image-src-change.html

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This WPT test may be referenced by the following Test IDs:
- /largest-contentful-paint/image-src-change.html - WPT Dashboard Interop Dashboard

<!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>