Improve KeyDownTriggerBehavior to support PreviewKeyDown, handled events, and modifier keys

Author: Iryna Konovalova

components/Behaviors/src/Keyboard/KeyDownTriggerBehavior.cs

@@ -8,53 +8,150 @@
 namespace CommunityToolkit.WinUI.Behaviors;
 
 /// <summary>
-/// This behavior listens to a key down event on the associated <see cref="UIElement"/> when it is loaded and executes an action.
+/// A behavior that listens to <see cref="UIElement.PreviewKeyDown"/> on the associated
+/// <see cref="FrameworkElement"/> and executes its actions when the specified key and
+/// optional modifier keys are pressed. Supports capturing handled events.
 /// </summary>
 [TypeConstraint(typeof(FrameworkElement))]
 public class KeyDownTriggerBehavior : Trigger<FrameworkElement>
 {
+    private KeyEventHandler? _handler;
     /// <summary>
-    /// Identifies the <see cref="Key"/> property.
+    /// Identifies the <see cref="Key"/> dependency property.
     /// </summary>
-    public static readonly DependencyProperty KeyProperty = DependencyProperty.Register(
-        nameof(Key),
-        typeof(VirtualKey),
-        typeof(KeyDownTriggerBehavior),
-        new PropertyMetadata(null));
+    public static readonly DependencyProperty KeyProperty =
+        DependencyProperty.Register(
+            nameof(Key),
+            typeof(VirtualKey),
+            typeof(KeyDownTriggerBehavior),
+            new PropertyMetadata(VirtualKey.None));
 
     /// <summary>
-    /// Gets or sets the key to listen when the associated object is loaded.
+    /// Gets or sets the key that triggers the behavior.
     /// </summary>
     public VirtualKey Key
     {
         get => (VirtualKey)GetValue(KeyProperty);
         set => SetValue(KeyProperty, value);
     }
 
+    /// <summary>
+    /// Identifies the <see cref="Modifiers"/> dependency property.
+    /// </summary>
+    public static readonly DependencyProperty ModifiersProperty =
+        DependencyProperty.Register(
+            nameof(Modifiers),
+            typeof(VirtualKeyModifiers),
+            typeof(KeyDownTriggerBehavior),
+            new PropertyMetadata(VirtualKeyModifiers.None));
+
+    /// <summary>
+    /// Gets or sets the modifier keys that must be pressed together with <see cref="Key"/>.
+    /// </summary>
+    public VirtualKeyModifiers Modifiers
+    {
+        get => (VirtualKeyModifiers)GetValue(ModifiersProperty);
+        set => SetValue(ModifiersProperty, value);
+    }
+
+    /// <summary>
+    /// Identifies the <see cref="HandledEventsToo"/> dependency property.
+    /// </summary>
+    public static readonly DependencyProperty HandledEventsTooProperty =
+        DependencyProperty.Register(
+            nameof(HandledEventsToo),
+            typeof(bool),
+            typeof(KeyDownTriggerBehavior),
+            new PropertyMetadata(true));
+
+    /// <summary>
+    /// Gets or sets a value indicating whether the behavior should receive
+    /// <see cref="UIElement.PreviewKeyDown"/> events even if they were already handled.
+    /// </summary>
+    public bool HandledEventsToo
+    {
+        get => (bool)GetValue(HandledEventsTooProperty);
+        set => SetValue(HandledEventsTooProperty, value);
+    }
+
     /// <inheritdoc/>
     protected override void OnAttached()
     {
-        AssociatedObject.KeyDown += OnAssociatedObjectKeyDown;
+        _handler = OnPreviewKeyDown;
+
+        AssociatedObject.AddHandler(
+            UIElement.PreviewKeyDownEvent,
+            _handler,
+            HandledEventsToo);
     }
 
     /// <inheritdoc/>
     protected override void OnDetaching()
     {
-        AssociatedObject.KeyDown -= OnAssociatedObjectKeyDown;
+        if (_handler is not null)
+        {
+            AssociatedObject.RemoveHandler(
+                UIElement.PreviewKeyDownEvent,
+                _handler);
+
+            _handler = null;
+        }
     }
 
     /// <summary>
-    /// Invokes the current actions when the <see cref="Key"/> is pressed.
+    /// Handles the <see cref="UIElement.PreviewKeyDown"/> event and executes the associated actions
+    /// when the specified <see cref="Key"/> and <see cref="Modifiers"/> match.
     /// </summary>
     /// <param name="sender">The source <see cref="UIElement"/> instance.</param>
     /// <param name="keyRoutedEventArgs">The arguments for the event (unused).</param>
-    private void OnAssociatedObjectKeyDown(object sender, KeyRoutedEventArgs keyRoutedEventArgs)
+    private void OnPreviewKeyDown(object sender, KeyRoutedEventArgs keyRoutedEventArgs)
     {
-        if (keyRoutedEventArgs.Key == Key)
+        if (keyRoutedEventArgs.Key != Key)
         {
-            keyRoutedEventArgs.Handled = true;
-            Interaction.ExecuteActions(sender, Actions, keyRoutedEventArgs);
+            return;
         }
+
+        if (!CheckModifiers())
+        {
+            return;
+        }
+
+        keyRoutedEventArgs.Handled = true;
+        Interaction.ExecuteActions(sender, Actions, keyRoutedEventArgs);
     }
-}
 
+    /// <summary>
+    /// Checks whether all required modifier keys specified in <see cref="Modifiers"/>
+    /// are currently pressed.
+    /// </summary>
+    /// <returns><see langword="true"/> if the modifier state matches; otherwise, <see langword="false"/>.</returns>
+
+    private bool CheckModifiers() =>
+        Match(VirtualKeyModifiers.Control, VirtualKey.Control) &&
+        Match(VirtualKeyModifiers.Shift, VirtualKey.Shift) &&
+        Match(VirtualKeyModifiers.Menu, VirtualKey.Menu);
+
+    /// <summary>
+    /// Determines whether a specific modifier key is required and whether it is currently pressed.
+    /// </summary>
+    /// <param name="mod">The modifier flag to test.</param>
+    /// <param name="key">The physical key corresponding to the modifier.</param>
+    /// <returns><see langword="true"/> if the modifier requirement matches the current key state; otherwise, <see langword="false"/>.</returns>
+    private bool Match(VirtualKeyModifiers mod, VirtualKey key)
+    {
+        bool required = (Modifiers & mod) != 0;
+        bool pressed = IsDown(key);
+        return required == pressed;
+    }
+
+    /// <summary>
+    /// Checks whether the specified key is currently in the <see cref="CoreVirtualKeyStates.Down"/> state.
+    /// </summary>
+    /// <param name="key">The key to test.</param>
+    /// <returns><see langword="true"/> if the key is pressed; otherwise, <see langword="false"/>.</returns>
+    private static bool IsDown(VirtualKey key)
+    {
+        var state = InputKeyboardSource.GetKeyStateForCurrentThread(key);
+        return state.HasFlag(CoreVirtualKeyStates.Down);
+    }
+}