/*
 * 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 './bfcache.js';
let firstHiddenTime = -1;
const initHiddenTime = () => {
    // If the document is hidden when this code runs, assume it was always
    // hidden and the page was loaded in the background, with the one exception
    // that visibility state is always 'hidden' during prerendering, so we have
    // to ignore that case until prerendering finishes (see: `prerenderingchange`
    // event logic below).
    return document.visibilityState === 'hidden' && !document.prerendering
        ? 0
        : Infinity;
};
const onVisibilityUpdate = (event) => {
    // If the document is 'hidden' and no previous hidden timestamp has been
    // set, update it based on the current event data.
    if (document.visibilityState === 'hidden' && firstHiddenTime > -1) {
        // If the event is a 'visibilitychange' event, it means the page was
        // visible prior to this change, so the event timestamp is the first
        // hidden time.
        // However, if the event is not a 'visibilitychange' event, then it must
        // be a 'prerenderingchange' event, and the fact that the document is
        // still 'hidden' from the above check means the tab was activated
        // in a background state and so has always been hidden.
        firstHiddenTime = event.type === 'visibilitychange' ? event.timeStamp : 0;
        // Remove all listeners now that a `firstHiddenTime` value has been set.
        removeChangeListeners();
    }
};
const addChangeListeners = () => {
    addEventListener('visibilitychange', onVisibilityUpdate, true);
    // IMPORTANT: when a page is prerendering, its `visibilityState` is
    // 'hidden', so in order to account for cases where this module checks for
    // visibility during prerendering, an additional check after prerendering
    // completes is also required.
    addEventListener('prerenderingchange', onVisibilityUpdate, true);
};
const removeChangeListeners = () => {
    removeEventListener('visibilitychange', onVisibilityUpdate, true);
    removeEventListener('prerenderingchange', onVisibilityUpdate, 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.
        firstHiddenTime = initHiddenTime();
        addChangeListeners();
        // 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();
                addChangeListeners();
            }, 0);
        });
    }
    return {
        get firstHiddenTime() {
            return firstHiddenTime;
        },
    };
};
