extensionUtils: Add InjectionManager

It is fairly common for extensions to monkey-patch existing
classes. Add a small helper class that makes this a tad bit
more convenient.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2809>

js/extensions/extension.js

@@ -29,3 +29,71 @@ export class Extension extends ExtensionBase {
 export const {
     gettext, ngettext, pgettext,
 } = Extension.defineTranslationFunctions();
+
+export class InjectionManager {
+    #savedMethods = new Map();
+
+    /**
+     * @callback CreateOverrideFunc
+     * @param {Function?} originalMethod - the original method if it exists
+     * @returns {Function} - a function to be used as override
+     */
+
+    /**
+     * Modify, replace or inject a method
+     *
+     * @param {object} prototype - the object (or prototype) that is modified
+     * @param {string} methodName - the name of the overwritten method
+     * @param {CreateOverrideFunc} createOverrideFunc - function to call to create the override
+     */
+    overrideMethod(prototype, methodName, createOverrideFunc) {
+        const originalMethod = this._saveMethod(prototype, methodName);
+        prototype[methodName] = createOverrideFunc(originalMethod);
+    }
+
+    /**
+     * Restore the original method
+     *
+     * @param {object} prototype - the object (or prototype) that is modified
+     * @param {string} methodName - the name of the method to restore
+     */
+    restoreMethod(prototype, methodName) {
+        const savedProtoMethods = this.#savedMethods.get(prototype);
+        if (!savedProtoMethods)
+            return;
+
+        const originalMethod = savedProtoMethods.get(methodName);
+        if (originalMethod === undefined)
+            delete prototype[methodName];
+        else
+            prototype[methodName] = originalMethod;
+
+        savedProtoMethods.delete(methodName);
+        if (savedProtoMethods.size === 0)
+            this.#savedMethods.delete(prototype);
+    }
+
+    /**
+     * Restore all original methods and clear overrides
+     */
+    clear() {
+        for (const [proto, map] of this.#savedMethods) {
+            map.forEach(
+                (_, methodName) => this.restoreMethod(proto, methodName));
+        }
+        console.assert(this.#savedMethods.size === 0,
+            `${this.#savedMethods.size} overrides left after clear()`);
+    }
+
+    _saveMethod(prototype, methodName) {
+        let savedProtoMethods = this.#savedMethods.get(prototype);
+        if (!savedProtoMethods) {
+            savedProtoMethods = new Map();
+            this.#savedMethods.set(prototype, savedProtoMethods);
+        }
+
+        const originalMethod = prototype[methodName];
+        savedProtoMethods.set(methodName, originalMethod);
+        return originalMethod;
+    }
+}