Learning notes of rxjs takeuntil operator

TakeUntil The official documentation for this operator is ：

Emit values until provided observable emits.

That is, it can be given another anchoring role Observable, When the anchor Observable emit When the value of , The original Observable Just stop firing , Get into complete operation .

Look at a practical example ：

import {
     interval, timer } from 'rxjs';
import {
     takeUntil } from 'rxjs/operators';

//emit value every 1s
const source = interval(1000);
//after 5 seconds, emit value
const timer$ = timer(5000);
//when timer emits after 5s, complete source
const example = source.pipe(takeUntil(timer$));
//output: 0,1,2,3
const subscribe = example.subscribe(val => console.log(val));

source Observable Every 1 Second time interval , Launch one from 0 The start increment interval is 1 Integer sequence of .

We constructed a timer Observable, The timeout interval is 5 second , in other words , At the fifth second , The Observable Will send a value . This timer Observable Pass in takeUntil, As a notification Observable, Five seconds later ,source Observable Will stop firing integers .

Finally, the output of the above program execution ：4 Print in seconds 0～4, Then the end .

Another example ：

const interval = interval(1000);
const clicks = fromEvent(document, 'click');
const result = interval.pipe(takeUntil(clicks));
result.subscribe(x => console.log(x));

In this case ,interval As a primitive Observable,clicks As notification Observable, The expression of the whole program is , Every 1 The second has an increment 1 Integer sequence printing , Until a click event occurs on the page , original interval Observable End .

Look at another example ：

// RxJS v6+
import {
     interval } from 'rxjs/observable/interval';
import {
     takeUntil, filter, scan, map, withLatestFrom } from 'rxjs/operators';

//emit value every 1s
const source = interval(1000);
//is number even?
const isEven = val => val % 2 === 0;
//only allow values that are even
const evenSource = source.pipe(filter(isEven));
//keep a running total of the number of even numbers out
const evenNumberCount = evenSource.pipe(scan((acc, _) => acc + 1, 0));
//do not emit until 5 even numbers have been emitted
const fiveEvenNumbers = evenNumberCount.pipe(filter(val => val > 5));

const example = evenSource.pipe(
  //also give me the current even number count for display
  withLatestFrom(evenNumberCount),
  map(([val, count]) => `Even number (${
      count}) : ${
      val}`),
  //when five even numbers have been emitted, complete source observable
  takeUntil(fiveEvenNumbers)
);
/* Even number (1) : 0, Even number (2) : 2 Even number (3) : 4 Even number (4) : 6 Even number (5) : 8 */
const subscribe = example.subscribe(val => console.log(val));

Let's analyze the logic of this example line by line ：

const evenSource = source.pipe(filter(isEven));

Generate an interval 1 An even number of seconds Observable.

const evenNumberCount = evenSource.pipe(scan((acc, _) => acc + 1, 0));

Accumulate the number of even numbers generated .

const fiveEvenNumbers = evenNumberCount.pipe(filter(val => val > 5));

When the number of even numbers generated is greater than 5 when , Emission value . This Observable As takeUntil Of notification Observable Use .

const example = evenSource.pipe(
  //also give me the current even number count for display
  withLatestFrom(evenNumberCount),
  map(([val, count]) => `Even number (${
      count}) : ${
      val}`),
  //when five even numbers have been emitted, complete source observable
  takeUntil(fiveEvenNumbers)
);

1. Use eventSource and eventNumberCount, adopt withLatestFrom Put two Observable Connect , Thus in map Operator in , You can print the even value and even total amount of the current emission at the same time . adopt takeUntil Pass in one only if the total number of even numbers is greater than 5 When the value is emitted Observable, The total number of even numbers can be greater than 5 after , Give Way interval Stop sending values .

The final execution effect ：