...

34 files changed, 203 insertions, 181 deletions

diff --git a/include/cru/ui/Base.h b/include/cru/ui/Base.h
index 44f76907..182c00f2 100644
--- a/include/cru/ui/Base.h
+++ b/include/cru/ui/Base.h
@@ -128,23 +128,6 @@ class CanvasPaintEventArgs {
   Size paint_size_;
 };
 
-enum class FlexDirection {
-  Horizontal,
-  HorizontalReverse,
-  Vertical,
-  VertivalReverse
-};
-
-using FlexMainAlignment = Alignment;
-using FlexCrossAlignment = Alignment;
-
-struct FlexChildLayoutData {
-  float expand_factor = 0;
-  float shrink_factor = 1;
-  // nullopt stands for looking at parent's setting
-  std::optional<FlexCrossAlignment> cross_alignment = std::nullopt;
-};
-
 struct StackChildLayoutData {
   std::optional<Alignment> horizontal;
   std::optional<Alignment> vertical;
diff --git a/include/cru/ui/controls/Button.h b/include/cru/ui/controls/Button.h
index ab1ef8eb..00253793 100644
--- a/include/cru/ui/controls/Button.h
+++ b/include/cru/ui/controls/Button.h
@@ -5,12 +5,13 @@
 #include "IBorderControl.h"
 #include "IClickableControl.h"
 #include "cru/common/Event.h"
+#include "cru/ui/render/BorderRenderObject.h"
 #include "cru/ui/style/ApplyBorderStyleInfo.h"
 
 namespace cru::ui::controls {
 class CRU_UI_API Button : public ContentControl,
-               public virtual IClickableControl,
-               public virtual IBorderControl {
+                          public virtual IClickableControl,
+                          public virtual IBorderControl {
  public:
   static constexpr StringView control_type = u"Button";
 
diff --git a/include/cru/ui/controls/Container.h b/include/cru/ui/controls/Container.h
index f250807d..926e395e 100644
--- a/include/cru/ui/controls/Container.h
+++ b/include/cru/ui/controls/Container.h
@@ -1,6 +1,8 @@
 #pragma once
 #include "ContentControl.h"
 
+#include "cru/ui/render/BorderRenderObject.h"
+
 namespace cru::ui::controls {
 class CRU_UI_API Container : public ContentControl {
   static constexpr StringView control_type = u"Container";
diff --git a/include/cru/ui/controls/ContentControl.h b/include/cru/ui/controls/ContentControl.h
index 86e90dae..5600ec25 100644
--- a/include/cru/ui/controls/ContentControl.h
+++ b/include/cru/ui/controls/ContentControl.h
@@ -1,7 +1,6 @@
 #pragma once
 #include "Control.h"
 
-#include "cru/ui/render/RenderObject.h"
 
 namespace cru::ui::controls {
 class CRU_UI_API ContentControl : public Control {
diff --git a/include/cru/ui/controls/Control.h b/include/cru/ui/controls/Control.h
index edbf5919..3927e604 100644
--- a/include/cru/ui/controls/Control.h
+++ b/include/cru/ui/controls/Control.h
@@ -2,8 +2,8 @@
 #include "Base.h"
 
 #include "../events/UiEvents.h"
-#include "../render/Base.h"
 #include "cru/common/Event.h"
+#include "cru/ui/render/RenderObject.h"
 
 namespace cru::ui::controls {
 class CRU_UI_API Control : public Object {
diff --git a/include/cru/ui/controls/FlexLayout.h b/include/cru/ui/controls/FlexLayout.h
index 1255b119..8fdb3aef 100644
--- a/include/cru/ui/controls/FlexLayout.h
+++ b/include/cru/ui/controls/FlexLayout.h
@@ -1,7 +1,15 @@
 #pragma once
 #include "LayoutControl.h"
 
+#include "cru/ui/render/FlexLayoutRenderObject.h"
+
 namespace cru::ui::controls {
+
+using render::FlexChildLayoutData;
+using render::FlexCrossAlignment;
+using render::FlexDirection;
+using render::FlexMainAlignment;
+
 class CRU_UI_API FlexLayout : public LayoutControl {
  public:
   static constexpr StringView control_type = u"FlexLayout";
diff --git a/include/cru/ui/controls/RootControl.h b/include/cru/ui/controls/RootControl.h
index c7ed916a..41424286 100644
--- a/include/cru/ui/controls/RootControl.h
+++ b/include/cru/ui/controls/RootControl.h
@@ -7,6 +7,7 @@
 #include "cru/platform/gui/Window.h"
 #include "cru/ui/Base.h"
 #include "cru/ui/host/WindowHost.h"
+#include "cru/ui/render/StackLayoutRenderObject.h"
 
 namespace cru::ui::controls {
 class CRU_UI_API RootControl : public LayoutControl {
diff --git a/include/cru/ui/controls/ScrollView.h b/include/cru/ui/controls/ScrollView.h
index 46c23691..7bc9b226 100644
--- a/include/cru/ui/controls/ScrollView.h
+++ b/include/cru/ui/controls/ScrollView.h
@@ -2,6 +2,7 @@
 #include "ContentControl.h"
 #include "cru/common/Base.h"
 #include "cru/ui/render/RenderObject.h"
+
 #include "cru/ui/render/ScrollRenderObject.h"
 
