41. share: one source for several subscribers

Cold vs Hot — an important pair of ideas

A cold Observable runs its logic anew for each subscriber. of, from, defer, an HTTP request — all cold. Subscribe twice and you get two requests.

A hot Observable is one source for everyone. Every subscriber sees the same values. A Subject is the typical example.

share — simple multicast

share() turns a cold Observable hot for as long as there's at least one subscriber. Under the hood it uses a Subject, but it spares you the manual work.

cold$.pipe(share())
// two subscribe → one cold run → both get the same values

Glossary

• Multicast — broadcasting one stream to many subscribers.
• Unicast — a separate stream per subscriber. Cold = unicast.

const { Observable, share } = Rx;

let starts = 0;

const cold$ = new Observable(subscriber => {
  starts += 1;
  const subscription = cold('20ms a|', { a: 'value' }).subscribe(subscriber);
  return () => subscription.unsubscribe();
});

const shared$ = cold$.pipe(
  share()
);