Platform accessibility text scaling

samples/ControlCatalog/MainView.xaml

@@ -6,7 +6,8 @@
              xmlns:models="using:ControlCatalog.Models"
              xmlns:pages="using:ControlCatalog.Pages"
              xmlns:viewModels="using:ControlCatalog.ViewModels"
-             x:DataType="viewModels:MainWindowViewModel">
+             x:DataType="viewModels:MainWindowViewModel"
+             TextScaling.IsEnabled="True">
   <Grid>
     <Grid.Styles>
       <Style Selector="TextBlock.h2">

src/Avalonia.Base/ITextScaler.cs

@@ -0,0 +1,16 @@
+using System;
+
+namespace Avalonia;
+
+/// <summary>
+/// Represents an object which can algoritmically scale text.
+/// </summary>
+public interface ITextScaler
+{
+    double GetScaledFontSize(Visual target, double baseFontSize);
+
+    /// <summary>
+    /// Raised when the text scaling algorithm has changed. Indicates that all text should be rescaled.
+    /// </summary>
+    event EventHandler<EventArgs>? TextScalingChanged;
+}

src/Avalonia.Base/Platform/DefaultPlatformSettings.cs

@@ -1,10 +1,8 @@
 using System;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
-using Avalonia.Media;
 using Avalonia.Metadata;
 using Avalonia.Threading;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Platform
 {
@@ -49,10 +47,16 @@ public virtual PlatformColorValues GetColorValues()
 
         public virtual event EventHandler<PlatformColorValues>? ColorValuesChanged;
 
-        protected void OnColorValuesChanged(PlatformColorValues colorValues)
+        protected virtual void OnColorValuesChanged(PlatformColorValues colorValues)
         {
             Dispatcher.UIThread.Send(
                 _ => ColorValuesChanged?.Invoke(this, colorValues));
         }
+
+        public event EventHandler<EventArgs>? TextScalingChanged;
+
+        protected virtual void OnTextScaleChanged() => Dispatcher.UIThread.Send(_ => TextScalingChanged?.Invoke(this, EventArgs.Empty));
+
+        public virtual double GetScaledFontSize(Visual target, double baseFontSize) => baseFontSize;
     }
 }

src/Avalonia.Base/Platform/IPlatformSettings.cs

@@ -10,7 +10,7 @@ namespace Avalonia.Platform
     /// Some of these settings might be changed by used globally in the OS in runtime.
     /// </summary>
     [NotClientImplementable]
-    public interface IPlatformSettings
+    public interface IPlatformSettings : ITextScaler
     {
         /// <summary>
         /// The size of the rectangle around the location of a pointer down that a pointer up
@@ -42,7 +42,7 @@ public interface IPlatformSettings
         /// Get a configuration for platform-specific hotkeys in an Avalonia application.
         /// </summary>
         PlatformHotkeyConfiguration HotkeyConfiguration { get; }
-        
+
         /// <summary>
         /// Gets current system color values including dark mode and accent colors.
         /// </summary>

src/Avalonia.Controls/Documents/IInlineHost.cs

@@ -3,7 +3,7 @@
 
 namespace Avalonia.Controls.Documents
 {
-    internal interface IInlineHost : ILogical
+    internal interface IInlineHost : ILogical, ITextScaleable
     {
         void Invalidate();
 

src/Avalonia.Controls/Documents/Inline.cs

@@ -81,7 +81,7 @@ protected TextRunProperties CreateTextRunProperties()
 
             return new GenericTextRunProperties(
                 typeface,
-                FontSize,
+                InlineHost is Visual hostVisual ? TextScaling.GetScaledFontSize(hostVisual, FontSize) : FontSize,
                 TextDecorations,
                 Foreground,
                 parentOrSelfBackground,

src/Avalonia.Controls/Documents/TextElement.cs

@@ -354,6 +354,12 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
         {
             base.OnPropertyChanged(change);
 
+            if (change.Property.OwnerType == typeof(TextScaling))
+            {
+                InlineHost?.Invalidate();
+                return;
+            }
+
             switch (change.Property.Name)
             {
                 case nameof(Background):
@@ -363,7 +369,7 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
                 case nameof(FontWeight):
                 case nameof(FontStretch):
                 case nameof(Foreground):
-                    InlineHost?.Invalidate();
+                InlineHost?.Invalidate();
                     break;
             }
         }

src/Avalonia.Controls/ITextScaleable.cs

@@ -0,0 +1,13 @@
+namespace Avalonia.Controls;
+
+/// <summary>
+/// Represents an object which particpates in <see cref="TextScaling"/>.
+/// </summary>
+/// <seealso cref="ITextScaler"/>
+public interface ITextScaleable
+{
+    /// <summary>
+    /// Called when the active text scaling algorithm for this object changes.
+    /// </summary>
+    void OnTextScalingChanged();
+}

