Added support for platform text scaling, disabled by default

Only TextBlock, Inline, and TextPresenter scale text by themselves
No change to low-level text rendering or FontSize values
Implemented for iOS and Windows
Windows scaling is currently uniform regardless of base font size, which is incorrect

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"
+             TextElement.IsPlatformTextScalingEnabled="True">
   <Grid>
     <Grid.Styles>
       <Style Selector="TextBlock.h2">

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 @@ namespace Avalonia.Platform
 
         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(double baseFontSize) => baseFontSize;
     }
 }

src/Avalonia.Base/Platform/IPlatformSettings.cs

@@ -42,7 +42,7 @@ namespace Avalonia.Platform
         /// 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>
@@ -52,5 +52,15 @@ namespace Avalonia.Platform
         /// Raises when current system color values are changed. Including changing of a dark mode and accent colors.
         /// </summary>
         event EventHandler<PlatformColorValues>? ColorValuesChanged;
+
+        /// <summary>
+        /// Raised when the system text scaling changes.
+        /// </summary>
+        event EventHandler<EventArgs>? TextScalingChanged;
+
+        /// <summary>
+        /// Scales a font size according to the current system text scaling rules.
+        /// </summary>
+        double GetScaledFontSize(double baseFontSize);
     }
 }

src/Avalonia.Controls/Documents/IInlineHost.cs

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

src/Avalonia.Controls/Documents/Inline.cs

@@ -83,7 +83,7 @@ namespace Avalonia.Controls.Documents
             return new GenericTextRunProperties(
                 typeface,
                 FontFeatures, 
-                FontSize,
+                InlineHost?.GetScaledFontSize(FontSize) ?? FontSize,
                 TextDecorations, 
                 Foreground,
                 parentOrSelfBackground,

src/Avalonia.Controls/Documents/TextElement.cs

@@ -14,6 +14,15 @@ namespace Avalonia.Controls.Documents
         public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<TextElement>();
 
+        /// <summary>
+        /// Defines the <see cref="IsPlatformTextScalingEnabled"/> property.
+        /// </summary>
+        public static readonly AttachedProperty<bool> IsPlatformTextScalingEnabledProperty =
+            AvaloniaProperty.RegisterAttached<TextElement, Control, bool>(
+                nameof(IsPlatformTextScalingEnabled),
+                defaultValue: false,
+                inherits: true);
+
         /// <summary>
         /// Defines the <see cref="FontFamily"/> property.
         /// </summary>
@@ -102,6 +111,16 @@ namespace Avalonia.Controls.Documents
             set => SetValue(BackgroundProperty, value);
         }
 
+        /// <summary>
+        /// Gets or sets whether the <see cref="FontSize"/> should be scaled according to platform text scaling
+        /// rules when measuring and rendering this control.
+        /// </summary>
+        public bool IsPlatformTextScalingEnabled
+        {
+            get => GetValue(IsPlatformTextScalingEnabledProperty);
+            set => SetValue(IsPlatformTextScalingEnabledProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the font family.
         /// </summary>
@@ -174,6 +193,9 @@ namespace Avalonia.Controls.Documents
             set => SetValue(LetterSpacingProperty, value);
         }
 
+        public static bool GetIsPlatformTextScalingEnabled(Control control) => control.GetValue(IsPlatformTextScalingEnabledProperty);
+        public static void SetIsPlatformTextScalingEnabled(Control control, bool value) => control.SetValue(IsPlatformTextScalingEnabledProperty, value);
+
         /// <summary>
         /// Gets the value of the attached <see cref="FontFamilyProperty"/> on a control.
         /// </summary>
@@ -359,6 +381,7 @@ namespace Avalonia.Controls.Documents
                 case nameof(Background):
                 case nameof(FontFamily):
                 case nameof(FontSize):
+                case nameof(IsPlatformTextScalingEnabled):
                 case nameof(FontStyle):
                 case nameof(FontWeight):
                 case nameof(FontStretch):

src/Avalonia.Controls/IPlatformTextScaleable.cs

@@ -0,0 +1,17 @@
+namespace Avalonia.Controls;
+
+/// <summary>
+/// Represents an object which particpates in platform text scaling. This is an accessibility feature
+/// which allows the user to request that text be drawn larger (or on some platforms smaller) than normal,
+/// without altering other UI elements.
+/// </summary>
+public interface IPlatformTextScaleable
+{
+    bool IsPlatformTextScalingEnabled { get; }
+    void OnPlatformTextScalingChanged();
+
+    /// <summary>
+    /// Scales a font size according to the current system text scaling rules and the value of <see cref="IsPlatformTextScalingEnabled"/>.
+    /// </summary>
+    double GetScaledFontSize(double baseFontSize);
+}

