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

ui/views/controls/button/md_text_button.cc

Index: ui/views/controls/button/md_text_button.cc
diff --git a/ui/views/controls/button/md_text_button.cc b/ui/views/controls/button/md_text_button.cc
index 9da1078f7692fe0cc322330f54828c6e15cb0066..217fad3cbe5a7ab474996555b0a840a3c6565a55 100644
--- a/ui/views/controls/button/md_text_button.cc
+++ b/ui/views/controls/button/md_text_button.cc
@@ -6,6 +6,8 @@
 
 #include "base/i18n/case_conversion.h"
 #include "ui/base/material_design/material_design_controller.h"
+#include "ui/compositor/paint_context.h"
+#include "ui/compositor/paint_recorder.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/color_palette.h"
 #include "ui/gfx/color_utils.h"
@@ -28,6 +30,29 @@ namespace {
 // Minimum size to reserve for the button contents.
 const int kMinWidth = 48;
 
+class VisibleMaskLayerDelegate : public ui::LayerDelegate {
+ public:
+  explicit VisibleMaskLayerDelegate(views::View* view, const ui::Layer* layer)
+      : view_(view), layer_(layer) {}
+  ~VisibleMaskLayerDelegate() override {}
+
+  // ui::LayerDelegate:
+  void OnPaintLayer(const ui::PaintContext& context) override {
+    ui::PaintRecorder recorder(context, layer_->size());
+    gfx::Rect bounds = view_->GetVisibleBounds();
+    recorder.canvas()->FillRect(bounds, SK_ColorBLACK);
+  }
+
+  void OnDelegatedFrameDamage(
+      const gfx::Rect& damage_rect_in_dip) override {}
+  void OnDeviceScaleFactorChanged(float device_scale_factor) override {}
+
+ private:
+views::View* view_;
+  const ui::Layer* const layer_;
+  DISALLOW_COPY_AND_ASSIGN(VisibleMaskLayerDelegate);
+};
+
 LabelButton* CreateButton(ButtonListener* listener,
                           const base::string16& text,
                           bool md) {
@@ -103,6 +128,22 @@ void MdTextButton::OnPaintBackground(gfx::Canvas* canvas) {
   }
 }
 
+bool MdTextButton::GetNeedsNotificationWhenVisibleBoundsChange() const {
+  return true;
+}
+
+void MdTextButton::OnVisibleBoundsChanged() {
+  gfx::Rect visible_bounds = GetVisibleBounds();
+  if (last_visible_bounds_ != visible_bounds)
+    SchedulePaint();
+  last_visible_bounds_ = visible_bounds;
+}
+
+void MdTextButton::OnLayerBoundsChanged(const gfx::Rect& old_bounds) {
+  LabelButton::OnLayerBoundsChanged(old_bounds);
+ // mask_layer_.SetBounds(layer()->bounds());
+}
+
 void MdTextButton::OnFocus() {
   LabelButton::OnFocus();
   FocusRing::Install(this);
@@ -206,6 +247,12 @@ MdTextButton::MdTextButton(ButtonListener* listener)
   // for fractional DSF).
   SetPaintToLayer(true);
   layer()->SetFillsBoundsOpaquely(false);
+/*
+  mask_layer_.SetFillsBoundsOpaquely(false);
+  mask_layer_delegate_.reset(new VisibleMaskLayerDelegate(this, &mask_layer_));
+  mask_layer_.set_delegate(mask_layer_delegate_.get());
+  layer()->SetMaskLayer(&mask_layer_);
+*/
 }
 
 void MdTextButton::UpdatePadding() {