Made the EndDrag logic clearer by using |drag_view_| as Drag state.

ui/app_list/views/apps_grid_view.cc

 // Copyright (c) 2012 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 "ui/app_list/views/apps_grid_view.h"
 
 #include <algorithm>
 
 #include "ui/app_list/app_list_item_model.h"
 #include "ui/app_list/apps_grid_view_delegate.h"
@@ skipping 174 matching lines @@
     return;
 
   Index index = GetIndexOfView(view);
   if (IsValidIndex(index))
     pagination_model_->SelectPage(index.page, false);
 }
 
 void AppsGridView::InitiateDrag(AppListItemView* view,
                                 Pointer pointer,
                                 const ui::LocatedEvent& event) {
-  if (drag_view_ || pulsing_blocks_model_.view_size())
+  if (drag_view_ || pulsing_blocks_model_.view_size() || !view)

[xiyuan, 2013/06/03 18:03:34]

nit: DCHECK(view) to catch the error as well since this should never happen and
should be fixed if it does happen.

[Mr4D (OOO till 08-26), 2013/06/03 18:35:16]

Done.

     return;
 
   drag_view_ = view;
 
   // When a drag and drop host is given, the item can be dragged out of the app
   // list window. In that case a proxy widget needs to be used.
   // Note: This code has very likely to be changed for Windows (non metro mode)
   // when a |drag_and_drop_host_| gets implemented.
   if (drag_and_drop_host_) {
     // Determine the mouse offset to the center of the icon so that the drag and
     // drop host follows this layer.
     gfx::Vector2d delta = event.root_location() -
                           drag_view_->GetBoundsInScreen().CenterPoint();
     delta.set_y(delta.y() + drag_view_->title()->size().height() / 2);
     // We have to hide the original item since the drag and drop host will do
     // the OS dependent code to "lift off the dragged item".
     drag_and_drop_host_->CreateDragIconProxy(event.root_location(),
                                              view->model()->icon(),
                                              drag_view_,
                                              delta,
                                              kDragAndDropProxyScale);
     HideView(drag_view_, true);
   }
   drag_start_ = event.location();
 }
 
 void AppsGridView::UpdateDrag(AppListItemView* view,
                               Pointer pointer,
                               const ui::LocatedEvent& event) {
-  if (!dragging() && drag_view_ &&
-      ExceededDragThreshold(event.location() - drag_start_)) {
+  // EndDrag was called before if |drag_view_| is NULL.
+  if (!drag_view_)
+    return;
+
+  if (!dragging() && ExceededDragThreshold(event.location() - drag_start_)) {
     drag_pointer_ = pointer;
     // Move the view to the front so that it appears on top of other views.
     ReorderChildView(drag_view_, -1);
     bounds_animator_.StopAnimatingView(drag_view_);
   }
   if (drag_pointer_ != pointer)
     return;
 
   ExtractDragLocation(event, &last_drag_point_);
   const Index last_drop_target = drop_target_;
@@ skipping 14 matching lines @@
     AnimateToIdealBounds();
 
   if (drag_and_drop_host_)
     drag_and_drop_host_->UpdateDragIconProxy(event.root_location());
 
   drag_view_->SetPosition(
       gfx::PointAtOffsetFromOrigin(last_drag_point_ - drag_start_));
 }
 
 void AppsGridView::EndDrag(bool cancel) {
+  // EndDrag was called before if |drag_view_| is NULL.
+  if (!drag_view_)
+    return;
+
   if (forward_events_to_drag_and_drop_host_) {
     forward_events_to_drag_and_drop_host_ = false;
     drag_and_drop_host_->EndDrag(cancel);
-  } else if (!cancel && dragging() && drag_view_) {
+  } else if (!cancel && dragging()) {
     CalculateDropTarget(last_drag_point_, true);
     if (IsValidIndex(drop_target_))
       MoveItemInModel(drag_view_, drop_target_);
   }
 
   if (drag_and_drop_host_) {
     // If we had a drag and drop proxy icon, we delete it and make the real
     // item visible again.
     drag_and_drop_host_->DestroyDragIconProxy();
     HideView(drag_view_, false);
   }
 
   drag_pointer_ = NONE;
   drop_target_ = Index();
-  if (drag_view_) {
-    drag_view_ = NULL;
-    AnimateToIdealBounds();
-  }
+  drag_view_ = NULL;
+  AnimateToIdealBounds();
 
   page_flip_timer_.Stop();
   page_flip_target_ = -1;
 }
 
 bool AppsGridView::IsDraggedView(const views::View* view) const {
   return drag_view_ == view;
 }
 
 void AppsGridView::SetDragAndDropHostOfCurrentAppList(
@@ skipping 629 matching lines @@
 
 void AppsGridView::HideView(views::View* view, bool hide) {
 #if defined(USE_AURA)
   ui::ScopedLayerAnimationSettings animator(view->layer()->GetAnimator());
   animator.SetPreemptionStrategy(ui::LayerAnimator::IMMEDIATELY_SET_NEW_TARGET);
   view->layer()->SetOpacity(hide ? 0 : 1);
 #endif
 }
 
 }  // namespace app_list