From 766a814f4e03afa0d99ad33e153d6502501a1c5a Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Sat, 5 Mar 2016 17:18:02 +0100
Subject: [PATCH] Check for correct state on attach/detach to tree.

Make sure that if we're attached to a logical tree, we can invoke
DetachedFromLogicalTree when Parent is set to null. This also has a side
effect in that Parent will now be non-null when DetachedFromLogicalTree
is called.
---
 src/Perspex.Base/Perspex.Base.csproj         |  1 +
 src/Perspex.Base/PerspexInternalException.cs | 22 +++++++++++++++
 src/Perspex.Controls/Control.cs              | 28 +++++++++++++-------
 3 files changed, 42 insertions(+), 9 deletions(-)
 create mode 100644 src/Perspex.Base/PerspexInternalException.cs

diff --git a/src/Perspex.Base/Perspex.Base.csproj b/src/Perspex.Base/Perspex.Base.csproj
index dd41f12e9e..9a6acd4048 100644
--- a/src/Perspex.Base/Perspex.Base.csproj
+++ b/src/Perspex.Base/Perspex.Base.csproj
@@ -63,6 +63,7 @@
     <Compile Include="Metadata\DependsOnAttribute.cs" />
     <Compile Include="Metadata\ContentAttribute.cs" />
     <Compile Include="PerspexDisposable.cs" />
+    <Compile Include="PerspexInternalException.cs" />
     <Compile Include="PerspexLocator.cs" />
     <Compile Include="Metadata\XmlnsDefinitionAttribute.cs" />
     <Compile Include="PerspexObjectExtensions.cs" />
diff --git a/src/Perspex.Base/PerspexInternalException.cs b/src/Perspex.Base/PerspexInternalException.cs
new file mode 100644
index 0000000000..5ab4341e1a
--- /dev/null
+++ b/src/Perspex.Base/PerspexInternalException.cs
@@ -0,0 +1,22 @@
+// Copyright (c) The Perspex Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+
+namespace Perspex
+{
+    /// <summary>
+    /// Exception signifying an internal logic error in Perspex.
+    /// </summary>
+    public class PerspexInternalException : Exception
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PerspexInternalException"/> class.
+        /// </summary>
+        /// <param name="message">The exception message.</param>
+        public PerspexInternalException(string message)
+            : base(message)
+        {
+        }
+    }
+}
diff --git a/src/Perspex.Controls/Control.cs b/src/Perspex.Controls/Control.cs
index 6aa7359783..eb096e2f0f 100644
--- a/src/Perspex.Controls/Control.cs
+++ b/src/Perspex.Controls/Control.cs
@@ -391,22 +391,32 @@ namespace Perspex.Controls
                     throw new InvalidOperationException("The Control already has a parent.");
                 }
 
-                InheritanceParent = parent as PerspexObject;
-                _parent = (IControl)parent;
+                if (_isAttachedToLogicalTree)
+                {
+                    var oldRoot = FindStyleRoot(old);
 
-                var root = FindStyleRoot(old);
+                    if (oldRoot == null)
+                    {
+                        throw new PerspexInternalException("Was attached to logical tree but cannot find root.");
+                    }
 
-                if (root != null)
-                {
-                    var e = new LogicalTreeAttachmentEventArgs(root);
+                    var e = new LogicalTreeAttachmentEventArgs(oldRoot);
                     OnDetachedFromLogicalTree(e);
                 }
 
-                root = FindStyleRoot(this);
+                InheritanceParent = parent as PerspexObject;
+                _parent = (IControl)parent;
 
-                if (root != null)
+                if (_parent is IStyleRoot || _parent?.IsAttachedToLogicalTree == true)
                 {
-                    var e = new LogicalTreeAttachmentEventArgs(root);
+                    var newRoot = FindStyleRoot(this);
+
+                    if (newRoot == null)
+                    {
+                        throw new PerspexInternalException("Parent is atttached to logical tree but cannot find root.");
+                    }
+
+                    var e = new LogicalTreeAttachmentEventArgs(newRoot);
                     OnAttachedToLogicalTree(e);
                 }