5 files changed, 55 insertions, 0 deletions
diff --git a/components/ui/zoom/BUILD.gn b/components/ui/zoom/BUILD.gn
index a3958b8..8774b0d 100644
--- a/components/ui/zoom/BUILD.gn
+++ b/components/ui/zoom/BUILD.gn
@@ -12,6 +12,7 @@ static_library("ui_zoom") {
     "zoom_controller.h",
     "zoom_event_manager.cc",
     "zoom_event_manager.h",
+    "zoom_event_manager_observer.h",
     "zoom_observer.h",
   ]
 
diff --git a/components/ui/zoom/zoom_event_manager.cc b/components/ui/zoom/zoom_event_manager.cc
index 7290627..f841123 100644
--- a/components/ui/zoom/zoom_event_manager.cc
+++ b/components/ui/zoom/zoom_event_manager.cc
@@ -4,6 +4,7 @@
 
 #include "components/ui/zoom/zoom_event_manager.h"
 
+#include "components/ui/zoom/zoom_event_manager_observer.h"
 #include "content/public/browser/browser_context.h"
 
 namespace {
@@ -37,4 +38,19 @@ ZoomEventManager::AddZoomLevelChangedCallback(
   return zoom_level_changed_callbacks_.Add(callback);
 }
 
+void ZoomEventManager::OnDefaultZoomLevelChanged() {
+  FOR_EACH_OBSERVER(ZoomEventManagerObserver, observers_,
+                    OnDefaultZoomLevelChanged());
+}
+
+void ZoomEventManager::AddZoomEventManagerObserver(
+    ZoomEventManagerObserver* observer) {
+  observers_.AddObserver(observer);
+}
+
+void ZoomEventManager::RemoveZoomEventManagerObserver(
+    ZoomEventManagerObserver* observer) {
+  observers_.RemoveObserver(observer);
+}
+
 }  // namespace ui_zoom
diff --git a/components/ui/zoom/zoom_event_manager.h b/components/ui/zoom/zoom_event_manager.h
index 3f0fcf0..72d335d 100644
--- a/components/ui/zoom/zoom_event_manager.h
+++ b/components/ui/zoom/zoom_event_manager.h
@@ -7,6 +7,7 @@
 
 #include "base/callback_list.h"
 #include "base/memory/weak_ptr.h"
+#include "base/observer_list.h"
 #include "base/supports_user_data.h"
 #include "content/public/browser/host_zoom_map.h"
 
@@ -16,6 +17,8 @@ class BrowserContext;
 
 namespace ui_zoom {
 
+class ZoomEventManagerObserver;
+
 // This class serves as a target for event notifications from all ZoomController
 // objects. Classes that need to know about browser-specific zoom events (e.g.
 // manual-mode zoom) should subscribe here.
@@ -34,9 +37,19 @@ class ZoomEventManager : public base::SupportsUserData::Data {
   void OnZoomLevelChanged(const content::HostZoomMap::ZoomLevelChange& change);
 
   // Add and remove zoom level changed callbacks.
+  // TODO(wjmaclean): Convert this callback mechanism to use
+  // ZoomEventManagerObserver instead.
   scoped_ptr<content::HostZoomMap::Subscription> AddZoomLevelChangedCallback(
       const content::HostZoomMap::ZoomLevelChangedCallback& callback);
 
+  // Called by ZoomLevelDelegates when changes are made to the default zoom
+  // level for their associated HostZoomMap.
+  void OnDefaultZoomLevelChanged();
+
+  // Add and remove observers.
+  void AddZoomEventManagerObserver(ZoomEventManagerObserver* observer);
+  void RemoveZoomEventManagerObserver(ZoomEventManagerObserver* observer);
+
   // Get a weak ptr to be used by clients who may themselves be UserData for
   // the context, since the order of destruction is undefined between the client
   // and this class.
@@ -47,6 +60,7 @@ class ZoomEventManager : public base::SupportsUserData::Data {
  private:
   base::CallbackList<void(const content::HostZoomMap::ZoomLevelChange&)>
       zoom_level_changed_callbacks_;
+  ObserverList<ZoomEventManagerObserver> observers_;
   base::WeakPtrFactory<ZoomEventManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ZoomEventManager);
diff --git a/components/ui/zoom/zoom_event_manager_observer.h b/components/ui/zoom/zoom_event_manager_observer.h
new file mode 100644
index 0000000..f3afedc
--- /dev/null
+++ b/components/ui/zoom/zoom_event_manager_observer.h
@@ -0,0 +1,23 @@
+// Copyright 2015 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.
+
+#ifndef COMPONENTS_UI_ZOOM_ZOOM_EVENT_MANAGER_OBSERVER_H_
+#define COMPONENTS_UI_ZOOM_ZOOM_EVENT_MANAGER_OBSERVER_H_
+
+namespace ui_zoom {
+
+class ZoomEventManagerObserver {
+ public:
+  // TODO(wjmaclean): convert existing ZoomLevelChangedCallbacks to be
+  // observers.
+  virtual void OnZoomLevelChanged() {}
+  virtual void OnDefaultZoomLevelChanged() {}
+
+ protected:
+  virtual ~ZoomEventManagerObserver() {}
+};
+
+}  // namespace ui_zoom
+
+#endif  // COMPONENTS_UI_ZOOM_ZOOM_EVENT_MANAGER_OBSERVER_H_
diff --git a/components/ui_zoom.gypi b/components/ui_zoom.gypi
index fc3ed76..15e988d 100644
--- a/components/ui_zoom.gypi
+++ b/components/ui_zoom.gypi
@@ -27,6 +27,7 @@
         'ui/zoom/zoom_controller.h',
         'ui/zoom/zoom_event_manager.cc',
         'ui/zoom/zoom_event_manager.h',
+        'ui/zoom/zoom_event_manager_observer.h',
         'ui/zoom/zoom_observer.h'
       ],
     }