mergeMap / flatMap

signature: mergeMap(project: function: Observable, resultSelector: function: any, concurrent: number): Observable

옵저버블과 매핑시킨 후 값을 발생시킵니다.

:bulb: flatMap과 mergeMap은 같은겁니다!

:bulb: 한번에 하나의 내부 subscription만 활성화해야하는 경우, switchMap을 살펴보세요!

:bulb: 내부 옵저버블의 subscription과 값 발생의 순서가 중요하다면, concatMap을 살펴보세요!

왜 mergeMap 을 사용할까요?

이 연산자는 내부 옵저버블은 평평하게 만들고 싶지만, 내부 subscription의 수는 조절하고싶을 때 가장 많이 쓰입니다.

예를 들어, when using switchMap each inner subscription is completed when the source emits, allowing only one active inner subscription. In contrast, mergeMap allows for multiple inner subscriptions to be active at a time. Because of this, one of the most common use-case for mergeMap is requests that should not be canceled, think writes rather than reads. Note that if order must be maintained concatMap is a better option.

Be aware that because mergeMap maintains multiple active inner subscriptions at once it's possible to create a memory leak through long-lived inner subscriptions. A basic example would be if you were mapping to an observable with an inner timer, or a stream of dom events. In these cases, if you still wish to utilize mergeMap you may want to take advantage of another operator to manage the completion of the inner subscription, think take or takeUntil. You can also limit the number of active inner subscriptions at a time with the concurrent parameter, seen in example 5.

Examples

Example 1: mergeMap simulating save of click locations

( StackBlitz )

// RxJS v6+
import { fromEvent, of } from 'rxjs';
import { mergeMap, delay } from 'rxjs/operators';

// faking network request for save
const saveLocation = location => {
  return of(location).pipe(delay(500));
};
// streams
const click$ = fromEvent(document, 'click');

click$
  .pipe(
    mergeMap((e: MouseEvent) => {
      return saveLocation({
        x: e.clientX,
        y: e.clientY,
        timestamp: Date.now()
      });
    })
  )
  // Saved! {x: 98, y: 170, ...}
  .subscribe(r => console.log('Saved!', r));

Example 2: mergeMap with ajax observable

( StackBlitz )

// RxJS v6+
import { fromEvent } from 'rxjs';
import { ajax } from 'rxjs/ajax';
import { mergeMap } from 'rxjs/operators';

// free api url
const API_URL = 'https://jsonplaceholder.typicode.com/todos/1';

// streams
const click$ = fromEvent(document, 'click');

click$
  .pipe(
    /*
     * Using mergeMap for example, but generally for GET requests
     * you will prefer switchMap.
     * Also, if you do not need the parameter like
     * below you could use mergeMapTo instead.
     * ex. mergeMapTo(ajax.getJSON(API_URL))
     */
    mergeMap(() => ajax.getJSON(API_URL))
  )
  // { userId: 1, id: 1, ...}
  .subscribe(console.log);

Example 3: mergeMap with promise (could also use from to convert to observable)

( StackBlitz )

// RxJS v6+
import { of } from 'rxjs';
import { mergeMap } from 'rxjs/operators';

// helper to create promise
const myPromise = val =>
  new Promise(resolve => resolve(`${val} World From Promise!`));

// emit 'Hello'
const source$ = of('Hello');

// map to promise and emit result
source$
  .pipe(mergeMap(val => myPromise(val)))
  // output: 'Hello World From Promise'
  .subscribe(val => console.log(val));

Example 4: mergeMap with resultSelector

( StackBlitz )

// RxJS v6+
import { of } from 'rxjs';
import { mergeMap } from 'rxjs/operators';

// helper to create promise
const myPromise = val =>
  new Promise(resolve => resolve(`${val} World From Promise!`));

// emit 'Hello'
const source$ = of('Hello');

source$
  .pipe(
    mergeMap(
      val => myPromise(val),
      /*
      you can also supply a second argument which receives the source value and emitted
      value of inner observable or promise
    */
      (valueFromSource, valueFromPromise) => {
        return `Source: ${valueFromSource}, Promise: ${valueFromPromise}`;
      }
    )
  )
  // output: "Source: Hello, Promise: Hello World From Promise!"
  .subscribe(val => console.log(val));

Example 5: mergeMap with concurrent value

( StackBlitz )

// RxJS v6+
import { interval } from 'rxjs';
import { mergeMap, take } from 'rxjs/operators';

// emit value every 1s
const source$ = interval(1000);

source$
  .pipe(
    mergeMap(
      // project
      val => interval(5000).pipe(take(2)),
      // resultSelector
      (oVal, iVal, oIndex, iIndex) => [oIndex, oVal, iIndex, iVal],
      // concurrent
      2
    )
  )
  /*
        Output:
        [0, 0, 0, 0] <--1st inner observable
        [1, 1, 0, 0] <--2nd inner observable
        [0, 0, 1, 1] <--1st inner observable
        [1, 1, 1, 1] <--2nd inner observable
        [2, 2, 0, 0] <--3rd inner observable
        [3, 3, 0, 0] <--4th inner observable
*/
  .subscribe(val => console.log(val));

Related Recipes

Additional Resources

:file_folder: Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/mergeMap.ts

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