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PubSub

Implementation of the PubSub pattern.

const ps = PubSub();
ps.sub(val => console.log('value: ' + val));

ps.pub(1);
// value: 1

ps.pub(2);
// value: 2

Overview

PubSub(
  cachedItemsNumber = 0,
  cachedInitItems = [],
  pubCachedOnSub = false
)

Parameters

  • cachedItemsNumber: number - a number of items to cache
  • cachedInitItems: any[] - initial content of cache; if it's an array whose length exceeds the cachedItemsNumber value, only last elements are taken
  • pubCachedOnSub: boolean - if true, newly added subscriber will be fired immediately with cached values

Return value

Returns object with following members:

  • subscribers: number - a number of subscribers
  • isCached: boolean - whether values are cached
  • cacheSize: number - a number of items being cached
  • valuesEmitted: number - a number of times any value was emitted
  • sub(subscriber: Function): Function - adds a subscriber and returns it
  • unsub(subscriber: Function): Function - removes the subscriber and returns it
  • unsubAll(): void - removes all subscribers
  • pub(value: any): void - emits a value to subscribers

subscriber is a function that takes two arguments - (value, meta):

  • value - emitted value
  • meta - object with following members:
    • pubOnSubCall: boolean - if pubCachedOnSub parameter of the PubSub() constructor was set to true, it states whether a current call of a subscriber is automatic and takes place due to addition of the subscriber
    • stop: Symbol - return it from a subscriber to stop the flow of a value and prevent remaining subscribers from handling the value

Examples

Use of the unsub() method

const ps = PubSub();

const subA$ = ps.sub(val => console.log('A: ' + val));
const subB$ = ps.sub(val => console.log('B: ' + val));

ps.pub(1);
// A: 1
// B: 1

ps.unsub(subA$);

ps.pub(2);
// B: 2

Use of the stop symbol

let i = 0;
const ps = PubSub();

ps.sub((val, meta) => {
  i++;
  console.log('A: ' + val);
  if (i === 2) return meta.stop;
});

ps.sub(val => {
  console.log('B: ' + val);
});

ps.pub(1);
// A: 1
// B: 1

ps.pub(2);
// A: 2

ps.pub(3);
// A: 3
// B: 3

Cache

const ps = PubSub(2, [10,20], true);

ps.sub(val => console.log('A: ' + val));
// A: 10,20

ps.pub(1);
// A: 20,1

ps.sub(val => console.log('B: ' + val));
// B: 20,1

ps.pub(2);
// A: 1,2
// B: 1,2

/*
exceptionally, do not emit cached items
on sub in this subscriber
*/
ps.sub((val, meta) => {
  if (!meta.pubOnSubCall) {
    console.log('C: ' + val);
  }
});

ps.pub(3);
// A: 2,3
// B: 2,3
// C: 2,3