 #include <memory>
diff --git a/include/cru/ui/controls/TextBlock.h b/include/cru/ui/controls/TextBlock.h
index 5e5674a2..3f87d96c 100644
--- a/include/cru/ui/controls/TextBlock.h
+++ b/include/cru/ui/controls/TextBlock.h
@@ -1,10 +1,12 @@
 #pragma once
 #include "NoChildControl.h"
 
+#include "../render/TextRenderObject.h"
 #include "TextHostControlService.h"
 
 namespace cru::ui::controls {
-class CRU_UI_API TextBlock : public NoChildControl, public virtual ITextHostControl {
+class CRU_UI_API TextBlock : public NoChildControl,
+                             public virtual ITextHostControl {
  public:
   static constexpr StringView control_type = u"TextBlock";
 
diff --git a/include/cru/ui/controls/TextBox.h b/include/cru/ui/controls/TextBox.h
index 2ed7db5e..10f67b72 100644
--- a/include/cru/ui/controls/TextBox.h
+++ b/include/cru/ui/controls/TextBox.h
@@ -1,6 +1,7 @@
 #pragma once
 #include "NoChildControl.h"
 
+#include "../render/TextRenderObject.h"
 #include "IBorderControl.h"
 #include "TextHostControlService.h"
 
@@ -8,8 +9,8 @@
 
 namespace cru::ui::controls {
 class CRU_UI_API TextBox : public NoChildControl,
-                public virtual IBorderControl,
-                public virtual ITextHostControl {
+                           public virtual IBorderControl,
+                           public virtual ITextHostControl {
  public:
   static constexpr StringView control_type = u"TextBox";
 
diff --git a/include/cru/ui/controls/TextHostControlService.h b/include/cru/ui/controls/TextHostControlService.h
index 5178107f..67b075c5 100644
--- a/include/cru/ui/controls/TextHostControlService.h
+++ b/include/cru/ui/controls/TextHostControlService.h
@@ -1,6 +1,7 @@
 #pragma once
 #include "Base.h"
 
+#include "../render/TextRenderObject.h"
 #include "cru/platform/gui/InputMethod.h"
 #include "cru/platform/gui/TimerHelper.h"
 #include "cru/platform/gui/UiApplication.h"
diff --git a/include/cru/ui/host/WindowHost.h b/include/cru/ui/host/WindowHost.h
index 8a8f1da5..80076afd 100644
--- a/include/cru/ui/host/WindowHost.h
+++ b/include/cru/ui/host/WindowHost.h
@@ -1,7 +1,6 @@
 #pragma once
 #include "../Base.h"
 
-#include "../render/Base.h"
 #include "cru/common/Event.h"
 #include "cru/platform/gui/Cursor.h"
 #include "cru/platform/gui/UiApplication.h"
diff --git a/include/cru/ui/render/Base.h b/include/cru/ui/render/Base.h
deleted file mode 100644
index fca67086..00000000
--- a/include/cru/ui/render/Base.h
+++ /dev/null
@@ -1,12 +0,0 @@
-#pragma once
-#include "../Base.h"
-
-namespace cru::ui::render {
-class RenderObject;
-class BorderRenderObject;
-class CanvasRenderObject;
-class FlexLayoutRenderObject;
-class ScrollRenderObject;
-class StackLayoutRenderObject;
-class TextRenderObject;
-}  // namespace cru::ui::render
diff --git a/include/cru/ui/render/BorderRenderObject.h b/include/cru/ui/render/BorderRenderObject.h
index 1720d680..2fc74f0f 100644
--- a/include/cru/ui/render/BorderRenderObject.h
+++ b/include/cru/ui/render/BorderRenderObject.h
@@ -1,10 +1,10 @@
 #pragma once
-#include "RenderObject.h"
+#include "SingleChildRenderObject.h"
 
 #include "../style/ApplyBorderStyleInfo.h"
 
 namespace cru::ui::render {
-class CRU_UI_API BorderRenderObject : public RenderObject {
+class CRU_UI_API BorderRenderObject : public SingleChildRenderObject {
   CRU_DEFINE_CLASS_LOG_TAG(u"cru::ui::render::BorderRenderObject")
 
  public:
@@ -13,57 +13,31 @@ class CRU_UI_API BorderRenderObject : public RenderObject {
   CRU_DELETE_MOVE(BorderRenderObject)
   ~BorderRenderObject() override;
 
-  RenderObject* GetChild() const { return child_; }
-  void SetChild(RenderObject* new_child);
-
   bool IsBorderEnabled() const { return is_border_enabled_; }
-  void SetBorderEnabled(bool enabled) { is_border_enabled_ = enabled; }
+  void SetBorderEnabled(bool enabled);
 
   std::shared_ptr<platform::graphics::IBrush> GetBorderBrush() {
     return border_brush_;
   }
-
-  void SetBorderBrush(std::shared_ptr<platform::graphics::IBrush> brush) {
-    if (brush == border_brush_) return;
-    border_brush_ = std::move(brush);
-    InvalidatePaint();
-  }
+  void SetBorderBrush(std::shared_ptr<platform::graphics::IBrush> brush);
 
