4 files changed, 16 insertions, 6 deletions
diff --git a/include/cru/ui/controls/Control.h b/include/cru/ui/controls/Control.h
index 97096e35..4dad4549 100644
--- a/include/cru/ui/controls/Control.h
+++ b/include/cru/ui/controls/Control.h
@@ -17,6 +17,8 @@ namespace cru::ui::controls {
  * former one is even not used.
  */
 class CRU_UI_API Control : public Object {
+  friend class RootControl;
+
  protected:
   Control();
 
@@ -32,7 +34,7 @@ class CRU_UI_API Control : public Object {
 
   //*************** region: tree ***************
  public:
-  virtual host::WindowHost* GetWindowHost() const;
+  host::WindowHost* GetWindowHost() const { return window_host_; }
 
   Control* GetParent() const { return parent_; }
   void SetParent(Control* parent);
@@ -143,12 +145,22 @@ class CRU_UI_API Control : public Object {
  protected:
   virtual void OnParentChanged(Control* old_parent, Control* new_parent) {}
 
+  virtual void OnWindowHostChanged(host::WindowHost* old_host,
+                                   host::WindowHost* new_host) {}
+
  protected:
   virtual void OnMouseHoverChange(bool newHover) { CRU_UNUSED(newHover) }
 
  private:
+  void OnParentChangedCore(Control* old_parent, Control* new_parent);
+  void OnWindowHostChangedCore(host::WindowHost* old_host,
+                               host::WindowHost* new_host);
+
+ private:
   Control* parent_ = nullptr;
 
+  host::WindowHost* window_host_ = nullptr;
+
  private:
   bool is_mouse_over_ = false;
 
diff --git a/include/cru/ui/controls/RootControl.h b/include/cru/ui/controls/RootControl.h
index a1878836..851b4db0 100644
--- a/include/cru/ui/controls/RootControl.h
+++ b/include/cru/ui/controls/RootControl.h
@@ -18,10 +18,6 @@ class CRU_UI_API RootControl
   ~RootControl() override;
 
  public:
-  host::WindowHost* GetWindowHost() const override {
-    return window_host_.get();
-  }
-
   platform::gui::INativeWindow* GetNativeWindow();
 
  protected:
diff --git a/include/cru/ui/controls/SingleChildControl.h b/include/cru/ui/controls/SingleChildControl.h
index 6bf150c0..e733c825 100644
--- a/include/cru/ui/controls/SingleChildControl.h
+++ b/include/cru/ui/controls/SingleChildControl.h
@@ -31,7 +31,8 @@ class SingleChildControl : public Control {
       child->SetParent(this);
     }
 
-    container_render_object_->SetChild(child->GetRenderObject());
+    container_render_object_->SetChild(
+        child == nullptr ? nullptr : child->GetRenderObject());
   }
 
   render::RenderObject* GetRenderObject() const override {
diff --git a/include/cru/ui/host/WindowHost.h b/include/cru/ui/host/WindowHost.h
index 1de915e0..9fe46ac0 100644
--- a/include/cru/ui/host/WindowHost.h
+++ b/include/cru/ui/host/WindowHost.h
@@ -18,6 +18,7 @@ struct AfterLayoutEventArgs {};
 
 // The bridge between control tree and native window.
 class CRU_UI_API WindowHost : public Object {
+  friend controls::Control;
   CRU_DEFINE_CLASS_LOG_TAG(u"cru::ui::host::WindowHost")
 
  public: