From 01937dc9d5215ef96ce756e3ccda51bf29032f58 Mon Sep 17 00:00:00 2001
From: Wojtek Kosior <wk@koszkonutek-tmp.pl.eu.org>
Date: Mon, 10 May 2021 18:07:05 +0200
Subject: initial commit

---
 common/once.mjs | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)
 create mode 100644 common/once.mjs

(limited to 'common/once.mjs')

diff --git a/common/once.mjs b/common/once.mjs
new file mode 100644
index 0000000..0f76366
--- /dev/null
+++ b/common/once.mjs
@@ -0,0 +1,42 @@
+/**
+* Myext feature initialization promise
+*
+* Copyright (C) 2021 Wojtek Kosior
+*
+* Dual-licensed under:
+*   - 0BSD license
+*   - GPLv3 or (at your option) any later version
+*/
+
+"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));
+}
+
+export function make_once(result_producer)
+{
+    let state = {waiting : [], ready : false, result : undefined};
+    assign_result(state, result_producer);
+    return () => get_result(state);
+}
+
+export default make_once;
-- 
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