Remove PlaceholderControl and custom parenting logic

New logic is to assume that if target control is already parented, no need to add it as a child control.

BlinForms.Framework/BlontrolAdapter.cs

@@ -135,22 +135,14 @@ namespace BlinForms.Framework
 
             TargetControl = nativeControl;
 
-            if (nativeControl is ICustomParentingBehavior customChildBehavior)
+            // TODO: Need a more reliable way to know whether the target control is already created, e.g. a return value
+            // from ControlFactory.CreateControl(). Right now the check assumes that if the target control is already parented,
+            // there is no need to parent it. Not an awful assumption, but looks odd.
+            if (TargetControl.Parent == null)
             {
-                customChildBehavior.SetActualParentControl(Parent.TargetControl);
-            }
-            else
-            {
-                if (Parent.TargetControl is ICustomParentingBehavior customParentBehavior)
-                {
-                    AddChildControl(customParentBehavior.GetEffectiveParentControl(), siblingIndex, TargetControl);
-                }
-                else
-                {
-                    // Add the new native control to the parent's child controls (the parent adapter is our
-                    // container, so the parent adapter's control is our control's container.
-                    AddChildControl(Parent.TargetControl, siblingIndex, TargetControl);
-                }
+                // Add the new native control to the parent's child controls (the parent adapter is our
+                // container, so the parent adapter's control is our control's container.
+                AddChildControl(Parent.TargetControl, siblingIndex, TargetControl);
             }
 
             var endIndexExcl = frameIndex + frames[frameIndex].ElementSubtreeLength;

BlinForms.Framework/Controls/ICustomParentingBehavior.cs

@@ -1,15 +0,0 @@
-using System.Windows.Forms;
-
-namespace BlinForms.Framework.Controls
-{
-    /// <summary>
-    /// Indicates that a control has a custom behavior for determining where child controls should be parented to.
-    /// For example, a SplitContainer control has built-in immutable children, and should not have those children
-    /// created directly.
-    /// </summary>
-    internal interface ICustomParentingBehavior
-    {
-        void SetActualParentControl(Control parentControl);
-        Control GetEffectiveParentControl();
-    }
-}

BlinForms.Framework/Controls/PlaceholderControl.cs

@@ -1,24 +0,0 @@
-using System;
-using System.Windows.Forms;
-
-namespace BlinForms.Framework.Controls
-{
-    public class PlaceholderControl : Control, ICustomParentingBehavior
-    {
-        private Control _parentControl;
-
-        public string State { get; set; }
-
-        public Control GetEffectiveParentControl()
-        {
-            var parentSplitter = (System.Windows.Forms.SplitContainer)_parentControl;
-            var containerPanel = State == "Panel1" ? parentSplitter.Panel1 : State == "Panel2" ? parentSplitter.Panel2 : throw new InvalidOperationException("Invalid panel state!!!");
-            return containerPanel;
-        }
-
-        public void SetActualParentControl(Control parentControl)
-        {
-            _parentControl = parentControl;
-        }
-    }
-}

BlinForms.Framework/Controls/SplitterPanel1.cs

@@ -1,20 +1,10 @@
-using Microsoft.AspNetCore.Components;
-using Microsoft.AspNetCore.Components.RenderTree;
-
-namespace BlinForms.Framework.Controls
+namespace BlinForms.Framework.Controls
 {
-    public class SplitterPanel1 : FormsComponentBase
+    public class SplitterPanel1 : SplitterPanelBase
     {
         static SplitterPanel1()
         {
-            BlontrolAdapter.KnownElements.Add(typeof(SplitterPanel1).FullName, new ComponentControlFactoryFunc((_, __) => new PlaceholderControl() { State = "Panel1", }));
-        }
-
-        [Parameter] public RenderFragment ChildContent { get; set; }
-
-        protected override void RenderContents(RenderTreeBuilder builder)
-        {
-            builder.AddContent(1000, ChildContent);
+            BlontrolAdapter.KnownElements.Add(typeof(SplitterPanel1).FullName, new ComponentControlFactoryFunc((_, parentControl) => GetSplitterPanel(parentControl, panelNumber: 1)));
         }
     }
 }

BlinForms.Framework/Controls/SplitterPanel2.cs

@@ -1,20 +1,10 @@
-using Microsoft.AspNetCore.Components;
-using Microsoft.AspNetCore.Components.RenderTree;
-
-namespace BlinForms.Framework.Controls
+namespace BlinForms.Framework.Controls
 {
-    public class SplitterPanel2 : FormsComponentBase
+    public class SplitterPanel2 : SplitterPanelBase
     {
         static SplitterPanel2()
         {
-            BlontrolAdapter.KnownElements.Add(typeof(SplitterPanel2).FullName, new ComponentControlFactoryFunc((_, __) => new PlaceholderControl() { State = "Panel2", }));
-        }
-
-        [Parameter] public RenderFragment ChildContent { get; set; }
-
-        protected override void RenderContents(RenderTreeBuilder builder)
-        {
-            builder.AddContent(1000, ChildContent);
+            BlontrolAdapter.KnownElements.Add(typeof(SplitterPanel2).FullName, new ComponentControlFactoryFunc((_, parentControl) => GetSplitterPanel(parentControl, panelNumber: 2)));
         }
     }
 }

BlinForms.Framework/Controls/SplitterPanelBase.cs

@@ -0,0 +1,32 @@
+using Microsoft.AspNetCore.Components;
+using Microsoft.AspNetCore.Components.RenderTree;
+using System;
+using System.Windows.Forms;
+
+namespace BlinForms.Framework.Controls
+{
+    public abstract class SplitterPanelBase : FormsComponentBase
+    {
+        private protected static Control GetSplitterPanel(Control parentControl, int panelNumber)
+        {
+            if (!(parentControl is System.Windows.Forms.SplitContainer splitContainer))
+            {
+                // This gets called from a static constructor, so we really don't want to throw from here
+                return null;
+            }
+            return panelNumber switch
+            {
+                1 => splitContainer.Panel1,
+                2 => splitContainer.Panel2,
+                _ => throw new InvalidOperationException($"Invalid SplitContainer panel number: {panelNumber}. The only valid values are '1' and '2'."),
+            };
+        }
+
+        [Parameter] public RenderFragment ChildContent { get; set; }
+
+        protected override void RenderContents(RenderTreeBuilder builder)
+        {
+            builder.AddContent(1000, ChildContent);
+        }
+    }
+}