common/once.js

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2021-07-20 Merge rebranding to "Hachette" Wojtek Kosior
/**
 * This file is part of Haketilo.
 *
 * Function: Wrap APIs that depend on some asynchronous initialization into
 *     promises.
 *
 * 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
 */