overview: Remove some observers from overview session.

We only have once window observer function OnWindowDestroying, and it
only removes the observer unless it is the restore focus window. So
there is no point observing the other windows or storing their pointers.

Bug: none
Test: none
Change-Id: Ib66cb0c34c316b1d7ed8c30f3d110e8b32d7b2b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533857Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826847}

ash/wm/overview/overview_session.cc

@@ -34,7 +34,6 @@
 #include "ash/wm/overview/scoped_overview_animation_settings.h"
 #include "ash/wm/splitview/split_view_controller.h"
 #include "ash/wm/splitview/split_view_utils.h"
-#include "ash/wm/switchable_windows.h"
 #include "ash/wm/window_state.h"
 #include "ash/wm/window_util.h"
 #include "base/auto_reset.h"
@@ -220,7 +219,6 @@ OverviewSession::OverviewSession(OverviewDelegate* delegate)
 }
 
 OverviewSession::~OverviewSession() {
-  DCHECK(observed_windows_.empty());
   // Don't delete |window_drag_controller_| yet since the stack might be still
   // using it.
   if (window_drag_controller_) {
@@ -254,14 +252,6 @@ void OverviewSession::Init(const WindowList& windows,
             });
 
   for (auto* root : root_windows) {
-    // Observed switchable containers for newly created windows on all root
-    // windows.
-    for (auto* container :
-         GetSwitchableContainersForRoot(root, /*active_desk_only=*/true)) {
-      container->AddObserver(this);
-      observed_windows_.insert(container);
-    }
-
     auto grid = std::make_unique<OverviewGrid>(root, windows, this);
     num_items_ += grid->size();
     grid_list_.push_back(std::move(grid));
@@ -586,11 +576,9 @@ void OverviewSession::RemoveItem(OverviewItem* overview_item) {
 void OverviewSession::RemoveItem(OverviewItem* overview_item,
                                  bool item_destroying,
                                  bool reposition) {
-  if (overview_item->GetWindow()->HasObserver(this)) {
-    overview_item->GetWindow()->RemoveObserver(this);
-    observed_windows_.erase(overview_item->GetWindow());
-    if (overview_item->GetWindow() == restore_focus_window_)
-      restore_focus_window_ = nullptr;
+  if (overview_item->GetWindow() == restore_focus_window_) {
+    restore_focus_window_->RemoveObserver(this);
+    restore_focus_window_ = nullptr;
   }
 
   overview_item->overview_grid()->RemoveItem(overview_item, item_destroying,
@@ -916,12 +904,8 @@ void OverviewSession::ResetFocusRestoreWindow(bool focus) {
     base::AutoReset<bool> restoring_focus(&ignore_activations_, true);
     wm::ActivateWindow(restore_focus_window_);
   }
-  // If the window is in the observed_windows_ list it needs to continue to be
-  // observed.
-  if (observed_windows_.find(restore_focus_window_) ==
-      observed_windows_.end()) {
-    restore_focus_window_->RemoveObserver(this);
-  }
+
+  restore_focus_window_->RemoveObserver(this);
   restore_focus_window_ = nullptr;
 }
 
@@ -1014,10 +998,9 @@ void OverviewSession::OnDisplayMetricsChanged(const display::Display& display,
 }
 
 void OverviewSession::OnWindowDestroying(aura::Window* window) {
-  window->RemoveObserver(this);
-  observed_windows_.erase(window);
-  if (window == restore_focus_window_)
-    restore_focus_window_ = nullptr;
+  DCHECK_EQ(restore_focus_window_, window);
+  restore_focus_window_->RemoveObserver(this);
+  restore_focus_window_ = nullptr;
 }
 
 void OverviewSession::OnKeyEvent(ui::KeyEvent* event) {
@@ -1212,13 +1195,10 @@ bool OverviewSession::ProcessForScrolling(const ui::KeyEvent& event) {
 }
 
 void OverviewSession::RemoveAllObservers() {
-  for (auto* window : observed_windows_)
-    window->RemoveObserver(this);
-  observed_windows_.clear();
-
   display::Screen::GetScreen()->RemoveObserver(this);
   if (restore_focus_window_)
     restore_focus_window_->RemoveObserver(this);
+  restore_focus_window_ = nullptr;
 }
 
 void OverviewSession::UpdateNoWindowsWidget() {

ash/wm/overview/overview_session.h

@@ -337,9 +337,6 @@ class ASH_EXPORT OverviewSession : public display::DisplayObserver,
 
   void OnItemAdded(aura::Window* window);
 
-  // Tracks observed windows.
-  base::flat_set<aura::Window*> observed_windows_;
-
   // Weak pointer to the overview delegate which will be called when a selection
   // is made.
   OverviewDelegate* delegate_;