Add project files.

UI/WPF/Markups/StateExtension.cs

@@ -0,0 +1,38 @@
+using System.Windows;
+using System.Windows.Data;
+
+namespace TheXamlGuy.UI.WPF;
+
+public class StateExtension : TriggerExtension
+{
+    private static readonly DependencyProperty PropertyTargetProperty =
+        DependencyProperty.RegisterAttached("PropertyTarget",
+            typeof(object), typeof(StateExtension));
+
+    private static readonly DependencyProperty StateProperty =
+        DependencyProperty.RegisterAttached("State",
+            typeof(object), typeof(StateExtension));
+
+    private readonly BindingBase targetBinding;
+
+    private readonly BindingBase stateBinding;
+
+    public StateExtension(object target, object state)
+    {
+        this.targetBinding = target is BindingBase targetBinding ? targetBinding : target.ToBinding();
+        this.stateBinding = state is BindingBase stateBinding ? stateBinding : state.ToBinding();
+    }
+
+    protected override void OnInvoked(object sender, EventArgs args)
+    {
+        BindingOperations.SetBinding(TargetObject, PropertyTargetProperty, targetBinding);
+        if (TargetObject?.GetValue(PropertyTargetProperty) is FrameworkElement target)
+        {
+            BindingOperations.SetBinding(target, StateProperty, stateBinding);
+            object? state = target.GetValue(StateProperty);
+            target.ApplyTemplate();
+            
+            VisualStateManager.GoToElementState(target, (string)state, true);
+        }
+    }
+}