Refactor text trimming and implement prefix trimming.

src/Avalonia.Controls/TextBlock.cs

@@ -134,7 +134,7 @@ public class TextBlock : Control
         /// Defines the <see cref="TextTrimming"/> property.
         /// </summary>
         public static readonly StyledProperty<TextTrimming> TextTrimmingProperty =
-            AvaloniaProperty.Register<TextBlock, TextTrimming>(nameof(TextTrimming));
+            AvaloniaProperty.Register<TextBlock, TextTrimming>(nameof(TextTrimming), defaultValue: TextTrimming.None);
 
         /// <summary>
         /// Defines the <see cref="TextDecorations"/> property.

src/Avalonia.Visuals/ApiCompatBaseline.txt

@@ -52,6 +52,12 @@ MembersMustExist : Member 'public void Avalonia.Media.TextHitTestResult..ctor()'
 MembersMustExist : Member 'public void Avalonia.Media.TextHitTestResult.IsInside.set(System.Boolean)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.TextHitTestResult.IsTrailing.set(System.Boolean)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.TextHitTestResult.TextPosition.set(System.Int32)' does not exist in the implementation but it does exist in the contract.
+CannotMakeTypeAbstract : Type 'Avalonia.Media.TextTrimming' is abstract in the implementation but is not abstract in the contract.
+TypeCannotChangeClassification : Type 'Avalonia.Media.TextTrimming' is a 'class' in the implementation but is a 'struct' in the contract.
+MembersMustExist : Member 'public Avalonia.Media.TextTrimming Avalonia.Media.TextTrimming Avalonia.Media.TextTrimming.CharacterEllipsis' does not exist in the implementation but it does exist in the contract.
+MembersMustExist : Member 'public Avalonia.Media.TextTrimming Avalonia.Media.TextTrimming Avalonia.Media.TextTrimming.None' does not exist in the implementation but it does exist in the contract.
+MembersMustExist : Member 'public Avalonia.Media.TextTrimming Avalonia.Media.TextTrimming Avalonia.Media.TextTrimming.WordEllipsis' does not exist in the implementation but it does exist in the contract.
+MembersMustExist : Member 'public System.Int32 System.Int32 Avalonia.Media.TextTrimming.value__' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.Typeface..ctor(Avalonia.Media.FontFamily, Avalonia.Media.FontStyle, Avalonia.Media.FontWeight)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.Typeface..ctor(System.String, Avalonia.Media.FontStyle, Avalonia.Media.FontWeight)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.Immutable.ImmutableConicGradientBrush..ctor(System.Collections.Generic.IReadOnlyList<Avalonia.Media.Immutable.ImmutableGradientStop>, System.Double, Avalonia.Media.GradientSpreadMethod, System.Nullable<Avalonia.RelativePoint>, System.Double)' does not exist in the implementation but it does exist in the contract.
@@ -79,6 +85,9 @@ MembersMustExist : Member 'public void Avalonia.Media.TextFormatting.ShapedTextC
 MembersMustExist : Member 'public Avalonia.Media.TextFormatting.ShapedTextCharacters.SplitTextCharactersResult Avalonia.Media.TextFormatting.ShapedTextCharacters.Split(System.Int32)' does not exist in the implementation but it does exist in the contract.
 TypesMustExist : Type 'Avalonia.Media.TextFormatting.ShapedTextCharacters.SplitTextCharactersResult' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'protected System.Boolean Avalonia.Media.TextFormatting.TextCharacters.TryGetRunProperties(Avalonia.Utilities.ReadOnlySlice<System.Char>, Avalonia.Media.Typeface, Avalonia.Media.Typeface, System.Int32)' does not exist in the implementation but it does exist in the contract.
+CannotAddAbstractMembers : Member 'public System.Collections.Generic.IReadOnlyList<Avalonia.Media.TextFormatting.TextRun> Avalonia.Media.TextFormatting.TextCollapsingProperties.Collapse(Avalonia.Media.TextFormatting.TextLine)' is abstract in the implementation but is missing in the contract.
+MembersMustExist : Member 'public Avalonia.Media.TextFormatting.TextCollapsingStyle Avalonia.Media.TextFormatting.TextCollapsingProperties.Style.get()' does not exist in the implementation but it does exist in the contract.
+TypesMustExist : Type 'Avalonia.Media.TextFormatting.TextCollapsingStyle' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.TextFormatting.TextEndOfLine..ctor()' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.TextFormatting.TextLayout..ctor(System.String, Avalonia.Media.Typeface, System.Double, Avalonia.Media.IBrush, Avalonia.Media.TextAlignment, Avalonia.Media.TextWrapping, Avalonia.Media.TextTrimming, Avalonia.Media.TextDecorationCollection, System.Double, System.Double, System.Double, System.Int32, System.Collections.Generic.IReadOnlyList<Avalonia.Utilities.ValueSpan<Avalonia.Media.TextFormatting.TextRunProperties>>)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Media.TextFormatting.TextLayout.Draw(Avalonia.Media.DrawingContext)' does not exist in the implementation but it does exist in the contract.
@@ -124,6 +133,12 @@ CannotAddAbstractMembers : Member 'public System.Double Avalonia.Media.TextForma
 CannotAddAbstractMembers : Member 'public Avalonia.Media.BaselineAlignment Avalonia.Media.TextFormatting.TextRunProperties.BaselineAlignment' is abstract in the implementation but is missing in the contract.
 CannotAddAbstractMembers : Member 'public Avalonia.Media.BaselineAlignment Avalonia.Media.TextFormatting.TextRunProperties.BaselineAlignment.get()' is abstract in the implementation but is missing in the contract.
 MembersMustExist : Member 'public Avalonia.Media.GlyphRun Avalonia.Media.TextFormatting.TextShaper.ShapeText(Avalonia.Utilities.ReadOnlySlice<System.Char>, Avalonia.Media.Typeface, System.Double, System.Globalization.CultureInfo)' does not exist in the implementation but it does exist in the contract.
+CannotSealType : Type 'Avalonia.Media.TextFormatting.TextTrailingCharacterEllipsis' is actually (has the sealed modifier) sealed in the implementation but not sealed in the contract.
+MembersMustExist : Member 'public void Avalonia.Media.TextFormatting.TextTrailingCharacterEllipsis..ctor(System.Double, Avalonia.Media.TextFormatting.TextRunProperties)' does not exist in the implementation but it does exist in the contract.
+MembersMustExist : Member 'public Avalonia.Media.TextFormatting.TextCollapsingStyle Avalonia.Media.TextFormatting.TextTrailingCharacterEllipsis.Style.get()' does not exist in the implementation but it does exist in the contract.
+CannotSealType : Type 'Avalonia.Media.TextFormatting.TextTrailingWordEllipsis' is actually (has the sealed modifier) sealed in the implementation but not sealed in the contract.
+MembersMustExist : Member 'public void Avalonia.Media.TextFormatting.TextTrailingWordEllipsis..ctor(System.Double, Avalonia.Media.TextFormatting.TextRunProperties)' does not exist in the implementation but it does exist in the contract.
+MembersMustExist : Member 'public Avalonia.Media.TextFormatting.TextCollapsingStyle Avalonia.Media.TextFormatting.TextTrailingWordEllipsis.Style.get()' does not exist in the implementation but it does exist in the contract.
 TypesMustExist : Type 'Avalonia.Media.TextFormatting.Unicode.BiDiClass' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public Avalonia.Media.TextFormatting.Unicode.BiDiClass Avalonia.Media.TextFormatting.Unicode.Codepoint.BiDiClass.get()' does not exist in the implementation but it does exist in the contract.
 TypesMustExist : Type 'Avalonia.Platform.ExportRenderingSubsystemAttribute' does not exist in the implementation but it does exist in the contract.
@@ -161,4 +176,4 @@ InterfacesShouldHaveSameMembers : Interface member 'public Avalonia.Media.GlyphR
 MembersMustExist : Member 'public Avalonia.Media.GlyphRun Avalonia.Platform.ITextShaperImpl.ShapeText(Avalonia.Utilities.ReadOnlySlice<System.Char>, Avalonia.Media.Typeface, System.Double, System.Globalization.CultureInfo)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'protected void Avalonia.Rendering.RendererBase.RenderFps(Avalonia.Platform.IDrawingContextImpl, Avalonia.Rect, System.Nullable<System.Int32>)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Utilities.ReadOnlySlice<T>..ctor(System.ReadOnlyMemory<T>, System.Int32, System.Int32)' does not exist in the implementation but it does exist in the contract.
-Total Issues: 162
+Total Issues: 177

