callbag-remember's Introduction

callbag-remember

Callbag operator which shares input stream between subscribers and emits last emitted value upon subscription. It behaves pretty much as Rx's .shareReplay(1).

Example

import fromEvent from 'callbag-from-event'
import forEach from 'callbag-for-each'
import map from 'callbag-map'
import merge from 'callbag-merge'
import pipe from 'callbag-pipe'
import remember from 'callbag-remember'

const focus$ = remember(
  merge(
    map(() => false)(fromEvent(window, 'blur')),
    map(() => true)(fromEvent(window, 'focus')),
  ),
)

pipe(
  focus$,
  forEach(tabFocused => {
    // ...
  }),
)

// ...

setTimeout(() => {
  pipe(
    focus$,
    forEach(tabFocused => {
      // ... will get last emitted value immediately
    }),
  )
}, 1000)

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callbag-remember's Issues

Cover with tests scenario with subscribing to completed source

Cover changes from this commit

Late-subscribing only works if early subscribing occurred

When a remembered source receives a value, that value is not emitted to late subscribers unless that source has already been subscribed to.

Perhaps this is the intended behavior, but to me this was unexpected. See: https://stackblitz.com/edit/js-t8rvxy?file=index.js

import remember from "callbag-remember";
import { pipe, forEach } from "callbag-basics";
import fromFunction from "callbag-from-function";

const { source, emitter } = fromFunction();

const rem$ = remember(source);

// Late subscription does not receive the value unless early subscription enabled
/*
pipe(
  rem$,
  forEach(value => {
    console.log("early-subscribe", value);
  })
);
*/

emitter(111);

setTimeout(() => {
  pipe(
    rem$,
    forEach(value => {
      console.log("late-subscribe", value);
    })
  );
}, 1000);

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