Separate out the activity watcher from presence code so I can hook read receipts into it without tangling it into the presence code.

src/UserActivity.js

@@ -0,0 +1,57 @@
+/*
+Copyright 2015 OpenMarket Ltd
+
+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
+
+    http://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.
+*/
+
+var dis = require("./dispatcher");
+
+var MIN_DISPATCH_INTERVAL = 1 * 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)
+ */
+class UserActivity {
+
+    /**
+     * Start listening to user activity
+     */
+    start() {
+        document.onmousemove = this._onUserActivity.bind(this);
+        document.onkeypress = this._onUserActivity.bind(this);
+        this.lastActivityAt = (new Date).getTime();
+        this.lastDispatchAt = 0;
+    }
+
+    /**
+     * Stop tracking user activity
+     */
+    stop() {
+        document.onmousemove = undefined;
+        document.onkeypress = undefined;
+    }
+
+    _onUserActivity() {
+        this.lastActivityAt = (new Date).getTime();
+        if (this.lastDispatchAt < this.lastActivityAt - MIN_DISPATCH_INTERVAL) {
+            this.lastDispatchAt = this.lastActivityAt;
+            dis.dispatch({
+                action: 'user_activity'
+            });
+        }
+    }
+}
+
+module.exports = new UserActivity();