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onFCP.ts
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onFCP.ts
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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 {onBFCacheRestore} from './lib/bfcache.js';
import {bindReporter} from './lib/bindReporter.js';
import {getActivationStart} from './lib/getActivationStart.js';
import {getVisibilityWatcher} from './lib/getVisibilityWatcher.js';
import {initMetric} from './lib/initMetric.js';
import {observe} from './lib/observe.js';
import {FCPMetric, FCPReportCallback, ReportOpts} from './types.js';
/**
* Calculates the [FCP](https://web.dev/fcp/) value for the current page and
* calls the `callback` function once the value is ready, along with the
* relevant `paint` performance entry used to determine the value. The reported
* value is a `DOMHighResTimeStamp`.
*/
export const onFCP = (onReport: FCPReportCallback, opts?: ReportOpts) => {
// Set defaults
opts = opts || {};
// https://web.dev/fcp/#what-is-a-good-fcp-score
const thresholds = [1800, 3000];
const visibilityWatcher = getVisibilityWatcher();
let metric = initMetric('FCP');
let report: ReturnType<typeof bindReporter>;
const handleEntries = (entries: FCPMetric['entries']) => {
(entries as PerformancePaintTiming[]).forEach((entry) => {
if (entry.name === 'first-contentful-paint') {
if (po) {
po.disconnect();
}
// Only report if the page wasn't hidden prior to the first paint.
if (entry.startTime < visibilityWatcher.firstHiddenTime) {
// The activationStart reference is used because FCP should be
// relative to page activation rather than navigation start if the
// page was prerendered. But in cases where `activationStart` occurs
// after the FCP, this time should be clamped at 0.
metric.value = Math.max(entry.startTime - getActivationStart(), 0);
metric.entries.push(entry);
report(true);
}
}
});
};
// TODO(philipwalton): remove the use of `fcpEntry` once this bug is fixed.
// https://bugs.webkit.org/show_bug.cgi?id=225305
// The check for `getEntriesByName` is needed to support Opera:
// https://github.com/GoogleChrome/web-vitals/issues/159
// The check for `window.performance` is needed to support Opera mini:
// https://github.com/GoogleChrome/web-vitals/issues/185
const fcpEntry = window.performance && window.performance.getEntriesByName &&
window.performance.getEntriesByName('first-contentful-paint')[0];
const po = fcpEntry ? null : observe('paint', handleEntries);
if (fcpEntry || po) {
report = bindReporter(onReport, metric, thresholds, opts.reportAllChanges);
if (fcpEntry) {
handleEntries([fcpEntry]);
}
// Only report after a bfcache restore if the `PerformanceObserver`
// successfully registered or the `paint` entry exists.
onBFCacheRestore((event) => {
metric = initMetric('FCP');
report = bindReporter(
onReport, metric, thresholds, opts!.reportAllChanges);
requestAnimationFrame(() => {
requestAnimationFrame(() => {
metric.value = performance.now() - event.timeStamp;
report(true);
});
});
});
}
};