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -15,7 +15,7 @@ using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Presenters
 {
-    public class TextPresenter : Control
+    public class TextPresenter : Control, IPlatformTextScaleable
     {
         public static readonly StyledProperty<bool> ShowSelectionHighlightProperty =
             AvaloniaProperty.Register<TextPresenter, bool>(nameof(ShowSelectionHighlight), defaultValue: true);
@@ -47,6 +47,12 @@ namespace Avalonia.Controls.Presenters
         public static readonly StyledProperty<int> SelectionEndProperty =
             TextBox.SelectionEndProperty.AddOwner<TextPresenter>(new(coerce: TextBox.CoerceCaretIndex));
 
+        /// <summary>
+        /// Defines the <see cref="IsPlatformTextScalingEnabled"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsPlatformTextScalingEnabledProperty =
+            TextElement.IsPlatformTextScalingEnabledProperty.AddOwner<TextPresenter>();
+
         /// <summary>
         /// Defines the <see cref="Text"/> property.
         /// </summary>
@@ -124,6 +130,13 @@ namespace Avalonia.Controls.Presenters
             set => SetValue(BackgroundProperty, value);
         }
 
+        /// <inheritdoc cref="TextElement.IsPlatformTextScalingEnabled"/>
+        public bool IsPlatformTextScalingEnabled
+        {
+            get => GetValue(IsPlatformTextScalingEnabledProperty);
+            set => SetValue(IsPlatformTextScalingEnabledProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets a value that determines whether the TextPresenter shows a selection highlight.
         /// </summary>
@@ -181,6 +194,13 @@ namespace Avalonia.Controls.Presenters
             get => TextElement.GetFontSize(this);
             set => TextElement.SetFontSize(this, value);
         }
+        
+        double IPlatformTextScaleable.GetScaledFontSize(double baseFontSize) => IsPlatformTextScalingEnabled && TopLevel.GetTopLevel(this) is { PlatformSettings: { } platformSettings } ? platformSettings.GetScaledFontSize(baseFontSize) : baseFontSize;
+        
+        /// <summary>
+        /// Gets <see cref="FontSize"/> scaled according to the platform's current text scaling rules.
+        /// </summary>
+        protected double EffectiveFontSize => ((IPlatformTextScaleable)this).GetScaledFontSize(FontSize);
 
         /// <summary>
         /// Gets or sets the font style.
@@ -335,6 +355,14 @@ namespace Avalonia.Controls.Presenters
 
         internal TextSelectionHandleCanvas? TextSelectionHandleCanvas { get; set; }
 
+        void IPlatformTextScaleable.OnPlatformTextScalingChanged()
+        {
+            if (IsPlatformTextScalingEnabled)
+            {
+                InvalidateMeasure();
+            }
+        }
+
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
         /// </summary>
@@ -350,7 +378,7 @@ namespace Avalonia.Controls.Presenters
             var maxWidth = MathUtilities.IsZero(constraint.Width) ? double.PositiveInfinity : constraint.Width;
             var maxHeight = MathUtilities.IsZero(constraint.Height) ? double.PositiveInfinity : constraint.Height;
 
-            var textLayout = new TextLayout(text, typeface, FontFeatures, FontSize, foreground, TextAlignment,
+            var textLayout = new TextLayout(text, typeface, FontFeatures, EffectiveFontSize, foreground, TextAlignment,
                 TextWrapping, maxWidth: maxWidth, maxHeight: maxHeight, textStyleOverrides: textStyleOverrides,
                 flowDirection: FlowDirection, lineHeight: LineHeight, letterSpacing: LetterSpacing);
 
@@ -553,7 +581,7 @@ namespace Avalonia.Controls.Presenters
             if (!string.IsNullOrEmpty(preeditText))
             {
                 var preeditHighlight = new ValueSpan<TextRunProperties>(caretIndex, preeditText.Length,
-                        new GenericTextRunProperties(typeface, FontFeatures, FontSize,
+                        new GenericTextRunProperties(typeface, FontFeatures, EffectiveFontSize,
                         foregroundBrush: foreground,
                         textDecorations: TextDecorations.Underline));
 
