Multiple subscriptions without recalculate common part

问题

const ex = ajax.getJSON('https://httpbin.org/get?a=24');
ex.pipe(pluck('args')).subscribe(x => console.log('Args: ', x));
ex.pipe(pluck('headers')).subscribe(x => console.log('Headers: ', x));

Code above(StackBlitz) initiates two ajax requests by one for each subscription.

What is correct way to initiate only one observable pipe/chain for all existing subscriptions?

I can achieve that with introduce a Promise for ajax(StackBlitz):

const p = new Promise(resolve => {
  ajax.getJSON('https://httpbin.org/get?a=24')
  .subscribe(res => resolve(res));
});

from(p).pipe(pluck('args')).subscribe(x => console.log('Args: ', x));
from(p).pipe(pluck('headers')).subscribe(x => console.log('Headers: ', x));

But I believe there must be some more convienient way.

回答1:

shareReplay from comments solves my issue:

const src = tag => of(42)
  .pipe(tap(_ => console.info(tag, 'source called')));

const ex = src('shareReplay').pipe(
  shareReplay(1)
);

ex.subscribe(x => console.log('sa:', x));
ex.subscribe(x => console.log('sb:', x));

shareReplay source called
sa: 42
sb: 42

Full demo

回答2:

You can use the publish operator for that:

const ex = ajax.getJSON('https://httpbin.org/get?a=24').pipe(publish());
ex.pipe(pluck('args')).subscribe(x => console.log('Args: ', x));
ex.pipe(pluck('headers')).subscribe(x => console.log('Headers: ', x));
ex.connect();

The ajax request will take place just once, and only after the connect() method is called (stackblitz demo).

来源：https://stackoverflow.com/questions/56990005/multiple-subscriptions-without-recalculate-common-part