   Thickness GetBorderThickness() const { return border_thickness_; }
-
-  void SetBorderThickness(const Thickness thickness) {
-    if (thickness == border_thickness_) return;
-    border_thickness_ = thickness;
-    InvalidateLayout();
-  }
+  void SetBorderThickness(const Thickness thickness);
 
   CornerRadius GetBorderRadius() { return border_radius_; }
-
-  void SetBorderRadius(const CornerRadius radius) {
-    if (radius == border_radius_) return;
-    border_radius_ = radius;
-    RecreateGeometry();
-  }
+  void SetBorderRadius(const CornerRadius radius);
 
   std::shared_ptr<platform::graphics::IBrush> GetForegroundBrush() {
     return foreground_brush_;
   }
 
-  void SetForegroundBrush(std::shared_ptr<platform::graphics::IBrush> brush) {
-    if (brush == foreground_brush_) return;
-    foreground_brush_ = std::move(brush);
-    InvalidatePaint();
-  }
+  void SetForegroundBrush(std::shared_ptr<platform::graphics::IBrush> brush);
 
   std::shared_ptr<platform::graphics::IBrush> GetBackgroundBrush() {
     return background_brush_;
   }
 
-  void SetBackgroundBrush(std::shared_ptr<platform::graphics::IBrush> brush) {
-    if (brush == background_brush_) return;
-    background_brush_ = std::move(brush);
-    InvalidatePaint();
-  }
+  void SetBackgroundBrush(std::shared_ptr<platform::graphics::IBrush> brush);
 
   void ApplyBorderStyle(const style::ApplyBorderStyleInfo& style);
 
@@ -74,7 +48,7 @@ class CRU_UI_API BorderRenderObject : public RenderObject {
   Rect GetPaddingRect() const override;
   Rect GetContentRect() const override;
 
-  String GetName() const override { return u"BorderRenderObject"; }
+  String GetName() const override;
 
  protected:
   Size OnMeasureContent(const MeasureRequirement& requirement,
@@ -87,8 +61,6 @@ class CRU_UI_API BorderRenderObject : public RenderObject {
   void RecreateGeometry();
 
  private:
-  RenderObject* child_ = nullptr;
-
   bool is_border_enabled_ = false;
 
   std::shared_ptr<platform::graphics::IBrush> border_brush_;
diff --git a/include/cru/ui/render/FlexLayoutRenderObject.h b/include/cru/ui/render/FlexLayoutRenderObject.h
index 164bae39..ff7340cb 100644
--- a/include/cru/ui/render/FlexLayoutRenderObject.h
+++ b/include/cru/ui/render/FlexLayoutRenderObject.h
@@ -2,6 +2,24 @@
 #include "LayoutRenderObject.h"
 
 namespace cru::ui::render {
+
+enum class FlexDirection {
+  Horizontal,
+  HorizontalReverse,
+  Vertical,
+  VertivalReverse
+};
+
+using FlexMainAlignment = Alignment;
+using FlexCrossAlignment = Alignment;
+
+struct FlexChildLayoutData {
+  float expand_factor = 0;
+  float shrink_factor = 1;
+  // nullopt stands for looking at parent's setting
+  std::optional<FlexCrossAlignment> cross_alignment = std::nullopt;
+};
+
 // Measure Logic (v0.1):
 // Cross axis measure logic is the same as stack layout.
 //
diff --git a/include/cru/ui/render/LayoutHelper.h b/include/cru/ui/render/LayoutHelper.h
index c2377066..05813489 100644
--- a/include/cru/ui/render/LayoutHelper.h
+++ b/include/cru/ui/render/LayoutHelper.h
@@ -1,14 +1,8 @@
 #pragma once
-#include "Base.h"
-
 #include "MeasureRequirement.h"
 
 namespace cru::ui::render {
 float CalculateAnchorByAlignment(Alignment alignment, float start_point,
                                  float content_length, float child_length);
 
-MeasureLength StackLayoutCalculateChildMaxLength(
-    MeasureLength parent_preferred_size, MeasureLength parent_max_size,
-    MeasureLength child_min_size, std::u16string_view log_tag,
-    std::u16string_view exceeds_message);
 }  // namespace cru::ui::render
diff --git a/include/cru/ui/render/LayoutRenderObject.h b/include/cru/ui/render/LayoutRenderObject.h
index 05da1bb8..62ee9005 100644
--- a/include/cru/ui/render/LayoutRenderObject.h
+++ b/include/cru/ui/render/LayoutRenderObject.h
@@ -1,7 +1,7 @@
 #pragma once
 #include "RenderObject.h"
 
-#include "cru/platform/graphics/util/Painter.h"
+#include "cru/platform/graphics/Painter.h"
 
 namespace cru::ui::render {
 template <typename TChildLayoutData>
@@ -54,6 +54,16 @@ class CRU_UI_API LayoutRenderObject : public RenderObject {
     return children_[position].layout_data;
   }
 
