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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);
+ });
+ });
+ }
+ });
+};