image-src-change.html | Explore in Territory

<!DOCTYPE HTML>
<meta charset=utf-8>
<title>Largest Contentful Paint: src change triggers new entry.</title>

<body>
  <script src="/resources/testharness.js"></script>
  <script src="/resources/testharnessreport.js"></script>
  <script src="resources/largest-contentful-paint-helpers.js"></script>
  <img src='' id='image_id' />
  <script>
    setup({ "hide_test_state": true });

    let first_image_src = '/images/black-rectangle.png';
    let second_image_src = '/images/blue.png';
    let image_id = 'image_id';

    // Add listener for load event that is fired when image is loaded.
    const image_load_promise = image_element => {
      return new Promise(resolve => {
        image_element.addEventListener('load', resolve);
      });
    }

    // Create a promise that resolves when an LCP is observed.
    const lcp_observation_promise = image_src => {
      return new Promise(resolve => {
        new PerformanceObserver((entryList) => {
          let lcpEntry = entryList.getEntries().find(e => e.url.includes(image_src));

          if (lcpEntry) {
            resolve(lcpEntry);
          }

        }).observe({ type: 'largest-contentful-paint' });
      });
    }

    const loadImageAndGetLCPEntry = async image_src => {
      let LCPObserverPromise = lcp_observation_promise(image_src);

      let image_element = document.getElementById(image_id);

      let promise = image_load_promise(image_element);

      image_element.src = image_src;

      await promise;

      return await LCPObserverPromise;
    }

    promise_test(async t => {

      assert_implements(window.LargestContentfulPaint, "LargestContentfulPaint is not implemented");


      // Load first image.
      let beforeLoad = performance.now();

      let first_LCP = await loadImageAndGetLCPEntry(first_image_src);

      // Verify first LCP entry correctness. The black-rectangle.png is 100 x 50.
      checkImage(first_LCP, window.location.origin + first_image_src, image_id, 100 * 50, beforeLoad);

      // Load second image.
      beforeLoad = performance.now();

      let second_LCP = await loadImageAndGetLCPEntry(second_image_src);

      // Verify second LCP entry correctness. The blue.png is 133 by 106.
      checkImage(second_LCP, window.location.origin + second_image_src, image_id, 133 * 106, beforeLoad);

    }, 'Largest Contentful Paint: changing src causes a new entry to be dispatched.');
  </script>
</body>
chromium/third_party/blink/web_tests/external/wpt/largest-contentful-paint/image-src-change.html