+  void Draw(platform::graphics::IPainter* painter) override {
+    for (auto& child : children_) {
+      painter->PushState();
+      painter->ConcatTransform(
+          Matrix::Translation(child.render_object->GetOffset()));
+      child.render_object->Draw(painter);
+      painter->PopState();
+    }
+  }
+
   RenderObject* HitTest(const Point& point) override {
     const auto child_count = GetChildCount();
     for (auto i = child_count - 1; i >= 0; --i) {
diff --git a/include/cru/ui/render/MeasureRequirement.h b/include/cru/ui/render/MeasureRequirement.h
index c740385b..832b936c 100644
--- a/include/cru/ui/render/MeasureRequirement.h
+++ b/include/cru/ui/render/MeasureRequirement.h
@@ -1,11 +1,10 @@
 #pragma once
-#include "Base.h"
+#include "../Base.h"
 
 #include "cru/common/String.h"
 
 #include <algorithm>
 #include <limits>
-#include <string>
 
 namespace cru::ui::render {
 constexpr Size Min(const Size& left, const Size& right) {
diff --git a/include/cru/ui/render/RenderObject.h b/include/cru/ui/render/RenderObject.h
index 67ef6162..cfa72b7d 100644
--- a/include/cru/ui/render/RenderObject.h
+++ b/include/cru/ui/render/RenderObject.h
@@ -1,10 +1,8 @@
 #pragma once
-#include "Base.h"
+#include "../Base.h"
 
 #include "MeasureRequirement.h"
-#include "cru/common/Base.h"
 #include "cru/common/String.h"
-#include "cru/ui/Base.h"
 
 namespace cru::ui::render {
 // Render object will not destroy its children when destroyed. Control must
diff --git a/include/cru/ui/render/ScrollBar.h b/include/cru/ui/render/ScrollBar.h
index 84d4375b..c65c0345 100644
--- a/include/cru/ui/render/ScrollBar.h
+++ b/include/cru/ui/render/ScrollBar.h
@@ -1,5 +1,4 @@
 #pragma once
-#include "Base.h"
 #include "cru/common/Base.h"
 #include "cru/common/Event.h"
 #include "cru/platform/graphics/Base.h"
diff --git a/include/cru/ui/render/ScrollRenderObject.h b/include/cru/ui/render/ScrollRenderObject.h
index 0890ec21..53c64c6c 100644
--- a/include/cru/ui/render/ScrollRenderObject.h
+++ b/include/cru/ui/render/ScrollRenderObject.h
@@ -1,10 +1,8 @@
 #pragma once
-#include "RenderObject.h"
+#include "SingleChildRenderObject.h"
 
+#include "ScrollBar.h"
 #include "cru/common/Event.h"
-#include "cru/platform/graphics/util/Painter.h"
-#include "cru/ui/Base.h"
-#include "cru/ui/render/ScrollBar.h"
 
 #include <memory>
 #include <optional>
@@ -18,7 +16,7 @@ namespace cru::ui::render {
 // Layout logic:
 // If child is smaller than content area, layout at lefttop.
 // Or layout by scroll state.
-class CRU_UI_API ScrollRenderObject : public RenderObject {
+class CRU_UI_API ScrollRenderObject : public SingleChildRenderObject {
  public:
   ScrollRenderObject();
 
@@ -27,9 +25,6 @@ class CRU_UI_API ScrollRenderObject : public RenderObject {
 
   ~ScrollRenderObject() override = default;
 
-  RenderObject* GetChild() const { return child_; }
-  void SetChild(RenderObject* new_child);
-
   RenderObject* HitTest(const Point& point) override;
 
   // Return the coerced scroll offset.
@@ -91,8 +86,6 @@ class CRU_UI_API ScrollRenderObject : public RenderObject {
   void InstallMouseWheelHandler(controls::Control* control);
 
  private:
-  RenderObject* child_;
-
   Point scroll_offset_;
 
   std::unique_ptr<ScrollBarDelegate> scroll_bar_delegate_;
diff --git a/include/cru/ui/render/SingleChildRenderObject.h b/include/cru/ui/render/SingleChildRenderObject.h
new file mode 100644
index 00000000..cc9e4126
--- /dev/null
+++ b/include/cru/ui/render/SingleChildRenderObject.h
@@ -0,0 +1,23 @@
+#pragma once
+#include "RenderObject.h"
+
+namespace cru::ui::render {
+class CRU_UI_API SingleChildRenderObject : public RenderObject {
+  CRU_DEFINE_CLASS_LOG_TAG(u"cru::ui::render::SingleChildRenderObject")
+
+ public:
+  SingleChildRenderObject() = default;
+  CRU_DELETE_COPY(SingleChildRenderObject)
+  CRU_DELETE_MOVE(SingleChildRenderObject)
+  ~SingleChildRenderObject() override = default;
+
+  RenderObject* GetChild() const { return child_; }
+  void SetChild(RenderObject* new_child);
+
+ protected:
+  virtual void OnChildChanged(RenderObject* old_child, RenderObject* new_child);
+
+ private:
+  RenderObject* child_ = nullptr;
+};
+}  // namespace cru::ui::render
diff --git a/include/cru/ui/render/StackLayoutRenderObject.h b/include/cru/ui/render/StackLayoutRenderObject.h
index 68b5b30c..65b81b3d 100644
--- a/include/cru/ui/render/StackLayoutRenderObject.h
+++ b/include/cru/ui/render/StackLayoutRenderObject.h
@@ -1,6 +1,5 @@
 #pragma once
 #include "LayoutRenderObject.h"
-#include "cru/ui/Base.h"
 
