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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 { doubleRAF } from './lib/doubleRAF.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 { whenActivated } from './lib/whenActivated.js';
/** Thresholds for FCP. See https://web.dev/articles/fcp#what_is_a_good_fcp_score */
export const FCPThresholds = [1800, 3000];
/**
* Calculates the [FCP](https://web.dev/articles/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, opts) => {
// Set defaults
opts = opts || {};
whenActivated(() => {
const visibilityWatcher = getVisibilityWatcher();
let metric = initMetric('FCP');
let report;
const handleEntries = (entries) => {
entries.forEach((entry) => {
if (entry.name === 'first-contentful-paint') {
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);
}
}
});
};
const po = observe('paint', handleEntries);
if (po) {
report = bindReporter(onReport, metric, FCPThresholds, opts.reportAllChanges);
// 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, FCPThresholds, opts.reportAllChanges);
doubleRAF(() => {
metric.value = performance.now() - event.timeStamp;
report(true);
});
});
}
});
};
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