/*
 * 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 './onBFCacheRestore.js';
import { onHidden } from './onHidden.js';
let firstHiddenTime = -1;
const initHiddenTime = () => {
    return document.visibilityState === 'hidden' ? 0 : Infinity;
};
const trackChanges = () => {
    // Update the time if/when the document becomes hidden.
    onHidden(({ timeStamp }) => {
        firstHiddenTime = timeStamp;
    }, true);
};
export const getVisibilityWatcher = () => {
    if (firstHiddenTime < 0) {
        // If the document is hidden when this code runs, assume it was hidden
        // since navigation start. This isn't a perfect heuristic, but it's the
        // best we can do until an API is available to support querying past
        // visibilityState.
        if (window.__WEB_VITALS_POLYFILL__) {
            firstHiddenTime = window.webVitals.firstHiddenTime;
            if (firstHiddenTime === Infinity) {
                trackChanges();
            }
        }
        else {
            firstHiddenTime = initHiddenTime();
            trackChanges();
        }
        // Reset the time on bfcache restores.
        onBFCacheRestore(() => {
            // Schedule a task in order to track the `visibilityState` once it's
            // had an opportunity to change to visible in all browsers.
            // https://bugs.chromium.org/p/chromium/issues/detail?id=1133363
            setTimeout(() => {
                firstHiddenTime = initHiddenTime();
                trackChanges();
            }, 0);
        });
    }
    return {
        get firstHiddenTime() {
            return firstHiddenTime;
        }
    };
};