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1/*
2 * Copyright 2020 Google LLC
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * https://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16import { onBFCacheRestore } from './lib/bfcache.js';
17import { bindReporter } from './lib/bindReporter.js';
18import { doubleRAF } from './lib/doubleRAF.js';
19import { getActivationStart } from './lib/getActivationStart.js';
20import { getVisibilityWatcher } from './lib/getVisibilityWatcher.js';
21import { initMetric } from './lib/initMetric.js';
22import { observe } from './lib/observe.js';
23import { whenActivated } from './lib/whenActivated.js';
24/** Thresholds for FCP. See https://web.dev/articles/fcp#what_is_a_good_fcp_score */
25export const FCPThresholds = [1800, 3000];
26/**
27 * Calculates the [FCP](https://web.dev/articles/fcp) value for the current page and
28 * calls the `callback` function once the value is ready, along with the
29 * relevant `paint` performance entry used to determine the value. The reported
30 * value is a `DOMHighResTimeStamp`.
31 */
32export const onFCP = (onReport, opts) => {
33 // Set defaults
34 opts = opts || {};
35 whenActivated(() => {
36 const visibilityWatcher = getVisibilityWatcher();
37 let metric = initMetric('FCP');
38 let report;
39 const handleEntries = (entries) => {
40 entries.forEach((entry) => {
41 if (entry.name === 'first-contentful-paint') {
42 po.disconnect();
43 // Only report if the page wasn't hidden prior to the first paint.
44 if (entry.startTime < visibilityWatcher.firstHiddenTime) {
45 // The activationStart reference is used because FCP should be
46 // relative to page activation rather than navigation start if the
47 // page was prerendered. But in cases where `activationStart` occurs
48 // after the FCP, this time should be clamped at 0.
49 metric.value = Math.max(entry.startTime - getActivationStart(), 0);
50 metric.entries.push(entry);
51 report(true);
52 }
53 }
54 });
55 };
56 const po = observe('paint', handleEntries);
57 if (po) {
58 report = bindReporter(onReport, metric, FCPThresholds, opts.reportAllChanges);
59 // Only report after a bfcache restore if the `PerformanceObserver`
60 // successfully registered or the `paint` entry exists.
61 onBFCacheRestore((event) => {
62 metric = initMetric('FCP');
63 report = bindReporter(onReport, metric, FCPThresholds, opts.reportAllChanges);
64 doubleRAF(() => {
65 metric.value = performance.now() - event.timeStamp;
66 report(true);
67 });
68 });
69 }
70 });
71};