add focus redirection

Author: emmauss

src/Avalonia.Base/Input/FocusChangingEventArgs.cs

@@ -19,12 +19,12 @@ internal FocusChangingEventArgs(RoutedEvent routedEvent) : base(routedEvent)
         /// <summary>
         /// Gets or sets the element that focus has moved to.
         /// </summary>
-        public IInputElement? NewFocus { get; init; }
+        public IInputElement? NewFocusedElement { get; internal set; }
 
         /// <summary>
         /// Gets or sets the element that previously had focus.
         /// </summary>
-        public IInputElement? OldFocus { get; init; }
+        public IInputElement? OldFocusedElement { get; init; }
 
         /// <summary>
         /// Gets or sets a value indicating how the change in focus occurred.
@@ -41,17 +41,30 @@ internal FocusChangingEventArgs(RoutedEvent routedEvent) : base(routedEvent)
         /// </summary>
         public bool Cancelled { get; private set; }
 
-        internal bool IsCancellable { get; init; }
+        internal bool CanCancelOrRedirectFocus { get; init; }
 
         /// <summary>
         /// Attempts to cancel the current focus change
         /// </summary>
         /// <returns>true if focus change was cancelled; otherwise, false</returns>
         public bool TryCancel()
         {
-            Cancelled = IsCancellable;
+            Cancelled = CanCancelOrRedirectFocus;
 
             return Cancelled;
         }
+
+        /// <summary>
+        /// Attempts to redirect focus from the targeted element to the specified element.
+        /// </summary>
+        public bool TrySetNewFocusedElement(IInputElement? inputElement)
+        {
+            if(CanCancelOrRedirectFocus)
+            {
+                NewFocusedElement = inputElement;
+            }
+
+            return inputElement == NewFocusedElement;
+        }
     }
 }

src/Avalonia.Base/Input/KeyboardDevice.cs

@@ -151,11 +151,11 @@ public void SetFocusedElement(
 
                 var losingFocus = new FocusChangingEventArgs(InputElement.LosingFocusEvent)
                 {
-                    OldFocus = FocusedElement,
-                    NewFocus = element,
+                    OldFocusedElement = FocusedElement,
+                    NewFocusedElement = element,
                     NavigationMethod = method,
                     KeyModifiers = keyModifiers,
-                    IsCancellable = isFocusChangeCancellable
+                    CanCancelOrRedirectFocus = isFocusChangeCancellable
                 };
 
                 interactive?.RaiseEvent(losingFocus);
@@ -165,15 +165,15 @@ public void SetFocusedElement(
                     changeFocus = false;
                 }
 
-                if (changeFocus && element is Interactive newFocus)
+                if (changeFocus && losingFocus.NewFocusedElement is Interactive newFocus)
                 {
                     var gettingFocus = new FocusChangingEventArgs(InputElement.GettingFocusEvent)
                     {
-                        OldFocus = FocusedElement,
-                        NewFocus = element,
+                        OldFocusedElement = FocusedElement,
+                        NewFocusedElement = losingFocus.NewFocusedElement,
                         NavigationMethod = method,
                         KeyModifiers = keyModifiers,
-                        IsCancellable = isFocusChangeCancellable
+                        CanCancelOrRedirectFocus = isFocusChangeCancellable
                     };
 
                     newFocus.RaiseEvent(gettingFocus);
@@ -182,6 +182,8 @@ public void SetFocusedElement(
                     {
                         changeFocus = false;
                     }
+
+                    element = gettingFocus.NewFocusedElement;
                 }
 
                 if (changeFocus)

tests/Avalonia.Base.UnitTests/Input/KeyboardDeviceTests.cs

@@ -185,5 +185,28 @@ public void Cancelled_Focus_Change_Should_Not_Send_Got_Focus_Event()
 
             Assert.True(focusCancelled);
         }
+
+        [Fact]
+        public void Redirected_Focus_Should_Change_Focused_Element()
+        {
+            var target = new KeyboardDevice();
+            var first = new Control();
+            var second = new Control();
+            var stack = new StackPanel();
+            stack.Children.AddRange(new[] { first, second });
+            var root = new TestRoot(stack);
+
+            first.GettingFocus += (s, e) =>
+            {
+                e.TrySetNewFocusedElement(second);
+            };
+
+            target.SetFocusedElement(
+                first,
+                NavigationMethod.Unspecified,
+                KeyModifiers.None);
+
+            Assert.True(second.IsFocused);
+        }
     }
 }