@@ -569,7 +597,7 @@ namespace Avalonia.Controls.Presenters
                     textStyleOverrides = new[]
                     {
                         new ValueSpan<TextRunProperties>(start, length,
-                        new GenericTextRunProperties(typeface, FontFeatures, FontSize,
+                        new GenericTextRunProperties(typeface, FontFeatures, EffectiveFontSize,
                             foregroundBrush: SelectionForegroundBrush))
                     };
                 }

src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs

@@ -154,7 +154,7 @@ namespace Avalonia.Controls.Primitives
                 }
 
                 var point = ToPresenter(handle.IndicatorPosition);
-                point = point.WithY(point.Y - _presenter.FontSize / 2);
+                point = point.WithY(point.Y - ((IPlatformTextScaleable)_presenter).GetScaledFontSize(_presenter.FontSize) / 2);
                 var hit = _presenter.TextLayout.HitTestPoint(point);
                 var position = hit.CharacterHit.FirstCharacterIndex + hit.CharacterHit.TrailingLength;
 

src/Avalonia.Controls/SelectableTextBlock.cs

@@ -187,7 +187,7 @@ namespace Avalonia.Controls
             var defaultProperties = new GenericTextRunProperties(
                 typeface,
                 FontFeatures,
-                FontSize,
+                EffectiveFontSize,
                 TextDecorations,
                 Foreground);
 
@@ -236,7 +236,7 @@ namespace Avalonia.Controls
                                 new GenericTextRunProperties(
                                     textRun.Properties?.Typeface ?? typeface,
                                     textRun.Properties?.FontFeatures ?? FontFeatures,
-                                    FontSize,
+                                    EffectiveFontSize,
                                     foregroundBrush: SelectionForegroundBrush)));
 
                         accumulatedLength += runLength;
@@ -247,7 +247,7 @@ namespace Avalonia.Controls
                     textStyleOverrides =
                     [
                         new ValueSpan<TextRunProperties>(start, length,
-                            new GenericTextRunProperties(typeface, FontFeatures, FontSize,
+                            new GenericTextRunProperties(typeface, FontFeatures, EffectiveFontSize,
                                 foregroundBrush: SelectionForegroundBrush))
                     ];
                 }

src/Avalonia.Controls/TextBlock.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// A control that displays a block of text.
     /// </summary>
-    public class TextBlock : Control, IInlineHost
+    public class TextBlock : Control, IInlineHost, IPlatformTextScaleable
     {
         /// <summary>
         /// Defines the <see cref="Background"/> property.
@@ -23,6 +23,12 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<TextBlock>();
 
+        /// <summary>
+        /// Defines the <see cref="IsPlatformTextScalingEnabled"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsPlatformTextScalingEnabledProperty =
+            TextElement.IsPlatformTextScalingEnabledProperty.AddOwner<TextBlock>();
+
         /// <summary>
         /// Defines the <see cref="Padding"/> property.
         /// </summary>
@@ -98,7 +104,7 @@ namespace Avalonia.Controls
         /// Defines the <see cref="LetterSpacing"/> property.
         /// </summary>
         /// <remarks>
-        /// This property uses <see cref="AvaloniaProperty.AddOwner{TOwner}(AvaloniaProperty)"/> to share the same
+        /// This property uses <see cref="StyledProperty{TValue}.AddOwner{TOwner}(StyledPropertyMetadata{TValue}?)"/> to share the same
         /// definition as <see cref="TextElement.LetterSpacingProperty"/>, ensuring consistent behavior across text
         /// elements and templated controls.
         /// </remarks>
@@ -192,6 +198,13 @@ namespace Avalonia.Controls
         /// </summary>
         public TextLayout TextLayout => _textLayout ??= CreateTextLayout(Text);
 
+        /// <inheritdoc cref="TextElement.IsPlatformTextScalingEnabled"/>
+        public bool IsPlatformTextScalingEnabled
+        {
+            get => GetValue(IsPlatformTextScalingEnabledProperty);
+            set => SetValue(IsPlatformTextScalingEnabledProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the padding to place around the <see cref="Text"/>.
         /// </summary>
@@ -237,6 +250,13 @@ namespace Avalonia.Controls
             set => SetValue(FontSizeProperty, value);
         }
 
