Initial Commit

node_modules/web-vitals/dist/modules/lib/getVisibilityWatcher.js

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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 './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;
+        },
+    };
+};