Timer in UserActivity, change semantics to "user probably looks at app"

Before, UserActivitity emitting actions meant that the user had very recently interaction with their hardware.
Now it means they are likely looking at the app.

You can attach a timer that is aborted when we think the user
stops looking at the page
(or hasn't touched their hardware for 2 minutes).

This works better than the previous approach for larger timeouts,
like the 30s we're about to implement for out-of-view RMs

src/UserActivity.js

@@ -15,32 +15,72 @@ limitations under the License.
 */
 
 import dis from './dispatcher';
+import Timer from './utils/Timer';
 
-const MIN_DISPATCH_INTERVAL_MS = 500;
-const CURRENTLY_ACTIVE_THRESHOLD_MS = 2000;
+// important this is larger than the timeouts of timers
+// used with UserActivity.timeWhileActive,
+// such as READ_MARKER_INVIEW_THRESHOLD_MS,
+// READ_MARKER_OUTOFVIEW_THRESHOLD_MS,
+// READ_RECEIPT_INTERVAL_MS in TimelinePanel
+const CURRENTLY_ACTIVE_THRESHOLD_MS = 2 * 60 * 1000;
 
 /**
  * This class watches for user activity (moving the mouse or pressing a key)
- * and dispatches the user_activity action at times when the user is interacting
- * with the app (but at a much lower frequency than mouse move events)
+ * and starts/stops attached timers while the user is active.
  */
 class UserActivity {
+    constructor() {
+        this._attachedTimers = [];
+        this._activityTimeout = new Timer(CURRENTLY_ACTIVE_THRESHOLD_MS);
+        this._onUserActivity = this._onUserActivity.bind(this);
+        this._onDocumentBlurred = this._onDocumentBlurred.bind(this);
+        this._onPageVisibilityChanged = this._onPageVisibilityChanged.bind(this);
+        this.lastScreenX = 0;
+        this.lastScreenY = 0;
+    }
+
+    /**
+     * Runs the given timer while the user is active, aborting when the user becomes inactive.
+     * Can be called multiple times with the same already running timer, which is a NO-OP.
+     * Can be called before the user becomes active, in which case it is only started
+     * later on when the user does become active.
+     */
+    timeWhileActive(timer) {
+        // important this happens first
+        const index = this._attachedTimers.indexOf(timer);
+        if (index === -1) {
+            this._attachedTimers.push(timer);
+            // remove when done or aborted
+            timer.finished().finally(() => {
+                const index = this._attachedTimers.indexOf(timer);
+                if (index !== -1) { // should never be -1
+                    this._attachedTimers.splice(index, 1);
+                }
+            // as we fork the promise here,
+            // avoid unhandled rejection warnings
+            }).catch((err) => {});
+        }
+        if (this.userCurrentlyActive()) {
+            timer.start();
+        }
+    }
+
     /**
      * Start listening to user activity
      */
     start() {
-        document.onmousedown = this._onUserActivity.bind(this);
-        document.onmousemove = this._onUserActivity.bind(this);
-        document.onkeydown = this._onUserActivity.bind(this);
+        document.onmousedown = this._onUserActivity;
+        document.onmousemove = this._onUserActivity;
+        document.onkeydown = this._onUserActivity;
+        document.addEventListener("visibilitychange", this._onPageVisibilityChanged);
+        document.addEventListener("blur", this._onDocumentBlurred);
+        document.addEventListener("focus", this._onUserActivity);
         // can't use document.scroll here because that's only the document
         // itself being scrolled. Need to use addEventListener's useCapture.
         // also this needs to be the wheel event, not scroll, as scroll is
         // fired when the view scrolls down for a new message.
-        window.addEventListener('wheel', this._onUserActivity.bind(this),
+        window.addEventListener('wheel', this._onUserActivity,
                                 { passive: true, capture: true });
-        this.lastActivityAtTs = new Date().getTime();
-        this.lastDispatchAtTs = 0;
-        this.activityEndTimer = undefined;
     }
 
     /**
@@ -50,8 +90,12 @@ class UserActivity {
         document.onmousedown = undefined;
         document.onmousemove = undefined;
         document.onkeydown = undefined;
-        window.removeEventListener('wheel', this._onUserActivity.bind(this),
+        window.removeEventListener('wheel', this._onUserActivity,
                                    { passive: true, capture: true });
+
+        document.removeEventListener("visibilitychange", this._onPageVisibilityChanged);
+        document.removeEventListener("blur", this._onDocumentBlurred);
+        document.removeEventListener("focus", this._onUserActivity);
     }
 
     /**
@@ -60,10 +104,22 @@ class UserActivity {
      * @returns {boolean} true if user is currently/very recently active
      */
     userCurrentlyActive() {
-        return this.lastActivityAtTs > new Date().getTime() - CURRENTLY_ACTIVE_THRESHOLD_MS;
+        return this._activityTimeout.isRunning();
     }
 
-    _onUserActivity(event) {
+    _onPageVisibilityChanged(e) {
+        if (document.visibilityState === "hidden") {
+            this._activityTimeout.abort();
+        } else {
+            this._onUserActivity(e);
+        }
+    }
+
+    _onDocumentBlurred() {
+        this._activityTimeout.abort();
+    }
+
+    async _onUserActivity(event) {
         if (event.screenX && event.type === "mousemove") {
             if (event.screenX === this.lastScreenX && event.screenY === this.lastScreenY) {
                 // mouse hasn't actually moved
@@ -73,30 +129,19 @@ class UserActivity {
             this.lastScreenY = event.screenY;
         }
 
-        this.lastActivityAtTs = new Date().getTime();
-        if (this.lastDispatchAtTs < this.lastActivityAtTs - MIN_DISPATCH_INTERVAL_MS) {
-            this.lastDispatchAtTs = this.lastActivityAtTs;
-            dis.dispatch({
-                action: 'user_activity',
-            });
-            if (!this.activityEndTimer) {
-                this.activityEndTimer = setTimeout(this._onActivityEndTimer.bind(this), MIN_DISPATCH_INTERVAL_MS);
+        if (!this._activityTimeout.isRunning()) {
+            this._activityTimeout.start();
+            dis.dispatch({action: 'user_activity_start'});
+            this._attachedTimers.forEach((t) => t.start());
+            try {
+                await this._activityTimeout.finished();
+            } catch (_e) { /* aborted */ }
+            this._attachedTimers.forEach((t) => t.abort());
+        } else {
+            this._activityTimeout.restart();
         }
     }
 }
 
-    _onActivityEndTimer() {
-        const now = new Date().getTime();
-        const targetTime = this.lastActivityAtTs + MIN_DISPATCH_INTERVAL_MS;
-        if (now >= targetTime) {
-            dis.dispatch({
-                action: 'user_activity_end',
-            });
-            this.activityEndTimer = undefined;
-        } else {
-            this.activityEndTimer = setTimeout(this._onActivityEndTimer.bind(this), targetTime - now);
-        }
-    }
-}
 
 module.exports = new UserActivity();