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -1,9 +1,7 @@
 using System;
 using System.Collections.Generic;
-using System.Data;
 using Avalonia.Controls.Documents;
 using Avalonia.Controls.Primitives;
-using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Media.Immutable;
@@ -15,7 +13,7 @@
 
 namespace Avalonia.Controls.Presenters
 {
-    public class TextPresenter : Control
+    public class TextPresenter : Control, ITextScaleable
     {
         public static readonly StyledProperty<bool> ShowSelectionHighlightProperty =
             AvaloniaProperty.Register<TextPresenter, bool>(nameof(ShowSelectionHighlight), defaultValue: true);
@@ -58,7 +56,7 @@ public class TextPresenter : Control
         /// </summary>
         public static readonly StyledProperty<string?> PreeditTextProperty =
             AvaloniaProperty.Register<TextPresenter, string?>(nameof(PreeditText));
-        
+
         /// <summary>
         /// Defines the <see cref="PreeditText"/> property.
         /// </summary>
@@ -148,7 +146,7 @@ public string? PreeditText
             get => GetValue(PreeditTextProperty);
             set => SetValue(PreeditTextProperty, value);
         }
-        
+
         public int? PreeditTextCursorPosition
         {
             get => GetValue(PreeditTextCursorPositionProperty);
@@ -335,6 +333,8 @@ public int SelectionEnd
 
         internal TextSelectionHandleCanvas? TextSelectionHandleCanvas { get; set; }
 
+        void ITextScaleable.OnTextScalingChanged() => InvalidateMeasure();
+
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
         /// </summary>
@@ -349,10 +349,13 @@ private TextLayout CreateTextLayoutInternal(Size constraint, string? text, Typef
             var maxWidth = MathUtilities.IsZero(constraint.Width) ? double.PositiveInfinity : constraint.Width;
             var maxHeight = MathUtilities.IsZero(constraint.Height) ? double.PositiveInfinity : constraint.Height;
 
+            var effectiveFontSize = TextScaling.GetScaledFontSize(this, FontSize);
+            var fontScaleFactor = effectiveFontSize / FontSize;
+
             var textLayout = new TextLayout(
                 text,
                 typeface,
-                FontSize,
+                effectiveFontSize,
                 Foreground,
                 TextAlignment,
                 TextWrapping,
@@ -361,8 +364,8 @@ private TextLayout CreateTextLayoutInternal(Size constraint, string? text, Typef
                 FlowDirection,
                 maxWidth,
                 maxHeight,
-                LineHeight,
-                LetterSpacing,
+                LineHeight * fontScaleFactor,
+                LetterSpacing * fontScaleFactor,
                 0,
                 FontFeatures,
                 textStyleOverrides);
@@ -554,6 +557,7 @@ protected virtual TextLayout CreateTextLayout()
             var preeditText = PreeditText;
             var text = GetCombinedText(Text, caretIndex, preeditText);
             var typeface = new Typeface(FontFamily, FontStyle, FontWeight, FontStretch);
+            var effectiveFontSize = TextScaling.GetScaledFontSize(this, FontSize);
             var selectionStart = SelectionStart;
             var selectionEnd = SelectionEnd;
             var start = Math.Min(selectionStart, selectionEnd);
@@ -568,7 +572,7 @@ protected virtual TextLayout CreateTextLayout()
                 var preeditHighlight = new ValueSpan<TextRunProperties>(caretIndex, preeditText.Length,
                         new GenericTextRunProperties(
                             typeface,
-                            FontSize,
+                            effectiveFontSize,
                             TextDecorations.Underline,
                             foreground,
                             fontFeatures: FontFeatures));
@@ -587,7 +591,7 @@ protected virtual TextLayout CreateTextLayout()
                         new ValueSpan<TextRunProperties>(start, length,
                         new GenericTextRunProperties(
                             typeface,
-                            FontSize,
+                            effectiveFontSize,
                             foregroundBrush: SelectionForegroundBrush,
                             fontFeatures: FontFeatures))
                     };
@@ -766,7 +770,7 @@ public void MoveCaretVertical(LogicalDirection direction = LogicalDirection.Forw
 
             CaretChanged();
         }
-        
+
         private void EnsureCaretTimer()
         {
             if (_caretTimer == null)
@@ -792,7 +796,7 @@ private void ResetCaretTimer()
                 _caretTimer = null;
             }
 
