Add ability to cancel focus change (#18373)

* implement pre events for focus change

* add cancelling api to focus change

* add overload for SetFocusedElement

* add focus redirection

* update with api change requests

src/Avalonia.Base/Input/FocusChangingEventArgs.cs

@@ -0,0 +1,70 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Avalonia.Interactivity;
+
+namespace Avalonia.Input
+{
+    public class FocusChangingEventArgs : RoutedEventArgs
+    {
+        /// <summary>
+        /// Provides data for focus changing.
+        /// </summary>
+        internal FocusChangingEventArgs(RoutedEvent routedEvent) : base(routedEvent)
+        {
+        }
+
+        /// <summary>
+        /// Gets or sets the element that focus has moved to.
+        /// </summary>
+        public IInputElement? NewFocusedElement { get; internal set; }
+
+        /// <summary>
+        /// Gets or sets the element that previously had focus.
+        /// </summary>
+        public IInputElement? OldFocusedElement { get; init; }
+
+        /// <summary>
+        /// Gets or sets a value indicating how the change in focus occurred.
+        /// </summary>
+        public NavigationMethod NavigationMethod { get; init; }
+
+        /// <summary>
+        /// Gets or sets any key modifiers active at the time of focus.
+        /// </summary>
+        public KeyModifiers KeyModifiers { get; init; }
+
+        /// <summary>
+        /// Gets whether focus change is canceled.
+        /// </summary>
+        public bool Canceled { get; private set; }
+
+        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()
+        {
+            Canceled = CanCancelOrRedirectFocus;
+
+            return Canceled;
+        }
+
+        /// <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/FocusManager.cs

@@ -78,7 +78,7 @@ namespace Avalonia.Input
             else
             {
                 _focusRoot = null;
-                keyboardDevice.SetFocusedElement(null, NavigationMethod.Unspecified, KeyModifiers.None);
+                keyboardDevice.SetFocusedElement(null, NavigationMethod.Unspecified, KeyModifiers.None, false);
                 return false;
             }
         }

src/Avalonia.Base/Input/InputElement.cs

@@ -82,12 +82,24 @@ namespace Avalonia.Input
         public static readonly RoutedEvent<GotFocusEventArgs> GotFocusEvent =
             RoutedEvent.Register<InputElement, GotFocusEventArgs>(nameof(GotFocus), RoutingStrategies.Bubble);
 
+        /// <summary>
+        /// Defines the <see cref="GettingFocus"/> event.
+        /// </summary>
+        public static readonly RoutedEvent<FocusChangingEventArgs> GettingFocusEvent =
+            RoutedEvent.Register<InputElement, FocusChangingEventArgs>(nameof(GettingFocus), RoutingStrategies.Bubble);
+
         /// <summary>
         /// Defines the <see cref="LostFocus"/> event.
         /// </summary>
         public static readonly RoutedEvent<RoutedEventArgs> LostFocusEvent =
             RoutedEvent.Register<InputElement, RoutedEventArgs>(nameof(LostFocus), RoutingStrategies.Bubble);
 
+        /// <summary>
+        /// Defines the <see cref="LosingFocus"/> event.
+        /// </summary>
+        public static readonly RoutedEvent<FocusChangingEventArgs> LosingFocusEvent =
+            RoutedEvent.Register<InputElement, FocusChangingEventArgs>(nameof(LosingFocus), RoutingStrategies.Bubble);
+
         /// <summary>
         /// Defines the <see cref="KeyDown"/> event.
         /// </summary>
@@ -213,6 +225,8 @@ namespace Avalonia.Input
 
             GotFocusEvent.AddClassHandler<InputElement>((x, e) => x.OnGotFocusCore(e));
             LostFocusEvent.AddClassHandler<InputElement>((x, e) => x.OnLostFocusCore(e));
+            GettingFocusEvent.AddClassHandler<InputElement>((x, e) => x.OnGettingFocus(e));
+            LosingFocusEvent.AddClassHandler<InputElement>((x, e) => x.OnLosingFocus(e));
             KeyDownEvent.AddClassHandler<InputElement>((x, e) => x.OnKeyDown(e));
             KeyUpEvent.AddClassHandler<InputElement>((x, e) => x.OnKeyUp(e));
             TextInputEvent.AddClassHandler<InputElement>((x, e) => x.OnTextInput(e));
@@ -249,6 +263,15 @@ namespace Avalonia.Input
             remove { RemoveHandler(GotFocusEvent, value); }
         }
 
+        /// <summary>
+        /// Occurs before the control receives focus.
+        /// </summary>
+        public event EventHandler<FocusChangingEventArgs>? GettingFocus
+        {
+            add { AddHandler(GettingFocusEvent, value); }
+            remove { RemoveHandler(GettingFocusEvent, value); }
+        }
+
         /// <summary>
         /// Occurs when the control loses focus.
         /// </summary>
@@ -258,6 +281,15 @@ namespace Avalonia.Input
             remove { RemoveHandler(LostFocusEvent, value); }
         }
 
+        /// <summary>
+        /// Occurs before the control loses focus.
+        /// </summary>
+        public event EventHandler<FocusChangingEventArgs>? LosingFocus
+        {
+            add { AddHandler(LosingFocusEvent, value); }
+            remove { RemoveHandler(LosingFocusEvent, value); }
+        }
+
         /// <summary>
         /// Occurs when a key is pressed while the control has focus.
         /// </summary>
@@ -543,6 +575,16 @@ namespace Avalonia.Input
             OnGotFocus(e);
         }
 
