72 lines
3.2 KiB
JavaScript
72 lines
3.2 KiB
JavaScript
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/*
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* Copyright 2020 Google LLC
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* https://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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import { onBFCacheRestore } from './lib/bfcache.js';
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import { bindReporter } from './lib/bindReporter.js';
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import { doubleRAF } from './lib/doubleRAF.js';
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import { getActivationStart } from './lib/getActivationStart.js';
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import { getVisibilityWatcher } from './lib/getVisibilityWatcher.js';
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import { initMetric } from './lib/initMetric.js';
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import { observe } from './lib/observe.js';
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import { whenActivated } from './lib/whenActivated.js';
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/** Thresholds for FCP. See https://web.dev/articles/fcp#what_is_a_good_fcp_score */
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export const FCPThresholds = [1800, 3000];
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/**
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* Calculates the [FCP](https://web.dev/articles/fcp) value for the current page and
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* calls the `callback` function once the value is ready, along with the
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* relevant `paint` performance entry used to determine the value. The reported
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* value is a `DOMHighResTimeStamp`.
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*/
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export const onFCP = (onReport, opts) => {
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// Set defaults
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opts = opts || {};
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whenActivated(() => {
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const visibilityWatcher = getVisibilityWatcher();
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let metric = initMetric('FCP');
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let report;
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const handleEntries = (entries) => {
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entries.forEach((entry) => {
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if (entry.name === 'first-contentful-paint') {
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po.disconnect();
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// Only report if the page wasn't hidden prior to the first paint.
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if (entry.startTime < visibilityWatcher.firstHiddenTime) {
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// The activationStart reference is used because FCP should be
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// relative to page activation rather than navigation start if the
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// page was prerendered. But in cases where `activationStart` occurs
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// after the FCP, this time should be clamped at 0.
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metric.value = Math.max(entry.startTime - getActivationStart(), 0);
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metric.entries.push(entry);
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report(true);
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}
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}
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});
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};
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const po = observe('paint', handleEntries);
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if (po) {
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report = bindReporter(onReport, metric, FCPThresholds, opts.reportAllChanges);
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// Only report after a bfcache restore if the `PerformanceObserver`
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// successfully registered or the `paint` entry exists.
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onBFCacheRestore((event) => {
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metric = initMetric('FCP');
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report = bindReporter(onReport, metric, FCPThresholds, opts.reportAllChanges);
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doubleRAF(() => {
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metric.value = performance.now() - event.timeStamp;
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report(true);
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});
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});
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}
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});
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};
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