+        double IPlatformTextScaleable.GetScaledFontSize(double baseFontSize) => IsPlatformTextScalingEnabled && TopLevel.GetTopLevel(this) is { PlatformSettings: { } platformSettings } ? platformSettings.GetScaledFontSize(baseFontSize) : baseFontSize;
+
+        /// <summary>
+        /// Gets <see cref="FontSize"/> scaled according to the platform's current text scaling rules.
+        /// </summary>
+        protected double EffectiveFontSize => ((IPlatformTextScaleable)this).GetScaledFontSize(FontSize);
+
         /// <summary>
         /// Gets or sets the font style used to draw the control's text.
         /// </summary>
@@ -652,6 +672,14 @@ namespace Avalonia.Controls
             }
         }
 
+        void IPlatformTextScaleable.OnPlatformTextScalingChanged()
+        {
+            if (IsPlatformTextScalingEnabled)
+            {
+                InvalidateMeasure();
+            }
+        }
+
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
         /// </summary>
@@ -663,7 +691,7 @@ namespace Avalonia.Controls
             var defaultProperties = new GenericTextRunProperties(
                 typeface,
                 FontFeatures,
-                FontSize,
+                EffectiveFontSize,
                 TextDecorations,
                 Foreground);
 
@@ -838,6 +866,7 @@ namespace Avalonia.Controls
             switch (change.Property.Name)
             {
                 case nameof(FontSize):
+                case nameof(IsPlatformTextScalingEnabled):
                 case nameof(FontWeight):
                 case nameof(FontStyle):
                 case nameof(FontFamily):

src/Avalonia.Controls/TopLevel.cs

@@ -26,6 +26,7 @@ using System.Threading.Tasks;
 using Avalonia.Diagnostics;
 using Avalonia.Rendering.Composition;
 using Avalonia.Threading;
+using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
 {
@@ -252,6 +253,8 @@ namespace Avalonia.Controls
                 PlatformImpl?.SetFrameThemeVariant((PlatformThemeVariant?)variant ?? PlatformThemeVariant.Light);
             });
 
+            PlatformSettings?.TextScalingChanged += OnPlatformTextScalingChanged;
+
             _keyboardNavigationHandler?.SetOwner(this);
             _accessKeyHandler?.SetOwner(this);
 
@@ -341,6 +344,15 @@ namespace Avalonia.Controls
                 }
             });
         }
+
+        private void OnPlatformTextScalingChanged(object? sender, EventArgs e)
+        {
+            foreach (var scaleable in this.GetVisualDescendants().OfType<IPlatformTextScaleable>())
+            {
+                scaleable.OnPlatformTextScalingChanged();
+            }
+        }
+
         /// <summary>
         /// Fired when the window is opened.
         /// </summary>

src/Windows/Avalonia.Win32/Win32PlatformSettings.cs

@@ -8,11 +8,22 @@ namespace Avalonia.Win32;
 
 internal class Win32PlatformSettings : DefaultPlatformSettings
 {
-    private static readonly Lazy<bool> s_uiSettingsSupported = new(() =>
-        WinRTApiInformation.IsTypePresent("Windows.UI.ViewManagement.UISettings")
-        && WinRTApiInformation.IsTypePresent("Windows.UI.ViewManagement.AccessibilitySettings")); 
-
     private PlatformColorValues? _lastColorValues;
+    private double _textScaleFactor = s_uiSettings2?.TextScaleFactor ?? 1;
+
+    private static readonly IUISettings2? s_uiSettings2;
+    private static readonly bool s_uiSettingsSupported;
+
+    static Win32PlatformSettings()
+    {
+        s_uiSettingsSupported = WinRTApiInformation.IsTypePresent("Windows.UI.ViewManagement.UISettings")
+        && WinRTApiInformation.IsTypePresent("Windows.UI.ViewManagement.AccessibilitySettings");
+
+        if (s_uiSettingsSupported)
+        {
+            s_uiSettings2 = NativeWinRTMethods.CreateInstance<IUISettings2>("Windows.UI.ViewManagement.UISettings");
+        }
+    }
 
     public override Size GetTapSize(PointerType type)
     {
@@ -33,10 +44,10 @@ internal class Win32PlatformSettings : DefaultPlatformSettings
     }
 