 namespace cru::ui::render {
 // Measure Logic:
diff --git a/include/cru/ui/render/TextRenderObject.h b/include/cru/ui/render/TextRenderObject.h
index f3a7332f..db8ff0d0 100644
--- a/include/cru/ui/render/TextRenderObject.h
+++ b/include/cru/ui/render/TextRenderObject.h
@@ -101,8 +101,6 @@ class CRU_UI_API TextRenderObject : public RenderObject {
                         const MeasureSize& preferred_size) override;
   void OnLayoutContent(const Rect& content_rect) override;
 
-  void OnResize(const Size& size) override;
-
  private:
   std::shared_ptr<platform::graphics::IBrush> brush_;
   std::shared_ptr<platform::graphics::IFont> font_;
diff --git a/src/ui/controls/FlexLayout.cpp b/src/ui/controls/FlexLayout.cpp
index 02ebb031..ecf4d428 100644
--- a/src/ui/controls/FlexLayout.cpp
+++ b/src/ui/controls/FlexLayout.cpp
@@ -1,7 +1,5 @@
 #include "cru/ui/controls/FlexLayout.h"
 
-#include "cru/ui/render/FlexLayoutRenderObject.h"
-
 namespace cru::ui::controls {
 using render::FlexLayoutRenderObject;
 
diff --git a/src/ui/controls/RootControl.cpp b/src/ui/controls/RootControl.cpp
index ac4d4d6d..828822c6 100644
--- a/src/ui/controls/RootControl.cpp
+++ b/src/ui/controls/RootControl.cpp
@@ -5,8 +5,6 @@
 #include "cru/platform/gui/Window.h"
 #include "cru/ui/Base.h"
 #include "cru/ui/host/WindowHost.h"
-#include "cru/ui/render/Base.h"
-#include "cru/ui/render/StackLayoutRenderObject.h"
 #include "gsl/pointers"
 
 #include <memory>
diff --git a/src/ui/render/BorderRenderObject.cpp b/src/ui/render/BorderRenderObject.cpp
index 5916c7aa..30493a49 100644
--- a/src/ui/render/BorderRenderObject.cpp
+++ b/src/ui/render/BorderRenderObject.cpp
@@ -4,10 +4,8 @@
 #include "cru/common/Logger.h"
 #include "cru/platform/graphics/Factory.h"
 #include "cru/platform/graphics/Geometry.h"
-#include "cru/platform/graphics/util/Painter.h"
-#include "cru/ui/Base.h"
+#include "cru/platform/graphics/Painter.h"
 #include "cru/ui/DebugFlags.h"
-#include "cru/ui/style/ApplyBorderStyleInfo.h"
 
 #include <algorithm>
 
@@ -16,14 +14,6 @@ BorderRenderObject::BorderRenderObject() { RecreateGeometry(); }
 
 BorderRenderObject::~BorderRenderObject() {}
 
-void BorderRenderObject::SetChild(RenderObject* new_child) {
-  if (child_ == new_child) return;
-  if (child_ != nullptr) child_->SetParent(nullptr);
-  child_ = new_child;
-  if (child_ != nullptr) child_->SetParent(this);
-  InvalidateLayout();
-}
-
 void BorderRenderObject::ApplyBorderStyle(
     const style::ApplyBorderStyleInfo& style) {
   if (style.border_brush) border_brush_ = *style.border_brush;
@@ -34,27 +24,57 @@ void BorderRenderObject::ApplyBorderStyle(
   InvalidateLayout();
 }
 
+void BorderRenderObject::SetBorderEnabled(bool enabled) {
+  if (is_border_enabled_ == enabled) return;
+  is_border_enabled_ = enabled;
+  InvalidateLayout();
+}
+
+void BorderRenderObject::SetBorderBrush(
+    std::shared_ptr<platform::graphics::IBrush> brush) {
+  if (brush == border_brush_) return;
+  border_brush_ = std::move(brush);
+  InvalidatePaint();
+}
+
+void BorderRenderObject::SetBorderThickness(const Thickness thickness) {
+  if (thickness == border_thickness_) return;
+  border_thickness_ = thickness;
+  InvalidateLayout();
+}
+
+void BorderRenderObject::SetBorderRadius(const CornerRadius radius) {
+  if (radius == border_radius_) return;
+  border_radius_ = radius;
+  RecreateGeometry();
+}
+
+void BorderRenderObject::SetForegroundBrush(
+    std::shared_ptr<platform::graphics::IBrush> brush) {
+  if (brush == foreground_brush_) return;
+  foreground_brush_ = std::move(brush);
+  InvalidatePaint();
+}
+
+void BorderRenderObject::SetBackgroundBrush(
+    std::shared_ptr<platform::graphics::IBrush> brush) {
+  if (brush == background_brush_) return;
+  background_brush_ = std::move(brush);
+  InvalidatePaint();
+}
+
 RenderObject* BorderRenderObject::HitTest(const Point& point) {
-  if (child_) {
-    const auto result = child_->HitTest(point - child_->GetOffset());
+  if (auto child = GetChild()) {
+    const auto result = child->HitTest(point - child->GetOffset());
     if (result != nullptr) {
       return result;
     }
   }
 
