web-vitals/src/getLCP.ts

/*
 * 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 {bindReporter} from './lib/bindReporter.js';
import {finalMetrics} from './lib/finalMetrics.js';
import {getVisibilityWatcher} from './lib/getVisibilityWatcher.js';
import {initMetric} from './lib/initMetric.js';
import {observe, PerformanceEntryHandler} from './lib/observe.js';
import {onBFCacheRestore} from './lib/onBFCacheRestore.js';
import {onHidden} from './lib/onHidden.js';
import {ReportHandler} from './types.js';


export const getLCP = (onReport: ReportHandler, reportAllChanges?: boolean) => {
  const visibilityWatcher = getVisibilityWatcher();
  let metric = initMetric('LCP');
  let report: ReturnType<typeof bindReporter>;

  const entryHandler = (entry: PerformanceEntry) => {
    // The startTime attribute returns the value of the renderTime if it is not 0,
    // and the value of the loadTime otherwise.
    const value = entry.startTime;

    // If the page was hidden prior to paint time of the entry,
    // ignore it and mark the metric as final, otherwise add the entry.
    if (value < visibilityWatcher.firstHiddenTime) {
      metric.value = value;
      metric.entries.push(entry);
    }

    report();
  };

  const po = observe('largest-contentful-paint', entryHandler);

  if (po) {
    report = bindReporter(onReport, metric, reportAllChanges);

    const stopListening = () => {
      if (!finalMetrics.has(metric)) {
        po.takeRecords().map(entryHandler as PerformanceEntryHandler);
        po.disconnect();
        finalMetrics.add(metric);
        report();
      }
    }

    // Stop listening after input. Note: while scrolling is an input that
    // stop LCP observation, it's unreliable since it can be programmatically
    // generated. See: https://github.com/GoogleChrome/web-vitals/issues/75
    ['keydown', 'click'].forEach((type) => {
      addEventListener(type, stopListening, {once: true, capture: true});
    });

    onHidden(stopListening, true);

    onBFCacheRestore((event) => {
      metric = initMetric('LCP');
      report = bindReporter(onReport, metric, reportAllChanges);
      requestAnimationFrame(() => {
        requestAnimationFrame(() => {
          metric.value = performance.now() - event.timeStamp;
          finalMetrics.add(metric);
          report();
        });
      });
    });
  }
};