element-timing/buffer-before-onload.html

<!DOCTYPE html>
<html>
<meta charset=utf-8 />
<title>Element Timing: buffer elements before onload</title>
<script src=/resources/testharness.js></script>
<script src=/resources/testharnessreport.js></script>
<script src="resources/element-timing-helpers.js"></script>
<body>
<img src=resources/slow-image.py?name=square20.png&sleep=500>
<script>
  /*
  In this test, a slow image is added to the frame to delay onload. The entry
  is available from the observer with the buffered flag set to true.
  */
  async_test(function(t) {
    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
    beforeRender = performance.now();
    const img = document.createElement('img');
    img.src = 'resources/square20.jpg';
    img.setAttribute('elementtiming', 'my_image');
    img.setAttribute('id', 'my_id');
    document.body.appendChild(img);

    // this PerformanceObserver should be notified about the previously
    // buffered element entry
    new PerformanceObserver(t.step_func((entryList, observer) => {
      assert_equals(entryList.getEntries().length, 1);
      entryList.getEntries().forEach(entry => {
        assert_equals(entry.entryType, "element");
        const pathname = window.location.origin + '/element-timing/resources/square20.jpg';
        checkElement(entry, pathname, 'my_image', 'my_id', beforeRender, img);
        checkNaturalSize(entry, 20, 20);
        observer.disconnect();
        t.done();
      });
    })).observe({
      type: "element",
      buffered: true
    });
  }, "Element Timing: image loads before onload available from buffered flag.");

</script>
</body>
</html>