Selenium提供了很多的event listening functions来跟踪脚本执行过程中的events.

How it works? 

在注册了listener的webDriver里面，这些listener可以listen events。events可以是任何类型的event 或者 action，比如element click，改变输入框的值，甚至是exceptions。

To enable this feature: 

1. 创建用户自定义的Event listener class. 

2. 通过实现webdriver的接口，来创建一个EventFiringWebDriverObject

3. And register the Listener to the EventFiringWebDriver instance.

Q: 如何创建自己的event listener class? 

A: By 实现WebDriverEventListener 接口（不推荐）。 

或者By 继承AbstractWedDriverEventListener 类。（推荐）

首先展示实现WebdriverEventListener 接口的方式（不推荐）： 

package main.test.org.seleniummonster.com.demo.eventlistenerdemo;import org.openqa.selenium.By;import org.openqa.selenium.WebDriver;import org.openqa.selenium.WebElement;import org.openqa.selenium.support.events.WebDriverEventListener; public class EventListenerType1 implements WebDriverEventListener {@Overridepublic void afterChangeValueOf(WebElement arg0, WebDriver arg1) {// Things to be done after changing the value in the webelement argo}@Overridepublic void afterClickOn(WebElement arg0, WebDriver arg1) {// Things to be done after clicking an element arg0}@Overridepublic void afterFindBy(By arg0, WebElement arg1, WebDriver arg2) {// Things to be done  after findby of webelement arg1}@Overridepublic void afterNavigateBack(WebDriver arg0) {// Things to be done after navigating back}@Overridepublic void afterNavigateForward(WebDriver arg0) {// Things to be done after navigating forward}@Overridepublic void afterNavigateTo(String arg0, WebDriver arg1) {// Things to be done after navigating to the given URL say, arg0}@Overridepublic void afterScript(String arg0, WebDriver arg1) {// Things to be done after script execution}@Overridepublic void beforeChangeValueOf(WebElement arg0, WebDriver arg1) {// Things to be done before Changing value in the webelement arg0}@Overridepublic void beforeClickOn(WebElement arg0, WebDriver arg1) {// Things to be done before Clicking an element arg0}@Overridepublic void beforeFindBy(By arg0, WebElement arg1, WebDriver arg2) {// Things to be done before a Findby of an web element arg1}@Overridepublic void beforeNavigateBack(WebDriver arg0) {// Things to be done before navigating back}@Overridepublic void beforeNavigateForward(WebDriver arg0) {// Things to be done before Navigating forward}@Overridepublic void beforeNavigateTo(String arg0, WebDriver arg1) {// Things to be done before navigating to a url arg0}@Overridepublicvoid beforeScript(String arg0, WebDriver arg1) {// Things to be done before javascript execution}@Overridepublic void onException(Throwable arg0, WebDriver arg1) {System.out.println("There is an exception in the script, please find the below error description" );arg0.printStackTrace();}}

在这种方法里，我们只重写了onException() ，其与方法都没做改动，但还是override了一遍。没办法啊。

然后就要介绍推荐的那种方法了。AbstractWebDriverEventListener 是一个抽象类，它实现了WebDriverEventListener接口，它override了WebDriverEventListener所有的方法（当然只是空的override了一下）。我们可以继承AbstractWebDriverEventListener class 来具体定义我们关注的方法，看如下例子：EventListenerType2.java

package main.test.org.seleniummonster.com.demo.eventlistenerdemo;import org.openqa.selenium.WebDriver;import org.openqa.selenium.support.events.AbstractWebDriverEventListener;public class EventListenerType2 extends AbstractWebDriverEventListener {/*** @author Selenium Monster*/@Overridepublic void onException(Throwable arg0, WebDriver arg1) {System.out.println("There is an exception in the script, please find the below error description" );arg0.printStackTrace();}}

代码简短了不少，接下来就是Registering the Listener with Webdriver instance

下面是个实例： 创建个EventFiringWebDriver 类的实例， 我们定义的listener类的实例。用register()方法把我们定义的listener注册到EventFiringWebDriver 对象上。可以注册As many listener as you whish. 

* If you have definitions for an action in all the listener, all of them will be executed.*

package main.test.org.seleniummonster.com.demo.eventlistenerdemo;import org.openqa.selenium.WebDriver;import org.openqa.selenium.firefox.FirefoxDriver;import org.openqa.selenium.support.events.EventFiringWebDriver;public class EventFiringWebdriverExample {/*** @author Selenium Monster*/public static void main(String[] args) {// 创建一个 webdriver实例WebDriver driver = new FirefoxDriver();// 创建EventFiringWebDriver 实例EventFiringWebDriver eventFiringWD= new EventFiringWebDriver(driver);// Creating instance of eventListener, that we just definedEventListenerType2 eventListener1=new EventListenerType2();// Register the Listener with the event firing drivereventFiringWD.register(eventListener1);eventFiringWD.get("https://www.google.co.in/");eventFiringWD.findElement(By.className("nosuchclassName"));}}

解除注册的方法：

eventFiringWD=eventFiringWD.unregister(eventListener1);

加监听的最大目的，就是减少维护脚本所花的力气， 让报错的信息，更容易定位到问题。