Example: add listen for custom event on page load (#529)

* Example: add listen for custom event on page load

* correct pptr path

examples/custom-event.js

@@ -0,0 +1,50 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+const puppeteer = require('puppeteer');
+
+(async() => {
+
+const browser = await puppeteer.launch();
+const page = await browser.newPage();
+
+// Define a window.onCustomEvent function on the page.
+await page.exposeFunction('onCustomEvent', e => {
+  console.log(`${e.type} fired`, e.detail || '');
+});
+
+/**
+ * Attach an event listener to page to capture a custom event on page load/navigation.
+ * @param {string} type Event name.
+ * @return {!Promise}
+ */
+function listenFor(type) {
+  return page.evaluateOnNewDocument(type => {
+    document.addEventListener(type, e => {
+      window.onCustomEvent({type, detail: e.detail});
+    });
+  }, type);
+}
+
+await listenFor('app-ready'); // Listen for "app-ready" custom event on page load.
+
+await page.goto('https://www.chromestatus.com/features', {waitUntil: 'networkidle'});
+
+browser.close();
+
+})();