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)
);


• 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 ：