/* * 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 { initMetric } from './lib/initMetric.js'; import { observe } from './lib/observe.js'; import { onHidden } from './lib/onHidden.js'; import { onBFCacheRestore } from './lib/onBFCacheRestore.js'; import { bindReporter } from './lib/bindReporter.js'; import { getFCP } from './getFCP.js'; let isMonitoringFCP = false; let fcpValue = -1; export const getCLS = (onReport, reportAllChanges) => { // Start monitoring FCP so we can only report CLS if FCP is also reported. // Note: this is done to match the current behavior of CrUX. if (!isMonitoringFCP) { getFCP((metric) => { fcpValue = metric.value; }); isMonitoringFCP = true; } const onReportWrapped = (arg) => { if (fcpValue > -1) { onReport(arg); } }; let metric = initMetric('CLS', 0); let report; let sessionValue = 0; let sessionEntries = []; const entryHandler = (entry) => { // Only count layout shifts without recent user input. if (!entry.hadRecentInput) { const firstSessionEntry = sessionEntries[0]; const lastSessionEntry = sessionEntries[sessionEntries.length - 1]; // If the entry occurred less than 1 second after the previous entry and // less than 5 seconds after the first entry in the session, include the // entry in the current session. Otherwise, start a new session. if (sessionValue && entry.startTime - lastSessionEntry.startTime < 1000 && entry.startTime - firstSessionEntry.startTime < 5000) { sessionValue += entry.value; sessionEntries.push(entry); } else { sessionValue = entry.value; sessionEntries = [entry]; } // If the current session value is larger than the current CLS value, // update CLS and the entries contributing to it. if (sessionValue > metric.value) { metric.value = sessionValue; metric.entries = sessionEntries; report(); } } }; const po = observe('layout-shift', entryHandler); if (po) { report = bindReporter(onReportWrapped, metric, reportAllChanges); onHidden(() => { po.takeRecords().map(entryHandler); report(true); }); onBFCacheRestore(() => { sessionValue = 0; fcpValue = -1; metric = initMetric('CLS', 0); report = bindReporter(onReportWrapped, metric, reportAllChanges); }); } };