1 files changed, 82 insertions, 0 deletions
diff --git a/browser/components/newtab/content-src/lib/detect-user-session-start.js b/browser/components/newtab/content-src/lib/detect-user-session-start.js
new file mode 100644
index 0000000000..43aa388967
--- /dev/null
+++ b/browser/components/newtab/content-src/lib/detect-user-session-start.js
@@ -0,0 +1,82 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import {
+  actionCreators as ac,
+  actionTypes as at,
+} from "common/Actions.sys.mjs";
+import { perfService as perfSvc } from "content-src/lib/perf-service";
+
+const VISIBLE = "visible";
+const VISIBILITY_CHANGE_EVENT = "visibilitychange";
+
+export class DetectUserSessionStart {
+  constructor(store, options = {}) {
+    this._store = store;
+    // Overrides for testing
+    this.document = options.document || global.document;
+    this._perfService = options.perfService || perfSvc;
+    this._onVisibilityChange = this._onVisibilityChange.bind(this);
+  }
+
+  /**
+   * sendEventOrAddListener - Notify immediately if the page is already visible,
+   *                    or else set up a listener for when visibility changes.
+   *                    This is needed for accurate session tracking for telemetry,
+   *                    because tabs are pre-loaded.
+   */
+  sendEventOrAddListener() {
+    if (this.document.visibilityState === VISIBLE) {
+      // If the document is already visible, to the user, send a notification
+      // immediately that a session has started.
+      this._sendEvent();
+    } else {
+      // If the document is not visible, listen for when it does become visible.
+      this.document.addEventListener(
+        VISIBILITY_CHANGE_EVENT,
+        this._onVisibilityChange
+      );
+    }
+  }
+
+  /**
+   * _sendEvent - Sends a message to the main process to indicate the current
+   *              tab is now visible to the user, includes the
+   *              visibility_event_rcvd_ts time in ms from the UNIX epoch.
+   */
+  _sendEvent() {
+    this._perfService.mark("visibility_event_rcvd_ts");
+
+    try {
+      let visibility_event_rcvd_ts =
+        this._perfService.getMostRecentAbsMarkStartByName(
+          "visibility_event_rcvd_ts"
+        );
+
+      this._store.dispatch(
+        ac.AlsoToMain({
+          type: at.SAVE_SESSION_PERF_DATA,
+          data: { visibility_event_rcvd_ts },
+        })
+      );
+    } catch (ex) {
+      // If this failed, it's likely because the `privacy.resistFingerprinting`
+      // pref is true.  We should at least not blow up.
+    }
+  }
+
+  /**
+   * _onVisibilityChange - If the visibility has changed to visible, sends a notification
+   *                      and removes the event listener. This should only be called once per tab.
+   */
+  _onVisibilityChange() {
+    if (this.document.visibilityState === VISIBLE) {
+      this._sendEvent();
+      this.document.removeEventListener(
+        VISIBILITY_CHANGE_EVENT,
+        this._onVisibilityChange
+      );
+    }
+  }
+}