src/Avalonia.Visuals/Media/FormattedText.cs

@@ -854,19 +854,9 @@ private TextLine FormatLine(ITextSource textSource, int textSourcePosition, doub
 
                     var lastRunProps = (GenericTextRunProperties)thatFormatRider.CurrentElement!;
 
-                    TextCollapsingProperties trailingEllipsis;
+                    TextCollapsingProperties collapsingProperties = _that._trimming.CreateCollapsingProperties(new TextCollapsingCreateInfo(maxLineLength, lastRunProps));
 
-                    if (_that._trimming == TextTrimming.CharacterEllipsis)
-                    {
-                        trailingEllipsis = new TextTrailingCharacterEllipsis(maxLineLength, lastRunProps);
-                    }
-                    else
-                    {
-                        Debug.Assert(_that._trimming == TextTrimming.WordEllipsis);
-                        trailingEllipsis = new TextTrailingWordEllipsis(maxLineLength, lastRunProps);
-                    }
-
-                    var collapsedLine = line.Collapse(trailingEllipsis);
+                    var collapsedLine = line.Collapse(collapsingProperties);
 
                     line = collapsedLine;
                 }
@@ -1121,11 +1111,6 @@ public TextTrimming Trimming
         {
             set
             {
-                if ((int)value < 0 || (int)value > (int)TextTrimming.WordEllipsis)
-                {
-                    throw new InvalidEnumArgumentException(nameof(value), (int)value, typeof(TextTrimming));
-                }
-
                 _trimming = value;
 