-            if (CaretBlinkInterval.TotalMilliseconds > 0) 
+            if (CaretBlinkInterval.TotalMilliseconds > 0)
             {
                 _caretTimer = new DispatcherTimer { Interval = CaretBlinkInterval };
                 _caretTimer.Tick += CaretTimerTick;
@@ -989,7 +993,7 @@ protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e
                 _caretTimer.Tick -= CaretTimerTick;
             }
         }
-        
+
         private void OnPreeditChanged(string? preeditText, int? cursorPosition)
         {
             if (string.IsNullOrEmpty(preeditText))
@@ -1020,7 +1024,7 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
             {
                 OnPreeditChanged(change.NewValue as string, PreeditTextCursorPosition);
             }
-            
+
             if(change.Property == PreeditTextCursorPositionProperty)
             {
                 OnPreeditChanged(PreeditText, PreeditTextCursorPosition);
@@ -1047,6 +1051,11 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
                 ResetCaretTimer();
             }
 
+            if (change.Property.OwnerType == typeof(TextScaling))
+            {
+                InvalidateTextLayout();
+            }
+
             switch (change.Property.Name)
             {
                 case nameof(PreeditText):

src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs

@@ -154,7 +154,7 @@ private void DragSelectionHandle(TextSelectionHandle handle)
                 }
 
                 var point = ToPresenter(handle.IndicatorPosition);
-                point = point.WithY(point.Y - _presenter.FontSize / 2);
+                point = point.WithY(point.Y - TextScaling.GetScaledFontSize(_presenter, _presenter.FontSize) / 2);
                 var hit = _presenter.TextLayout.HitTestPoint(point);
                 var position = hit.CharacterHit.FirstCharacterIndex + hit.CharacterHit.TrailingLength;
 
@@ -327,9 +327,6 @@ internal bool ShowContextMenu()
             {
                 if (_textBox.ContextFlyout is PopupFlyoutBase flyout)
                 {
-                    var verticalOffset = (double.IsNaN(_textBox.LineHeight) ? _textBox.FontSize : _textBox.LineHeight) +
-                                         ContextMenuPadding;
-
                     TextSelectionHandle? handle = null;
 
                     if (_textBox.SelectionStart != _textBox.SelectionEnd)
@@ -349,6 +346,7 @@ internal bool ShowContextMenu()
 
                     if (handle != null)
                     {
+                        var verticalOffset = (_textBox.GetLineHeight() ?? 0) + ContextMenuPadding;
                         var topLeft = ToTextBox(handle.GetTopLeft());
                         flyout.VerticalOffset = topLeft.Y - verticalOffset;
                         flyout.HorizontalOffset = topLeft.X;

src/Avalonia.Controls/SelectableTextBlock.cs

@@ -183,18 +183,20 @@ protected override void OnLostFocus(RoutedEventArgs e)
         protected override TextLayout CreateTextLayout(string? text)
         {
             var typeface = new Typeface(FontFamily, FontStyle, FontWeight, FontStretch);
+            var effectiveFontSize = TextScaling.GetScaledFontSize(this, FontSize);
+            var fontScaleFactor = effectiveFontSize / FontSize;
 
             var defaultProperties = new GenericTextRunProperties(
                 typeface,
-                FontSize,
+                effectiveFontSize,
                 TextDecorations,
                 Foreground,
                 fontFeatures: FontFeatures);
 
             var paragraphProperties = new GenericTextParagraphProperties(FlowDirection, TextAlignment, true, false,
-                defaultProperties, TextWrapping, LineHeight, 0, LetterSpacing)
+                defaultProperties, TextWrapping, LineHeight * fontScaleFactor, 0, LetterSpacing * fontScaleFactor)
             {
-                LineSpacing = LineSpacing
+                LineSpacing = LineSpacing * fontScaleFactor,
             };
 
             List<ValueSpan<TextRunProperties>>? textStyleOverrides = null;
@@ -235,7 +237,7 @@ protected override TextLayout CreateTextLayout(string? text)
                                 overlapLength,
                                 new GenericTextRunProperties(
                                     textRun.Properties?.Typeface ?? typeface,
-                                    FontSize,
+                                    effectiveFontSize,
                                     foregroundBrush: SelectionForegroundBrush,
                                     fontFeatures: textRun.Properties?.FontFeatures ?? FontFeatures)));
 
@@ -249,7 +251,7 @@ protected override TextLayout CreateTextLayout(string? text)
                         new ValueSpan<TextRunProperties>(start, length,
                             new GenericTextRunProperties(
                                 typeface,
-                                FontSize,
+                                effectiveFontSize,
                                 foregroundBrush: SelectionForegroundBrush,
                                 fontFeatures: FontFeatures))
                     ];