Use libWrapper for function hooks (resolves #5)

lib/libwrapper_shim.js

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2021 fvtt-lib-wrapper Rui Pinheiro
+
+'use strict';
+
+// A shim for the libWrapper library
+export let libWrapper = undefined;
+
+Hooks.once('init', () => {
+	// Check if the real module is already loaded - if so, use it
+	if(globalThis.libWrapper && !(globalThis.libWrapper.is_fallback ?? true)) {
+		libWrapper = globalThis.libWrapper;
+		return;
+	}
+
+	// Fallback implementation
+	libWrapper = class {
+		static get is_fallback() { return true };
+
+		static register(module, target, fn, type="MIXED", {chain=undefined}={}) {
+			const is_setter = target.endsWith('#set');
+			target = !is_setter ? target : target.slice(0, -4);
+			const split = target.split('.');
+			const fn_name = split.pop();
+			const root_nm = split.splice(0,1)[0];
+			const _eval = eval; // The browser doesn't expose all global variables (e.g. 'Game') inside globalThis, but it does to an eval. We copy it to a variable to have it run in global scope.
+			const obj = split.reduce((x,y)=>x[y], globalThis[root_nm] ?? _eval(root_nm));
+
+			let iObj = obj;
+			let descriptor = null;
+			while(iObj) {
+				descriptor = Object.getOwnPropertyDescriptor(iObj, fn_name);
+				if(descriptor) break;
+				iObj = Object.getPrototypeOf(iObj);
+			}
+			if(!descriptor || descriptor?.configurable === false) throw `libWrapper Shim: '${target}' does not exist, could not be found, or has a non-configurable descriptor.`;
+
+			let original = null;
+			const wrapper = (chain ?? type != 'OVERRIDE') ? function() { return fn.call(this, original.bind(this), ...arguments); } : function() { return fn.apply(this, arguments); };
+
+			if(!is_setter) {
+				if(descriptor.value) {
+					original = descriptor.value;
+					descriptor.value = wrapper;
+				}
+				else {
+					original = descriptor.get;
+					descriptor.get = wrapper;
+				}
+			}
+			else {
+				if(!descriptor.set) throw `libWrapper Shim: '${target}' does not have a setter`;
+				original = descriptor.set;
+				descriptor.set = wrapper;
+			}
+
+			descriptor.configurable = true;
+			Object.defineProperty(obj, fn_name, descriptor);
+		}
+	}
+});