web-vitals
Version:
Easily measure performance metrics in JavaScript
64 lines (63 loc) • 2.37 kB
JavaScript
/*
* Copyright 2020 Google LLC
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import { onBFCacheRestore } from './bfcache.js';
import { onHidden } from './onHidden.js';
let firstHiddenTime = -1;
const initHiddenTime = () => {
// If the document is hidden and not prerendering, assume it was always
// hidden and the page was loaded in the background.
return document.visibilityState === 'hidden' &&
!document.prerendering ? 0 : Infinity;
};
const trackChanges = () => {
// Update the time if/when the document becomes hidden.
onHidden(({ timeStamp }) => {
firstHiddenTime = timeStamp;
}, true);
};
export const getVisibilityWatcher = () => {
if (firstHiddenTime < 0) {
// If the document is hidden when this code runs, assume it was hidden
// since navigation start. This isn't a perfect heuristic, but it's the
// best we can do until an API is available to support querying past
// visibilityState.
if (window.__WEB_VITALS_POLYFILL__) {
firstHiddenTime = window.webVitals.firstHiddenTime;
if (firstHiddenTime === Infinity) {
trackChanges();
}
}
else {
firstHiddenTime = initHiddenTime();
trackChanges();
}
// Reset the time on bfcache restores.
onBFCacheRestore(() => {
// Schedule a task in order to track the `visibilityState` once it's
// had an opportunity to change to visible in all browsers.
// https://bugs.chromium.org/p/chromium/issues/detail?id=1133363
setTimeout(() => {
firstHiddenTime = initHiddenTime();
trackChanges();
}, 0);
});
}
return {
get firstHiddenTime() {
return firstHiddenTime;
}
};
};