Visual regression testing with WebdriverIO, Jasmine and Allure

2021, Mar 29    

We have recently started using the Visual Regression Testing Service, and I thought I would share how we incorporate it in our test automation.

It’s quite straightforward to capture an image, then run the test again and perform an assertion, checking there are no differences between the captured basline image and the newly captured image, for example:

// check screen
expect(browser.checkScreen('example page')).toEqual(0);

// Check an element
expect(browser.checkElement($('#element-id'), 'example element')).toEqual(0);

Having previously had some experience with writing custom matchers for Jasmine, I thought it would make sense to wrap some of the functionality inside a matcher, for the following reasons:

  • Firstly, we could set a consistent screen size before we do the comparison

  • Secondly, by setting the plugin option returnAllCompareData: true, it would allow us to return more information that would be useful when a test fails:

const checkResult = {
  // The formatted filename, this depends on the options `formatImageName`
  fileName: 'examplePage-chrome-headless-latest-1366x768.png',
  folders: {
      // The actual folder and the file name
      actual: '/Users/wswebcreation/Git/wdio-image-comparison-service/.tmp/actual/desktop_chrome/examplePage-chrome-headless-latest-1366x768.png',
      // The baseline folder and the file name
      baseline: '/Users/wswebcreation/Git/wdio-image-comparison-service/localBaseline/desktop_chrome/examplePage-chrome-headless-latest-1366x768.png',
      // This following folder is optional and only if there is a mismatch
      // The folder that holds the diffs and the file name
      diff: '/Users/wswebcreation/Git/wdio-image-comparison-service/.tmp/diff/desktop_chrome/examplePage-chrome-headless-latest-1366x768.png',
    },
    // The mismatch percentage
    misMatchPercentage: 2.34
};

Show difference Show overlay

Bringing this all together we end up with:

jasmine.addMatchers({
    toMatchImageSnapshot: function(util, customEqualityTesters) {
        "use strict";
        return {
            compare: function(actual, expected) {
                browser.setWindowSize(1200, 900);

                let checkResult;
                if(actual.hasOwnProperty('element')) {
                  actual.element.waitForDisplayed();
                  checkResult = browser.checkElement(actual.element, actual.tag);
                }
                else {
                  checkResult = browser.checkScreen(actual.tag);
                }

                const pass = checkResult.misMatchPercentage === 0;

                if (!pass) { 
                  const actualImage = fs.readFileSync(checkResult.folders.actual);
                  const expectedImage = fs.readFileSync(checkResult.folders.baseline);
                  const differenceImage = fs.readFileSync(checkResult.folders.diff);
                  allure.addLabel('testType', 'screenshotDiff');
                  allure.addAttachment('diff', differenceImage, 'image/png');
                  allure.addAttachment('actual', actualImage, 'image/png');
                  allure.addAttachment('expected', expectedImage, 'image/png');
                }

                return {
                    pass,
                    message: `Expected to have matched image at ${checkResult.folders.actual}, but there was a difference of ${checkResult.misMatchPercentage}%. 
                    Difference can be viewed at ${checkResult.folders.diff}. Original can be viewed at ${checkResult.folders.baseline}`
                };
            }
        };
    }
});

This allows us to re-write the example assertions above as:

// check screen
expect({ tag: 'example page' }).toMatchImageSnapshot();

// Check an element
expect({ element: $('#element-id'), tag: 'example element' }).toMatchImageSnapshot();

What’s next

We plan to add functionality to allow for updating the baseline, rather than having to manaully update the baselines.