common/once.js


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/**
 * Myext feature initialization promise
 *
 * Copyright (C) 2021 Wojtek Kosior
 * Redistribution terms are gathered in the `copyright' file.
 */

"use strict";

/*
 * 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);
    }

    window.make_once = make_once;
})();