+        protected virtual void OnGettingFocus(FocusChangingEventArgs e)
+        {
+
+        }
+
+        protected virtual void OnLosingFocus(FocusChangingEventArgs e)
+        {
+
+        }
+
         /// <summary>
         /// Invoked when an unhandled <see cref="GotFocusEvent"/> reaches an element in its 
         /// route that is derived from this class. Implement this method to add class handling 

src/Avalonia.Base/Input/KeyboardDevice.cs

@@ -129,41 +129,89 @@ namespace Avalonia.Input
         }
 
         public void SetFocusedElement(
-            IInputElement? element, 
+            IInputElement? element,
             NavigationMethod method,
             KeyModifiers keyModifiers)
+        {
+            SetFocusedElement(element, method, keyModifiers, true);
+        }
+
+
+        public void SetFocusedElement(
+            IInputElement? element,
+            NavigationMethod method,
+            KeyModifiers keyModifiers,
+            bool isFocusChangeCancellable)
         {
             if (element != FocusedElement)
             {
                 var interactive = FocusedElement as Interactive;
 
-                if (FocusedElement != null && 
-                    (!((Visual)FocusedElement).IsAttachedToVisualTree ||
-                     _focusedRoot != ((Visual?)element)?.VisualRoot as IInputRoot) &&
-                    _focusedRoot != null)
+                bool changeFocus = true;
+
+                var losingFocus = new FocusChangingEventArgs(InputElement.LosingFocusEvent)
                 {
-                    ClearChildrenFocusWithin(_focusedRoot, true);
+                    OldFocusedElement = FocusedElement,
+                    NewFocusedElement = element,
+                    NavigationMethod = method,
+                    KeyModifiers = keyModifiers,
+                    CanCancelOrRedirectFocus = isFocusChangeCancellable
+                };
+
+                interactive?.RaiseEvent(losingFocus);
+
+                if (losingFocus.Canceled)
+                {
+                    changeFocus = false;
                 }
-                
-                SetIsFocusWithin(FocusedElement, element);
-                _focusedElement = element;
-                _focusedRoot = ((Visual?)_focusedElement)?.VisualRoot as IInputRoot;
 
-                interactive?.RaiseEvent(new RoutedEventArgs
+                if (changeFocus && losingFocus.NewFocusedElement is Interactive newFocus)
                 {
-                    RoutedEvent = InputElement.LostFocusEvent,
-                });
+                    var gettingFocus = new FocusChangingEventArgs(InputElement.GettingFocusEvent)
+                    {
+                        OldFocusedElement = FocusedElement,
+                        NewFocusedElement = losingFocus.NewFocusedElement,
+                        NavigationMethod = method,
+                        KeyModifiers = keyModifiers,
+                        CanCancelOrRedirectFocus = isFocusChangeCancellable
+                    };
 
-                interactive = element as Interactive;
+                    newFocus.RaiseEvent(gettingFocus);
 
-                interactive?.RaiseEvent(new GotFocusEventArgs
+                    if (gettingFocus.Canceled)
+                    {
+                        changeFocus = false;
+                    }
+
+                    element = gettingFocus.NewFocusedElement;
+                }
+
+                if (changeFocus)
                 {
-                    NavigationMethod = method,
-                    KeyModifiers = keyModifiers,
-                });
+                    // Clear keyboard focus from currently focused element
+                    if (FocusedElement != null &&
+                        (!((Visual)FocusedElement).IsAttachedToVisualTree ||
+                         _focusedRoot != ((Visual?)element)?.VisualRoot as IInputRoot) &&
+                        _focusedRoot != null)
+                    {
+                        ClearChildrenFocusWithin(_focusedRoot, true);
+                    }
+
+                    SetIsFocusWithin(FocusedElement, element);
+                    _focusedElement = element;
+                    _focusedRoot = ((Visual?)_focusedElement)?.VisualRoot as IInputRoot;
+
+                    interactive?.RaiseEvent(new RoutedEventArgs(InputElement.LostFocusEvent));
 
-                _textInputManager.SetFocusedElement(element);
-                RaisePropertyChanged(nameof(FocusedElement));
+                    (element as Interactive)?.RaiseEvent(new GotFocusEventArgs
+                    {
+                        NavigationMethod = method,
+                        KeyModifiers = keyModifiers,
+                    });
+
+                    _textInputManager.SetFocusedElement(element);
+                    RaisePropertyChanged(nameof(FocusedElement));
+                }
             }
         }
 

src/Avalonia.Controls/TopLevel.cs

@@ -965,7 +965,7 @@ namespace Avalonia.Controls
                 focused = focused.VisualParent;
 
             if (focused == this)
-                KeyboardDevice.Instance?.SetFocusedElement(null, NavigationMethod.Unspecified, KeyModifiers.None);
+                KeyboardDevice.Instance?.SetFocusedElement(null, NavigationMethod.Unspecified, KeyModifiers.None, false);
         }
 
         protected override bool BypassFlowDirectionPolicies => true;

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

@@ -159,5 +159,54 @@ namespace Avalonia.Base.UnitTests.Input
 
             Assert.Equal(1, propertyChangedRaised);
         }
+
+        [Fact]
+        public void Cancelled_Focus_Change_Should_Not_Send_Got_Focus_Event()
+        {
+            var target = new KeyboardDevice();
+            var focused = new Control();
+            var root = new TestRoot();
+            bool focusCancelled = false;
+
+            focused.GettingFocus += (s, e) =>
+            {
+                focusCancelled = e.TryCancel();
+            };
+
+            focused.GotFocus += (s, e) =>
+            {
+                focusCancelled = false;
+            };
+
+            target.SetFocusedElement(
+                focused,
+                NavigationMethod.Unspecified,
+                KeyModifiers.None);
+
+            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);
+        }
     }
 }