Focus next/previous input element with Tab

src/Library/TerminalUI/FocusManager.cs

@@ -1,9 +1,13 @@
-using TerminalUI.Traits;
+using System.Diagnostics.Contracts;
+using GeneralInputKey;
+using TerminalUI.Controls;
+using TerminalUI.Traits;
 
 namespace TerminalUI;
 
 public class FocusManager : IFocusManager
 {
+    private readonly IRenderEngine _renderEngine;
     private IFocusable? _focused;
 
     public IFocusable? Focused
@@ -32,6 +36,11 @@ public class FocusManager : IFocusManager
         private set => _focused = value;
     }
 
+    public FocusManager(IRenderEngine renderEngine)
+    {
+        _renderEngine = renderEngine;
+    }
+
     public void SetFocus(IFocusable focusable) => Focused = focusable;
 
     public void UnFocus(IFocusable focusable)
@@ -39,4 +48,95 @@ public class FocusManager : IFocusManager
         if (Focused == focusable)
             Focused = null;
     }
+
+    public void HandleKeyInput(GeneralKeyEventArgs keyEventArgs)
+    {
+        if (keyEventArgs.Handled || Focused is null) return;
+
+        if (keyEventArgs.Key == Keys.Tab && keyEventArgs.SpecialKeysStatus.IsShiftPressed)
+        {
+            FocusElement(
+                (views, from) => views.TakeWhile(x => x != from).Reverse(),
+                c => c.Reverse()
+            );
+            keyEventArgs.Handled = true;
+        }
+        else if (keyEventArgs.Key == Keys.Tab)
+        {
+            FocusElement(
+                (views, from) => views.SkipWhile(x => x != from).Skip(1),
+                c => c
+            );
+            keyEventArgs.Handled = true;
+        }
+    }
+
+
+    private void FocusElement(
+        Func<IEnumerable<IView>, IView, IEnumerable<IView>> fromChildSelector,
+        Func<IEnumerable<IView>, IEnumerable<IView>> childSelector
+    )
+    {
+        if (Focused is null) return;
+
+        var element = FindElement(Focused,
+            Focused,
+            fromChildSelector
+        );
+
+        if (element is null)
+        {
+            var topParent = FindLastFocusParent(Focused);
+            element = FindElement(
+                topParent,
+                Focused,
+                fromChildSelector,
+                childSelector
+            );
+        }
+
+        if (element is null) return;
+
+        _renderEngine.RequestRerender(element);
+        _renderEngine.RequestRerender(Focused);
+        Focused = element;
+    }
+
+    [Pure]
+    private static IFocusable? FindElement(IView view, IView original,
+        Func<IEnumerable<IView>, IView, IEnumerable<IView>> fromChildSelector,
+        Func<IEnumerable<IView>, IEnumerable<IView>>? childSelector = null,
+        IView? from = null)
+    {
+        if (!view.IsVisible) return null;
+        childSelector ??= views => views;
+
+        if (view != original && view is IFocusable focusable)
+            return focusable;
+
+        var visualChildren = from != null && view.VisualChildren.Contains(from)
+            ? fromChildSelector(view.VisualChildren, from)
+            : childSelector(view.VisualChildren);
+
+        foreach (var viewVisualChild in visualChildren)
+        {
+            var result = FindElement(viewVisualChild, original, fromChildSelector, childSelector);
+            if (result is not null)
+            {
+                return result;
+            }
+        }
+
+        if (view.VisualParent is null || view.IsFocusBoundary)
+            return null;
+
+        return FindElement(view.VisualParent, original, fromChildSelector, childSelector, view);
+    }
+
+    [Pure]
+    private static IView FindLastFocusParent(IView view)
+    {
+        if (view.IsFocusBoundary || view.VisualParent is null) return view;
+        return FindLastFocusParent(view.VisualParent);
+    }
 }