summaryrefslogtreecommitdiff
path: root/common/once.js
blob: e3e6dbec042718936159c67d4c7414f031d909f7 (about) (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
/**
 * Myext feature initialization promise
 *
 * Copyright (C) 2021 Wojtek Kosior
 * Redistribution terms are gathered in the `copyright' file.
 */

/*
 * This module provides an easy way to wrap an async function into a promise
 * so that it only gets executed once.
 */

async function assign_result(state, result_producer)
{
    state.result = await result_producer();
    state.ready = true;
    for (let cb of state.waiting)
	setTimeout(cb, 0, state.result);
    state.waiting = undefined;
}

async function get_result(state)
{
    if (state.ready)
	return state.result;

    return new Promise((resolve, reject) => state.waiting.push(resolve));
}

function make_once(result_producer)
{
    let state = {waiting : [], ready : false, result : undefined};
    assign_result(state, result_producer);
    return () => get_result(state);
}

/*
 * EXPORTS_START
 * EXPORT make_once
 * EXPORTS_END
 */