feat: add check if listener exists

src/index.js

@@ -27,7 +27,7 @@ export {
   createProxyWindow,
   toProxyWindow,
 } from "./setup";
-export { on, once, send } from "./public";
+export { on, once, send, listenerExists } from "./public";
 export { markWindowKnown } from "./lib";
 export { cleanUpWindow } from "./clean";
 

src/public/index.js

@@ -2,3 +2,4 @@
 
 export * from "./on";
 export * from "./send";
+export * from "./listenerExists";

src/public/listenerExists.js

@@ -0,0 +1,34 @@
+/* @flow */
+
+import { getRequestListener } from "../drivers";
+import type { ServerOptionsType } from "../types";
+
+const getDefaultServerOptions = (): ServerOptionsType => {
+  // $FlowFixMe
+  return {};
+};
+
+export function listenerExists(
+  name: string,
+  options: ?ServerOptionsType
+): boolean {
+  if (!name) {
+    throw new Error("Expected name");
+  }
+
+  options = options || getDefaultServerOptions();
+
+  const win = options.window;
+  const domain = options.domain;
+
+  if (Array.isArray(domain)) {
+    for (const item of domain) {
+      if (getRequestListener({ name, win, domain: item })) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  return Boolean(getRequestListener({ name, win, domain }));
+}