   if (is_border_enabled_) {
-    const auto contains =
-        border_outer_geometry_->FillContains(Point{point.x, point.y});
-    return contains ? this : nullptr;
+    return border_outer_geometry_->FillContains(point) ? this : nullptr;
   } else {
-    const auto margin = GetMargin();
-    const auto size = GetDesiredSize();
-    return Rect{margin.left, margin.top,
-                std::max(size.width - margin.GetHorizontalTotal(), 0.0f),
-                std::max(size.height - margin.GetVerticalTotal(), 0.0f)}
-                   .IsPointInside(point)
-               ? this
-               : nullptr;
+    return GetPaddingRect().IsPointInside(point) ? this : nullptr;
   }
 }
 
@@ -80,10 +100,10 @@ void BorderRenderObject::Draw(platform::graphics::IPainter* painter) {
     }
   }
 
-  if (child_) {
+  if (auto child = GetChild()) {
     painter->PushState();
-    painter->ConcatTransform(Matrix::Translation(child_->GetOffset()));
-    child_->Draw(painter);
+    painter->ConcatTransform(Matrix::Translation(child->GetOffset()));
+    child->Draw(painter);
     painter->PopState();
   }
 
@@ -94,17 +114,17 @@ void BorderRenderObject::Draw(platform::graphics::IPainter* painter) {
 
 Size BorderRenderObject::OnMeasureContent(const MeasureRequirement& requirement,
                                           const MeasureSize& preferred_size) {
-  if (child_) {
-    child_->Measure(requirement, preferred_size);
-    return child_->GetDesiredSize();
+  if (auto child = GetChild()) {
+    child->Measure(requirement, preferred_size);
+    return child->GetDesiredSize();
   } else {
     return Size{};
   }
 }
 
 void BorderRenderObject::OnLayoutContent(const Rect& content_rect) {
-  if (child_) {
-    child_->Layout(content_rect.GetLeftTop());
+  if (auto child = GetChild()) {
+    child->Layout(content_rect.GetLeftTop());
   }
 }
 
@@ -117,7 +137,7 @@ Thickness BorderRenderObject::GetOuterSpaceThickness() const {
 }
 
 Rect BorderRenderObject::GetPaddingRect() const {
-  const auto size = GetDesiredSize();
+  const auto size = GetSize();
   Rect rect{Point{}, size};
   rect = rect.Shrink(GetMargin());
   if (is_border_enabled_) rect = rect.Shrink(border_thickness_);
@@ -209,4 +229,6 @@ void BorderRenderObject::RecreateGeometry() {
   geometry_ = builder->Build();
   builder.reset();
 }
+
+String BorderRenderObject::GetName() const { return u"BorderRenderObject"; }
 }  // namespace cru::ui::render
diff --git a/src/ui/render/CanvasRenderObject.cpp b/src/ui/render/CanvasRenderObject.cpp
index e818333c..431b8f56 100644
--- a/src/ui/render/CanvasRenderObject.cpp
+++ b/src/ui/render/CanvasRenderObject.cpp
@@ -1,7 +1,7 @@
 #include "cru/ui/render/CanvasRenderObject.h"
 
 namespace cru::ui::render {
-CanvasRenderObject::CanvasRenderObject() : RenderObject() {}
+CanvasRenderObject::CanvasRenderObject()  {}
 
 CanvasRenderObject::~CanvasRenderObject() = default;
 
diff --git a/src/ui/render/FlexLayoutRenderObject.cpp b/src/ui/render/FlexLayoutRenderObject.cpp
index 261a0f43..ebe60395 100644
--- a/src/ui/render/FlexLayoutRenderObject.cpp
+++ b/src/ui/render/FlexLayoutRenderObject.cpp
@@ -1,10 +1,7 @@
 #include "cru/ui/render/FlexLayoutRenderObject.h"
 
 #include "cru/common/Logger.h"
-#include "cru/platform/graphics/util/Painter.h"
-#include "cru/ui/Base.h"
 #include "cru/ui/render/LayoutHelper.h"
-#include "cru/ui/render/MeasureRequirement.h"
 
