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/*
* 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);
});
}
};