WIP - mask a view's layer by its view coordinates

ui/views/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.
 
 #define _USE_MATH_DEFINES // For VC++ to get M_PI. This has to be first.
 
 #include "ui/views/view.h"
 
 #include <algorithm>
 #include <cmath>
@@ skipping 803 matching lines @@
     // been set. Note that the X (or left) position we pass to ClipRect takes
     // into consideration whether or not the View uses a right-to-left layout so
     // that we paint the View in its mirrored position if need be.
     if (clip_path_.isEmpty()) {
       clip_recorder.ClipRect(GetMirroredBounds());
     } else {
       gfx::Path clip_path_in_parent = clip_path_;
       clip_path_in_parent.offset(GetMirroredX(), y());
       clip_recorder.ClipPathWithAntiAliasing(clip_path_in_parent);
     }
+  } else {
+    DCHECK(layer());
+    clip_recorder.ClipRect(GetVisibleBounds());

[sky, 2016/11/29 18:08:13]

Is this really enough? What happens if an animation occurs that effectively
moves the layer. Wouldn't the clip move with the layer, which isn't really waht
you want.

[Evan Stade, 2016/11/29 18:38:01]

when the layer is moved that affects the view's transform (as per GetTransform),
which is taken into account in GetVisibleBounds.

[sky, 2016/11/29 20:27:09]

That's assuming you end up here, right? If the layer is moved that is not
necessarily the case.

   }
 
   ui::TransformRecorder transform_recorder(context);
   if (paint_relative_to_parent) {
     // Translate the graphics such that 0,0 corresponds to where
     // this View is located relative to its parent.
     gfx::Transform transform_from_parent;
     gfx::Vector2d offset_from_parent = GetMirroredPosition().OffsetFromOrigin();
     transform_from_parent.Translate(offset_from_parent.x(),
                                     offset_from_parent.y());
@@ skipping 538 matching lines @@
 void View::OnBoundsChanged(const gfx::Rect& previous_bounds) {
 }
 
 void View::PreferredSizeChanged() {
   InvalidateLayout();
   if (parent_)
     parent_->ChildPreferredSizeChanged(this);
 }
 
 bool View::GetNeedsNotificationWhenVisibleBoundsChange() const {
-  return false;
+  return true;
 }
 
 void View::OnVisibleBoundsChanged() {
+  if (layer())
+    SchedulePaint();
 }
 
 // Tree operations -------------------------------------------------------------
 
 void View::ViewHierarchyChanged(const ViewHierarchyChangedDetails& details) {
 }
 
 void View::VisibilityChanged(View* starting_from, bool is_visible) {
 }
 
@@ skipping 1019 matching lines @@
   // Message the RootView to do the drag and drop. That way if we're removed
   // the RootView can detect it and avoid calling us back.
   gfx::Point widget_location(event.location());
   ConvertPointToWidget(this, &widget_location);
   widget->RunShellDrag(this, data, widget_location, drag_operations, source);
   // WARNING: we may have been deleted.
   return true;
 }
 
 }  // namespace views