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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 { onHidden } from './lib/onHidden.js';
+import { runOnce } from './lib/runOnce.js';
+import { whenActivated } from './lib/whenActivated.js';
+import { whenIdle } from './lib/whenIdle.js';
+/** Thresholds for LCP. See https://web.dev/articles/lcp#what_is_a_good_lcp_score */
+export const LCPThresholds = [2500, 4000];
+const reportedMetricIDs = {};
+/**
+ * Calculates the [LCP](https://web.dev/articles/lcp) value for the current page and
+ * calls the `callback` function once the value is ready (along with the
+ * relevant `largest-contentful-paint` performance entry used to determine the
+ * value). The reported value is a `DOMHighResTimeStamp`.
+ *
+ * If the `reportAllChanges` configuration option is set to `true`, the
+ * `callback` function will be called any time a new `largest-contentful-paint`
+ * performance entry is dispatched, or once the final value of the metric has
+ * been determined.
+ */
+export const onLCP = (onReport, opts) => {
+ // Set defaults
+ opts = opts || {};
+ whenActivated(() => {
+ const visibilityWatcher = getVisibilityWatcher();
+ let metric = initMetric('LCP');
+ let report;
+ const handleEntries = (entries) => {
+ // If reportAllChanges is set then call this function for each entry,
+ // otherwise only consider the last one.
+ if (!opts.reportAllChanges) {
+ entries = entries.slice(-1);
+ }
+ entries.forEach((entry) => {
+ // Only report if the page wasn't hidden prior to LCP.
+ if (entry.startTime < visibilityWatcher.firstHiddenTime) {
+ // The startTime attribute returns the value of the renderTime if it is
+ // not 0, and the value of the loadTime otherwise. The activationStart
+ // reference is used because LCP should be relative to page activation
+ // rather than navigation start if the page was prerendered. But in cases
+ // where `activationStart` occurs after the LCP, this time should be
+ // clamped at 0.
+ metric.value = Math.max(entry.startTime - getActivationStart(), 0);
+ metric.entries = [entry];
+ report();
+ }
+ });
+ };
+ const po = observe('largest-contentful-paint', handleEntries);
+ if (po) {
+ report = bindReporter(onReport, metric, LCPThresholds, opts.reportAllChanges);
+ const stopListening = runOnce(() => {
+ if (!reportedMetricIDs[metric.id]) {
+ handleEntries(po.takeRecords());
+ po.disconnect();
+ reportedMetricIDs[metric.id] = true;
+ report(true);
+ }
+ });
+ // Stop listening after input. Note: while scrolling is an input that
+ // stops LCP observation, it's unreliable since it can be programmatically
+ // generated. See: https://github.com/GoogleChrome/web-vitals/issues/75
+ ['keydown', 'click'].forEach((type) => {
+ // Wrap in a setTimeout so the callback is run in a separate task
+ // to avoid extending the keyboard/click handler to reduce INP impact
+ // https://github.com/GoogleChrome/web-vitals/issues/383
+ addEventListener(type, () => whenIdle(stopListening), {
+ once: true,
+ capture: true,
+ });
+ });
+ onHidden(stopListening);
+ // Only report after a bfcache restore if the `PerformanceObserver`
+ // successfully registered.
+ onBFCacheRestore((event) => {
+ metric = initMetric('LCP');
+ report = bindReporter(onReport, metric, LCPThresholds, opts.reportAllChanges);
+ doubleRAF(() => {
+ metric.value = performance.now() - event.timeStamp;
+ reportedMetricIDs[metric.id] = true;
+ report(true);
+ });
+ });
+ }
+ });
+};