How To Test Canvas Element In Selenium

by

Canvas elements are extensively utilized in contemporary web applications for generating graphics, animations, and numerous other features. However, testing these canvas elements can be challenging due to the ineffectiveness of conventional selection methods. In such cases, Selenium, a commonly used automated testing framework, proves to be useful.

The Canvas is a HTML5 element used to draw graphics on a web page. The graphics can be dynamic and interactive, but it is a bit harder to automate the testing process. Canvas doesn’t have any separate elements to interact with unlike other HTML tags. Everything drawn on canvas is a pixel and doesn’t have any ID or class or tag. Selenium won’t be able to identify any unique properties for such elements. We have to use JavaScript along with Selenium to test canvas elements.

Testing Canvas Elements Using JavaScript Executor

A useful class that Selenium provides for handling such canvas elements is the JavaScript Executor. We can use JavaScript to access the canvas and extract information from it which can then be used for verification purposes.

This can be achieved using the following steps:

  1. Get the canvas element using Selenium. You can do this using the findElement() method.
  2. Use JavaScript Executor to get the Canvas pixel data.
  3. Verify the pixel data based on your requirements. 
WebDriver driver = new ChromeDriver();
JavascriptExecutor js = (JavascriptExecutor) driver;

// Get the canvas element
WebElement canvas = driver.findElement(By.id("canvas"));

// Execute JavaScript to get the pixel data from canvas
String pixelData = (String) js.executeScript("return arguments[0].getContext('2d').getImageData(0, 0, 1, 1).data.toString();", canvas);

// Verify pixel data
assertTrue("Pixel data is not as expected", pixelData.equals(expectedPixelData));

The JavaScript code arguments[0].getContext(‘2d’).getImageData(0, 0, 1, 1).data.toString(); gets the pixel data of the first pixel on the canvas. The getImageData(x, y, width, height) method returns an ImageData object representing the underlying pixel data for the area of the canvas.

Remember, Selenium alone cannot interact directly with the canvas element. Selenium combined with JavaScript can be a powerful tool for testing the canvas elements in your web application.

Always ensure the quality of your web application by properly testing all elements, including the often overlooked canvas element.