1 files changed, 44 insertions, 0 deletions
diff --git a/common/once.js b/common/once.js
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+++ b/common/once.js
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+/**
+* 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));
+    }
+
+    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;
+})();