče (idealno za iskanje), `concatMap` uredi, `mergeMap` paralelizira in `exhaustMap` blokira dvojnike, je bistveno za pravilno asinkrono obravnavo v katerem koli RxJS-intenzivnem kodu Angularja.