 #include <algorithm>
 #include <functional>
diff --git a/src/ui/render/LayoutHelper.cpp b/src/ui/render/LayoutHelper.cpp
index d7f6df8a..7a82bb2d 100644
--- a/src/ui/render/LayoutHelper.cpp
+++ b/src/ui/render/LayoutHelper.cpp
@@ -6,11 +6,11 @@ namespace cru::ui::render {
 float CalculateAnchorByAlignment(Alignment alignment, float start_point,
                                  float content_length, float child_length) {
   switch (alignment) {
-    case FlexCrossAlignment::Start:
+    case Alignment::Start:
       return start_point;
-    case FlexCrossAlignment::Center:
+    case Alignment::Center:
       return start_point + (content_length - child_length) / 2.0f;
-    case FlexCrossAlignment::End:
+    case Alignment::End:
       return start_point + content_length - child_length;
     default:
       return start_point;
diff --git a/src/ui/render/ScrollBar.cpp b/src/ui/render/ScrollBar.cpp
index e6389e16..695caa03 100644
--- a/src/ui/render/ScrollBar.cpp
+++ b/src/ui/render/ScrollBar.cpp
@@ -448,7 +448,7 @@ void HorizontalScrollBar::DrawDownArrow(
 }
 
 bool HorizontalScrollBar::IsShowBar() {
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
   if (child == nullptr) return false;
 
   const auto view_rect = render_object_->GetViewRect();
@@ -466,7 +466,7 @@ std::optional<Rect> HorizontalScrollBar::GetExpandedAreaRect(
 
   const auto padding_rect = render_object_->GetPaddingRect();
 
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
 
   const auto view_rect = render_object_->GetViewRect();
   const auto child_size = child->GetDesiredSize();
@@ -518,7 +518,7 @@ std::optional<Rect> HorizontalScrollBar::GetCollapsedThumbRect() {
   auto show = IsShowBar();
   if (!show) return std::nullopt;
 
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
 
   const auto view_rect = render_object_->GetViewRect();
   const auto child_size = child->GetDesiredSize();
@@ -546,7 +546,7 @@ float HorizontalScrollBar::CalculateNewScrollPosition(
 
   auto thumb_head_end = scroll_area_end - thumb_original_rect.width;
 
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
   const auto child_size = child->GetDesiredSize();
 
   new_thumb_start =
@@ -597,7 +597,7 @@ void VerticalScrollBar::DrawDownArrow(
 }
 
 bool VerticalScrollBar::IsShowBar() {
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
   if (child == nullptr) return false;
 
   const auto view_rect = render_object_->GetViewRect();
@@ -615,7 +615,7 @@ std::optional<Rect> VerticalScrollBar::GetExpandedAreaRect(
 
   const auto padding_rect = render_object_->GetPaddingRect();
 
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
 
   const auto view_rect = render_object_->GetViewRect();
   const auto child_size = child->GetDesiredSize();
@@ -663,7 +663,7 @@ std::optional<Rect> VerticalScrollBar::GetCollapsedTriggerExpandAreaRect() {
 }
 
 std::optional<Rect> VerticalScrollBar::GetCollapsedThumbRect() {
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
   if (child == nullptr) return std::nullopt;
 
   const auto view_rect = render_object_->GetViewRect();
@@ -693,7 +693,7 @@ float VerticalScrollBar::CalculateNewScrollPosition(
 
   auto thumb_head_end = scroll_area_end - thumb_original_rect.height;
 
-  const auto child = render_object_->GetFirstChild();
+  const auto child = render_object_->GetChild();
   const auto child_size = child->GetDesiredSize();
 
   new_thumb_start =
diff --git a/src/ui/render/ScrollRenderObject.cpp b/src/ui/render/ScrollRenderObject.cpp
index e2710de4..03bb1a2d 100644
--- a/src/ui/render/ScrollRenderObject.cpp
+++ b/src/ui/render/ScrollRenderObject.cpp
@@ -1,8 +1,6 @@
 #include "cru/ui/render/ScrollRenderObject.h"
 
 #include "cru/platform/graphics/Painter.h"
-#include "cru/platform/graphics/util/Painter.h"
-#include "cru/ui/Base.h"
 #include "cru/ui/controls/Control.h"
 #include "cru/ui/render/ScrollBar.h"
 
@@ -38,19 +36,12 @@ Point CoerceScroll(const Point& scroll_offset, const Size& content_size,
 }
 }  // namespace
 
-ScrollRenderObject::ScrollRenderObject() : RenderObject() {
+ScrollRenderObject::ScrollRenderObject() {
   scroll_bar_delegate_ = std::make_unique<ScrollBarDelegate>(this);
   scroll_bar_delegate_->ScrollAttemptEvent()->AddHandler(
       [this](const struct Scroll& scroll) { this->ApplyScroll(scroll); });
 }
 
-void ScrollRenderObject::SetChild(RenderObject* new_child) {
-  if (child_ == new_child) return;
-  if (child_) child_->SetParent(nullptr);
-  child_ = new_child;
-  if (child_) child_->SetParent(this);
-}
-
 void ScrollRenderObject::ApplyScroll(const struct Scroll& scroll) {
   auto direction = scroll.direction;
 
@@ -79,9 +70,9 @@ void ScrollRenderObject::ApplyScroll(const struct Scroll& scroll) {
 }
 
 RenderObject* ScrollRenderObject::HitTest(const Point& point) {
-  if (child_) {
-    const auto offset = child_->GetOffset();
-    const auto r = child_->HitTest(point - offset);
+  if (auto child = GetChild()) {
+    const auto offset = child->GetOffset();
+    const auto r = child->HitTest(point - offset);
     if (r != nullptr) return r;
   }
 
