If you’re testing a web application, you can’t go far wrong with Selenium WebDriver. But in this web 2.0 world of ajax-y goodness, it can be a pain dealing with the asynchronous nature of modern sites. Back when all we had was web 1.0 you clicked a button and eventually you got a new page, or if you were unlucky: an error message. But now when you click links all sorts of funky things happen – some of which happen faster than others. From the user’s perspective this creates a great UI. But if you’re trying to automate testing this you can get all sorts of horrible race conditions.

Thread.sleep

The naive approach is to write your tests the same way you did before: you click buttons and assert that what you expected to happen actually happened. For the most part, this works. Sites are normally fast enough, even in a continuous integration environment, that by the time the test harness looks for a change it’s already happened.

But then… things slow down a little and you start getting flickers - tests that sometimes pass and sometimes fail. So you add a little delay. Just 500 milliseconds should do it, while you wait for the server to respond and update the page. Then a month later it’s flickering again, so you make it 1 second. Then two… then twenty.

The trouble is, each test runs at the pace that it runs at its slowest. If login normally takes 0.1 seconds, but sometimes takes 10 seconds when the environment’s overloaded – the test has to wait for 10 seconds so as not to flicker. This means even though the app often runs faster,  the test has to wait just in case.

Before you know it, your tests are crawling and take hours to run – you’ve lost your fast feedback loop and developers no longer trust the tests.

An Example

Thankfully WebDriver has a solution to this. It allows you to wait for some condition to pass, so you can use it to control the pace of your tests. To demonstrate this, I’ve created a simple web application with a login form – the source is available on github. The login takes a stupid amount of time, so the tests need to react to this so as not to introduce arbitrary waits.

The application is very simple – a username and password field with an authenticate button that makes an ajax request to log the user in. If the login is successful, we update the screen to let the user know.

The first thing is to write our test (obviously in the real world we’d have written the test before our production code, but its the test that’s interesting here not what we’re testing – so we’ll do it in the wrong order just this once):

@Test
public void authenticatesUser()
{
    driver.get("http://localhost:8080/");

    LoginPage loginPage = LoginPage.open(driver);
    loginPage.setUsername("admin");
    loginPage.setPassword("password");
    loginPage.clickAuthenticate();
    Assert.assertEquals("Logged in as admin", loginPage.welcomeMessage());
}

We have a page object that encapsulates the login functionality. We provide the username & password then click authenticate. Finally we check that the page has updated with the user message. But how have we dealt with the asynchronous nature of this application?

WebDriverWait

Through the magic of WebDriverWait we can wait for a function to return true before we continue:

public void clickAuthenticate() {
    this.authenticateButton.click();
    new WebDriverWait(driver, 30).until(accountPanelIsVisible());
}

private Predicate<WebDriver> accountPanelIsVisible() {
    return new Predicate<WebDriver>() {
        @Override public boolean apply(WebDriver driver) {
            return isAccountPanelVisible();
        }
    };
}
private boolean isAccountPanelVisible() {
    return accountPanel.isDisplayed();
}

Our clickAuthenticate method clicks the button then instructs WebDriver to wait for our condition to pass. The condition is defined via a predicate (c’mon Java where’s the closures?). The predicate is simply a method that will run to determine whether or not the condition is true yet. In this case, we delegate to the isAccountPanelVisible method on the page object. This does exactly what it says on the tin, it uses the page element to check whether it’s visible yet. Simple, no?

In this way we can define a condition we want to be true before we continue. In this case, the exit condition of the clickAuthenticate method is that the asynchronous authentication process has completed. This means that tests don’t need to worry about the internal mechanics of the page – about whether the operation is asynchronous or not. The test merely specifies what to test, the page object encapsulates how to do it.

Javascript

It’s all well and good waiting for elements to be visible or certain text to be present, but sometimes we might want more subtle control. A good approach is to update Javascript state when an action has finished. This means that tests can inspect javascript variables to determine whether something has completed or not – allowing very clear and simple coordination between production code and test.

Continuing with our login example, instead of relying on a <div> becoming visible, we could instead have set a Javascript variable. The code in fact does both, so we can have two tests. The second looks as follows:

public void authenticate() {
    this.authenticateButton.click();
    new WebDriverWait(driver, 30).until(authenticated());
}

private Predicate<WebDriver> authenticated() {
    return new Predicate<WebDriver>() {
        @Override public boolean apply(WebDriver driver) {
            return isAuthenticated();
        }
    };
}

private boolean isAuthenticated() {
    return (Boolean) executor().executeScript("return authenticated;");
}
private JavascriptExecutor executor() {
    return (JavascriptExecutor) driver;
}

This example follows the same basic pattern as the test before, but we use a different predicate. Instead of checking whether an element is visible or not, we instead get the status of a Javascript variable. We can do this because each WebDriver also implements the JavascriptExecutor allowing us to run Javascript inside the browser within the context of the test. I.e. the script “return authenticated” runs within the browser, but the result is returned to our test. We simply inspect the state of a variable, which is false initially and set to true once the authentication process has finished.

This allows us to closely coordinate our production and test code without the risk of flickering tests because of race conditions.

From http://blog.activelylazy.co.uk/2012/01/29/testing-asynchronous-applications-with-webdriverwait/