chore: separated System.Windows.Interactivity code

src/Shared/System.Windows.Interactivity/BehaviorCollection.cs

@@ -0,0 +1,41 @@
+using System.Windows;
+
+namespace HandyControl.Interactivity;
+
+public sealed class BehaviorCollection : AttachableCollection<Behavior>
+{
+    internal BehaviorCollection()
+    {
+    }
+
+    protected override void OnAttached()
+    {
+        foreach (var behavior in this)
+            behavior.Attach(AssociatedObject);
+    }
+
+    protected override void OnDetaching()
+    {
+        foreach (var behavior in this)
+            behavior.Detach();
+    }
+
+    internal override void ItemAdded(Behavior item)
+    {
+        if (item == null || AssociatedObject == null)
+            return;
+        item.Attach(AssociatedObject);
+    }
+
+    internal override void ItemRemoved(Behavior item)
+    {
+        if (((IAttachedObject) item)?.AssociatedObject == null)
+            return;
+        item.Detach();
+    }
+
+    protected override Freezable CreateInstanceCore()
+    {
+        return new BehaviorCollection();
+    }
+}