Adds ability to suspend notifying observers when display list is changed

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index 43124a9d76f342be6111678c7dd0b8bd3b9e097c..cdcbeb5215db1d6451419c678f47a51145c13c73 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -137,15 +137,20 @@ void WindowManager::RemoveObserver(WindowManagerObserver* observer) {
 RootWindowController* WindowManager::CreateRootWindowController(
     ui::Window* window,
     const display::Display& display) {
-  // TODO(sky): should be passed whether display is primary.
-
-  // There needs to be at least one display before creating
-  // RootWindowController, otherwise initializing the compositor fails.
+  // CreateRootWindowController() means a new display is being added, so the
+  // DisplayList needs to be updated. Calling AddDisplay() results in
+  // notifying DisplayObservers. Ash code assumes when this happens their is

[James Cook, 2016/09/29 04:15:05]

nit: their -> there

[sky, 2016/09/29 18:04:44]

Done.

+  // a valid RootWindowController for the new display. Suspend notifying
+  // observers, add the Display, create the RootWindowController, and then
+  // notify DisplayObservers. Classic ash does this by making sure
+  // WindowTreeHostManager is added as a DisplayObserver early on.
+  std::unique_ptr<display::DisplayListObserverLock> display_lock =
+      screen_->display_list()->SuspendObserverUpdates();
   const bool was_displays_empty = screen_->display_list()->displays().empty();

[James Cook, 2016/09/29 04:15:05]

optional: |is_first_display| ?

[sky, 2016/09/29 18:04:44]

Done.

-  if (was_displays_empty) {
-    screen_->display_list()->AddDisplay(display,
-                                        display::DisplayList::Type::PRIMARY);
-  }
+  // TODO(sky): should be passed whether display is primary.

[James Cook, 2016/09/29 04:15:05]

Is this todo still relevant?

[sky, 2016/09/29 18:04:44]

I *think* so, but I'm not 100% positive. I'm not sure if a new display can be
added and made primary in one distinct step.

+  screen_->display_list()->AddDisplay(
+      display, was_displays_empty ? display::DisplayList::Type::PRIMARY
+                                  : display::DisplayList::Type::NOT_PRIMARY);
 
   std::unique_ptr<RootWindowController> root_window_controller_ptr(
       new RootWindowController(this, window, display));
@@ -156,15 +161,6 @@ RootWindowController* WindowManager::CreateRootWindowController(
   FOR_EACH_OBSERVER(WindowManagerObserver, observers_,
                     OnRootWindowControllerAdded(root_window_controller));
 
-  if (!was_displays_empty) {
-    // If this isn't the initial display then add the display to Screen after
-    // creating the RootWindowController. We need to do this after creating the
-    // RootWindowController as adding the display triggers OnDisplayAdded(),
-    // which triggers some overrides asking for the RootWindowController for the
-    // new display.
-    screen_->display_list()->AddDisplay(
-        display, display::DisplayList::Type::NOT_PRIMARY);
-  }
   return root_window_controller;
 }