     public override TimeSpan GetDoubleTapTime(PointerType type) => TimeSpan.FromMilliseconds(GetDoubleClickTime());
-    
+
     public override PlatformColorValues GetColorValues()
     {
-        if (!s_uiSettingsSupported.Value)
+        if (!s_uiSettingsSupported)
         {
             return base.GetColorValues();
         }
@@ -74,10 +85,10 @@ internal class Win32PlatformSettings : DefaultPlatformSettings
                     PlatformThemeVariant.Light,
                 ContrastPreference = ColorContrastPreference.NoPreference,
                 AccentColor1 = accent
-            };   
+            };
         }
     }
-    
+
     internal void OnColorValuesChanged()
     {
         var oldColorValues = _lastColorValues;
@@ -87,5 +98,25 @@ internal class Win32PlatformSettings : DefaultPlatformSettings
         {
             OnColorValuesChanged(colorValues);
         }
+
+        var newTextScaleFactor = s_uiSettings2?.TextScaleFactor ?? 1;
+        if (newTextScaleFactor != _textScaleFactor)
+        {
+            _textScaleFactor = newTextScaleFactor;
+            OnTextScaleChanged();
+        }
+    }
+
+    /// <summary>
+    /// The algorithm used is undocumented, but <see href="https://github.com/microsoft/microsoft-ui-xaml/blob/5788dee271452753d5b2c70179f976c3e96a45c7/src/dxaml/xcp/core/text/common/TextFormatting.cpp#L181-L204">defined in Microsoft's source code</see>.
+    /// </summary>
+    public override double GetScaledFontSize(double baseFontSize)
+    {
+        if (baseFontSize <= 0)
+        {
+            return baseFontSize;
+        }
+
+        return Math.Max(1, baseFontSize) + Math.Max(-Math.Exp(1) * Math.Log(baseFontSize) + 18, 0.0) * (_textScaleFactor - 1);
     }
 }

src/Windows/Avalonia.Win32/WinRT/winrt.idl

@@ -859,6 +859,15 @@ enum UIColorType
     Complement = 9
 }
 
+[uuid(BAD82401-2721-44F9-BB91-2BB228BE442F)]
+interface IUISettings2 : IInspectable
+{
+    [propget] HRESULT GetTextScaleFactor([out][retval] DOUBLE* value);
+    
+    [eventadd] HRESULT AddTextScaleFactorChanged([in] void* handler, [out][retval] int* token);
+    [eventremove] HRESULT RemoveTextScaleFactorChanged([in] int token);
+}
+
 [uuid(03021BE4-5254-4781-8194-5168F7D06D7B)]
 interface IUISettings3 : IInspectable
 {

src/iOS/Avalonia.iOS/PlatformSettings.cs

@@ -1,6 +1,7 @@
 using System;
 using Avalonia.Media;
 using Avalonia.Platform;
+using Foundation;
 using UIKit;
 
 namespace Avalonia.iOS;
@@ -8,8 +9,25 @@ namespace Avalonia.iOS;
 // TODO: ideally should be created per view/activity.
 internal class PlatformSettings : DefaultPlatformSettings
 {
+    private readonly NSObject _contentSizeChangedToken;
+
     private PlatformColorValues? _lastColorValues;
 
+    public PlatformSettings()
+    {
+        _contentSizeChangedToken = UIApplication.Notifications.ObserveContentSizeCategoryChanged(OnContentSizeCategoryChanged);
+    }
+
+    ~PlatformSettings()
+    {
+        _contentSizeChangedToken.Dispose();
+    }
+
+    private void OnContentSizeCategoryChanged(object? sender, UIContentSizeCategoryChangedEventArgs e)
+    {
+        OnTextScaleChanged();
+    }
+
     public override PlatformColorValues GetColorValues()
     {
         var themeVariant = UITraitCollection.CurrentTraitCollection.UserInterfaceStyle == UIUserInterfaceStyle.Dark ?
@@ -61,4 +79,6 @@ internal class PlatformSettings : DefaultPlatformSettings
             OnColorValuesChanged(colorValues);
         }
     }
+
+    public override double GetScaledFontSize(double baseFontSize) => UIFontMetrics.DefaultMetrics.GetScaledValue((nfloat)baseFontSize);
 }