How to highlight elements in use with Selenium WebDriver?

How can we highlight the elements in use with Selenium WebDriver?

A reader asked for recommendations on how to use Selenium to make the elements in use visible.

Note: The earlier version of Selenium had a “Highlight” command. This blog post was originally written on 20100630 describing how to use that. The post has been updated in 2020 for Selenium WebDriver.

The Question

Selenium-RC had highlight which flashed the control yellow. A small helper function help debugging so people could see some indication of what elements your script touches.

I can see why you might want to to highlight the locator elements in use at the time and error happens, if you take screenshots automatically when a test fails.

And now, with the notion of “Observability”, we might consider, how can I make my automated execution visible to an external observer.

An Approach

I would start by first creating a simple class which can highlight elements.

Given a driver and a desired background colour

public class HighlightElement {
    private final WebDriver driver;
    private final String backgroundColour;
    WebElement lastElement;
    String lastElementOrigColour;


    public HighlightElement(final WebDriver driver, 
                            final String backgroundColour) {
        this.driver = driver;
        this.backgroundColour = backgroundColour;
    }

have a method which can highlight a given element
it also remembers which element we last added a highlight to, so that we don’t litter the page with highlighted elements
I’m using a very simple approach of changing the background colour of the elements.

    public WebElement highlight(WebElement newElement){

        // turn off last element
        if(lastElement!=null){
            setBackgroundColourOfElement(lastElement, 
                    driver, this.lastElementOrigColour);
        }

        lastElement = newElement;

        if(newElement!=null){

            this.lastElementOrigColour = 
                        newElement.getCssValue("backgroundColor");

            setBackgroundColourOfElement(newElement, driver,
                                         backgroundColour);
        }

        return newElement;
    }

the actual work of setting the background colour is done by a simple JavaScript script
Casting the driver to JavascriptExecutor allows running the script to change the background colour of the element
catch any errors just in case something goes wrong

    private void setBackgroundColourOfElement(final WebElement element, 
                                              WebDriver driver, 
                                              final String desiredColour) {

        try {
            
            ((JavascriptExecutor) driver).executeScript(
                    "arguments[0].style.backgroundColor=arguments[1]",
                            element, desiredColour);
            
        }catch(Exception e){
            if(e instanceof StaleElementReferenceException){
                // ignore
            }else {
                System.out.println("Error setting background colour of element");
                e.printStackTrace();
            }
        }
    }
}

And this could then be used in a test:

driver.get("https://testpages.eviltester.com/pages/forms/html-form/");

highlighter.highlight(
        driver.findElement(By.name("username"))
    ).sendKeys("Bob");

This would allow adhoc highlighting if needed.

What if highlighting was always on?

To make highlighting always on, I would want to do that at a driver level and could use an EventFiringWebDriver with a listener which trapped:

Some interaction events:

beforeClickOn
beforeChangeValueOf

And the ‘find’ event:

afterFindBy

And this would basically delgate off to the HighlightElement class we created earlier.

public class ElementHighlighter implements WrapsDriver {
    EventFiringWebDriver driver;

    public ElementHighlighter(final WebDriver aDriver, 
                              final String desiredBackgroundColour) {
        
        driver = new EventFiringWebDriver(aDriver);
        driver.register(new ElementHighlighterListener(
                                    driver, desiredBackgroundColour));
    }

    @Override
    public WebDriver getWrappedDriver() {
        return driver;
    }

    private class ElementHighlighterListener extends
                             AbstractWebDriverEventListener {

        HighlightElement highlighter;

        public ElementHighlighterListener(WebDriver driver, 
                                          final String highlightColour) {
            
            highlighter = new HighlightElement(driver, highlightColour);
        }

        @Override
        public void beforeClickOn(final WebElement element,
                                final WebDriver driver) {
            
            highlighter.highlight(element);
            super.beforeClickOn(element, driver);
        }

        @Override
        public void beforeChangeValueOf(final WebElement element, 
                                        final WebDriver driver, 
                                        final CharSequence[] keysToSend) {
            
            highlighter.highlight(element);
            super.beforeChangeValueOf(element, driver, keysToSend);
        }

        @Override
        public void afterFindBy(final By by, final WebElement element, 
                                final WebDriver driver) {
            
            highlighter.highlight(element);
            super.afterFindBy(by, element, driver);
        }
    }
}