web-vitals/src/getFCP.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 {getFirstHidden} from './lib/getFirstHidden.js';
import {initMetric} from './lib/initMetric.js';
import {observe} from './lib/observe.js';
import {onBFCacheRestore} from './lib/onBFCacheRestore.js';
import {ReportHandler} from './types.js';


export const getFCP = (onReport: ReportHandler, reportAllChanges?: boolean) => {
  const firstHidden = getFirstHidden();
  let metric = initMetric('FCP');
  let report: ReturnType<typeof bindReporter>;

  const entryHandler = (entry: PerformanceEntry) => {
    if (entry.name === 'first-contentful-paint') {
      if (po) {
        po.disconnect();
      }

      // Only report if the page wasn't hidden prior to the first paint.
      if (entry.startTime < firstHidden.timeStamp) {
        metric.value = entry.startTime;
        metric.entries.push(entry);
        finalMetrics.add(metric);
        report();
      }
    }
  };

  const po = observe('paint', entryHandler);
  if (po) {
    report = bindReporter(onReport, metric, reportAllChanges);

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