More user id tracking for mus:

components/mus/ws/user_id_tracker.cc

Index: components/mus/ws/user_id_tracker.cc
diff --git a/components/mus/ws/user_id_tracker.cc b/components/mus/ws/user_id_tracker.cc
new file mode 100644
index 0000000000000000000000000000000000000000..912de1e9476a47e2fb05686136d0e1255b486a04
--- /dev/null
+++ b/components/mus/ws/user_id_tracker.cc
@@ -0,0 +1,53 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/mus/ws/user_id_tracker.h"
+
+#include "components/mus/ws/user_id_tracker_observer.h"
+#include "mojo/shell/public/interfaces/shell.mojom.h"
+
+namespace mus {
+namespace ws {
+
+UserIdTracker::UserIdTracker()
+    : active_id_(mojo::shell::mojom::Connector::kInvalidApplicationID) {}
+UserIdTracker::~UserIdTracker() {
+  DCHECK(ids_.empty());
+}
+
+bool UserIdTracker::IsValidUserId(UserId id) const {
+  return ids_.count(id) > 0;
+}
+
+void UserIdTracker::SetActiveUserId(UserId id) {
+  if (id == active_id_)
+    return;
+
+  active_id_ = id;
+  FOR_EACH_OBSERVER(UserIdTrackerObserver, observers_,
+                    OnActiveUserIdChanged(id));
+}
+
+void UserIdTracker::AddUserId(UserId id) {
+  DCHECK(!IsValidUserId(id));
+  ids_.insert(id);
+  FOR_EACH_OBSERVER(UserIdTrackerObserver, observers_, OnUserIdAdded(id));
+}
+
+void UserIdTracker::RemoveUserId(UserId id) {
+  DCHECK(IsValidUserId(id));
+  ids_.erase(id);
+  FOR_EACH_OBSERVER(UserIdTrackerObserver, observers_, OnUserIdRemoved(id));
+}
+
+void UserIdTracker::AddObserver(UserIdTrackerObserver* observer) {
+  observers_.AddObserver(observer);
+}
+
+void UserIdTracker::RemoveObserver(UserIdTrackerObserver* observer) {
+  observers_.RemoveObserver(observer);
+}
+
+}  // namespace ws
+}  // namespace mus