@@ -90,12 +81,12 @@ RenderObject* ScrollRenderObject::HitTest(const Point& point) {
 }  // namespace cru::ui::render
 
 void ScrollRenderObject::Draw(platform::graphics::IPainter* painter) {
-  if (child_) {
+  if (auto child = GetChild()) {
     painter->PushLayer(this->GetContentRect());
-    const auto offset = child_->GetOffset();
+    const auto offset = child->GetOffset();
     painter->PushState();
     painter->ConcatTransform(Matrix::Translation(offset));
-    child_->Draw(painter);
+    child->Draw(painter);
     painter->PopState();
     painter->PopLayer();
   }
@@ -103,11 +94,12 @@ void ScrollRenderObject::Draw(platform::graphics::IPainter* painter) {
 }
 
 Point ScrollRenderObject::GetScrollOffset() {
-  if (child_)
+  if (auto child = GetChild()) {
     return CoerceScroll(scroll_offset_, GetContentRect().GetSize(),
-                        child_->GetDesiredSize());
-  else
+                        child->GetSize());
+  } else {
     return scroll_offset_;
+  }
 }
 
 void ScrollRenderObject::SetScrollOffset(const Point& offset) {
@@ -171,12 +163,12 @@ void ScrollRenderObject::SetMouseWheelScrollEnabled(bool enable) {
 
 Size ScrollRenderObject::OnMeasureContent(const MeasureRequirement& requirement,
                                           const MeasureSize& preferred_size) {
-  if (child_) {
-    child_->Measure(MeasureRequirement{MeasureSize::NotSpecified(),
-                                       MeasureSize::NotSpecified()},
-                    MeasureSize::NotSpecified());
+  if (auto child = GetChild()) {
+    child->Measure(MeasureRequirement{MeasureSize::NotSpecified(),
+                                      MeasureSize::NotSpecified()},
+                   MeasureSize::NotSpecified());
 
-    Size result = requirement.Coerce(child_->GetDesiredSize());
+    Size result = requirement.Coerce(child->GetDesiredSize());
     if (preferred_size.width.IsSpecified()) {
       result.width = preferred_size.width.GetLengthOrUndefined();
     }
@@ -196,8 +188,8 @@ Size ScrollRenderObject::OnMeasureContent(const MeasureRequirement& requirement,
 }  // namespace cru::ui::render
 
 void ScrollRenderObject::OnLayoutContent(const Rect& content_rect) {
-  if (child_) {
-    child_->Layout(content_rect.GetLeftTop() - GetScrollOffset());
+  if (auto child = GetChild()) {
+    child->Layout(content_rect.GetLeftTop() - GetScrollOffset());
   }
 }
 
@@ -255,8 +247,10 @@ bool ScrollRenderObject::HorizontalCanScrollUp() {
 }
 
 bool ScrollRenderObject::HorizontalCanScrollDown() {
+  auto child = GetChild();
+  if (child == nullptr) return false;
   return GetScrollOffset().x <
-         child_->GetDesiredSize().width - GetViewRect().width;
+         child->GetDesiredSize().width - GetViewRect().width;
 }
 
 bool ScrollRenderObject::VerticalCanScrollUp() {
@@ -264,7 +258,9 @@ bool ScrollRenderObject::VerticalCanScrollUp() {
 }
 
 bool ScrollRenderObject::VerticalCanScrollDown() {
+  auto child = GetChild();
+  if (child == nullptr) return false;
   return GetScrollOffset().y <
-         child_->GetDesiredSize().height - GetViewRect().height;
+         child->GetDesiredSize().height - GetViewRect().height;
 }
 }  // namespace cru::ui::render
diff --git a/src/ui/render/SingleChildRenderObject.cpp b/src/ui/render/SingleChildRenderObject.cpp
new file mode 100644
index 00000000..7b10b343
--- /dev/null
+++ b/src/ui/render/SingleChildRenderObject.cpp
@@ -0,0 +1,22 @@
+#include "cru/ui/render/SingleChildRenderObject.h"
+
+namespace cru::ui::render {
+void SingleChildRenderObject::SetChild(RenderObject *new_child) {
+  assert(new_child->GetParent() == nullptr);
+  if (child_ == new_child) return;
+  auto old_child = child_;
+  if (child_) {
+    child_->SetParent(nullptr);
+  }
+  child_ = new_child;
+  if (child_) {
+    child_->SetParent(this);
+  }
+  OnChildChanged(old_child, new_child);
+}
+
+void SingleChildRenderObject::OnChildChanged(RenderObject *old_child,
+                                             RenderObject *new_child) {
+  InvalidateLayout();
+}
+}  // namespace cru::ui::render
diff --git a/src/ui/render/TextRenderObject.cpp b/src/ui/render/TextRenderObject.cpp
index ec5ce662..6f4a8320 100644
--- a/src/ui/render/TextRenderObject.cpp
+++ b/src/ui/render/TextRenderObject.cpp
@@ -3,8 +3,8 @@
 #include "../Helper.h"
 #include "cru/common/Logger.h"
 #include "cru/platform/graphics/Factory.h"
+#include "cru/platform/graphics/Painter.h"
 #include "cru/platform/graphics/TextLayout.h"
-#include "cru/platform/graphics/util/Painter.h"
 #include "cru/ui/DebugFlags.h"
 
 #include <algorithm>