Add example using MV3 userScripts API (#576)

userScripts-mv3/userscript_api.js

@@ -0,0 +1,50 @@
+"use strict";
+
+// userscript_api.js defines a single function that generates the global APIs
+// that should be made available to scripts running in this USER_SCRIPT sandbox.
+// This script is scheduled to run before user scripts by the parseUserScript
+// function in userscript_manager_logic.mjs
+
+globalThis.initCustomAPIForUserScripts = grants => {
+  // Allow initialization only once.
+  delete globalThis.initCustomAPIForUserScripts;
+
+  // background.js calls userScripts.configureWorld({ messaging: true }), which
+  // exposes runtime.sendMessage here. When this method is called, the
+  // runtime.onUserScriptMessage listener (in background.js) is called, which
+  // in turn calls handleUserScriptMessage in userscript_manager_logic.mjs.
+  const sendMessage = browser.runtime.sendMessage;
+
+  // Clear access to privileged API to prevent userscripts from communicating
+  // to the privileged backend.
+  globalThis.browser = undefined;
+
+  if (grants.includes("GM_info")) {
+    // Example of an API that retrieves information:
+    // https://www.tampermonkey.net/documentation.php#api:GM_info
+    // https://violentmonkey.github.io/api/gm/#gm_info
+    // https://wiki.greasespot.net/GM.info
+    // NOTE: The following implementation of GM_info is async to demonstrate
+    // how one can retrieve information on demand. The actual GM_info function
+    // as defined by full-featured user script managers is synchronous.
+    globalThis.GM_info = async () => {
+      return sendMessage({ userscript_api_name: "GM_info" });
+    };
+  }
+
+  if (grants.includes("GM_openInTab")) {
+    // Example of an API that sends information:
+    // https://www.tampermonkey.net/documentation.php#api:GM_openInTab
+    // https://violentmonkey.github.io/api/gm/#gm_openintab
+    // https://wiki.greasespot.net/GM.openInTab
+    globalThis.GM_openInTab = async (url) => {
+      await sendMessage({ userscript_api_name: "GM_openInTab", args: [url] });
+    };
+  }
+
+  // TODO: Implement more APIs.
+
+  // After this function returns, the userscript may execute and potentially
+  // change built-in prototypes. To protect against that, make sure to store
+  // any functions that you want to use in a local variable!
+};