refactor shared utils to more obviously separate contexts

src/shared/onceDefined.ts

@@ -0,0 +1,47 @@
+/*
+ * Vencord, a modification for Discord's desktop app
+ * Copyright (c) 2022 Vendicated and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+import type { LiteralUnion } from "type-fest";
+
+/**
+ * Wait for a property to be defined on the target, then call the callback with
+ * the value
+ * @param target Object
+ * @param property Property to be defined
+ * @param callback Callback
+ *
+ * @example onceDefined(window, "webpackChunkdiscord_app", wpInstance => wpInstance.push(...));
+ */
+export function onceDefined<T extends object, P extends LiteralUnion<keyof T, PropertyKey>>(
+    target: T, property: P, callback: (v: P extends keyof T ? T[P] : any) => void
+): void {
+    const propertyAsAny = property as any;
+
+    if (property in target)
+        return void callback(target[propertyAsAny]);
+
+    Object.defineProperty(target, property, {
+        set(v) {
+            delete target[propertyAsAny];
+            target[propertyAsAny] = v;
+            callback(v);
+        },
+        configurable: true,
+        enumerable: false
+    });
+}