                 _defaultParaProps.SetTextWrapping(_trimming == TextTrimming.None ?

src/Avalonia.Visuals/Media/TextCollapsingCreateInfo.cs

@@ -0,0 +1,16 @@
+using Avalonia.Media.TextFormatting;
+
+namespace Avalonia.Media
+{
+    public readonly struct TextCollapsingCreateInfo
+    {
+        public readonly double Width;
+        public readonly TextRunProperties TextRunProperties;
+
+        public TextCollapsingCreateInfo(double width, TextRunProperties textRunProperties)
+        {
+            Width = width;
+            TextRunProperties = textRunProperties;
+        }
+    }
+}

src/Avalonia.Visuals/Media/TextFormatting/ShapedTextCharacters.cs

@@ -132,6 +132,29 @@ internal bool TryMeasureCharacters(double availableWidth, out int length)
             return length > 0;
         }
 
+        internal bool TryMeasureCharactersBackwards(double availableWidth, out int length, out double width)
+        {
+            length = 0;
+            width = 0;
+
+            for (var i = ShapedBuffer.Length - 1; i >= 0; i--)
+            {
+                var advance = ShapedBuffer.GlyphAdvances[i];
+
+                if (width + advance > availableWidth)
+                {
+                    break;
+                }
+
+                Codepoint.ReadAt(GlyphRun.Characters, length, out var count);
+
+                length += count;
+                width += advance;
+            }
+
+            return length > 0;
+        }
+
         internal SplitResult<ShapedTextCharacters> Split(int length)
         {
             if (IsReversed)

src/Avalonia.Visuals/Media/TextFormatting/TextCollapsingProperties.cs

@@ -1,23 +1,26 @@
-namespace Avalonia.Media.TextFormatting
+using System.Collections.Generic;
+
+namespace Avalonia.Media.TextFormatting
 {
     /// <summary>
-    /// Properties of text collapsing
+    /// Properties of text collapsing.
     /// </summary>
     public abstract class TextCollapsingProperties
     {
         /// <summary>
-        /// Gets the width in which the collapsible range is constrained to
+        /// Gets the width in which the collapsible range is constrained to.
         /// </summary>
         public abstract double Width { get; }
 
         /// <summary>
-        /// Gets the text run that is used as collapsing symbol
+        /// Gets the text run that is used as collapsing symbol.
         /// </summary>
         public abstract TextRun Symbol { get; }
 
         /// <summary>
-        /// Gets the style of collapsing
+        /// Collapses given text line.
         /// </summary>
-        public abstract TextCollapsingStyle Style { get; }
+        /// <param name="textLine">Text line to collapse.</param>
+        public abstract IReadOnlyList<TextRun>? Collapse(TextLine textLine);
     }
 }

src/Avalonia.Visuals/Media/TextFormatting/TextEllipsisHelper.cs

@@ -0,0 +1,95 @@
+using System.Collections.Generic;
+using Avalonia.Media.TextFormatting.Unicode;
+
+namespace Avalonia.Media.TextFormatting
+{
+    internal class TextEllipsisHelper
+    {
+        public static List<ShapedTextCharacters>? Collapse(TextLine textLine, TextCollapsingProperties properties, bool isWordEllipsis)
+        {
+            var shapedTextRuns = textLine.TextRuns as List<ShapedTextCharacters>;
+
+            if (shapedTextRuns is null)
+            {
+                return null;
+            }
+
+            var runIndex = 0;
+            var currentWidth = 0.0;
+            var collapsedLength = 0;
+            var textRange = textLine.TextRange;
+            var shapedSymbol = TextFormatterImpl.CreateSymbol(properties.Symbol, FlowDirection.LeftToRight);
+
+            if (properties.Width < shapedSymbol.GlyphRun.Size.Width)
+            {
+                return new List<ShapedTextCharacters>(0);
+            }
+
+            var availableWidth = properties.Width - shapedSymbol.Size.Width;
+
+            while (runIndex < shapedTextRuns.Count)
+            {
+                var currentRun = shapedTextRuns[runIndex];
+
+                currentWidth += currentRun.Size.Width;
+
+                if (currentWidth > availableWidth)
+                {
+                    if (currentRun.TryMeasureCharacters(availableWidth, out var measuredLength))
+                    {
+                        if (isWordEllipsis && measuredLength < textRange.End)
+                        {
+                            var currentBreakPosition = 0;
+
+                            var lineBreaker = new LineBreakEnumerator(currentRun.Text);
+
+                            while (currentBreakPosition < measuredLength && lineBreaker.MoveNext())
+                            {
+                                var nextBreakPosition = lineBreaker.Current.PositionMeasure;
+
+                                if (nextBreakPosition == 0)
+                                {
+                                    break;
+                                }
+
+                                if (nextBreakPosition >= measuredLength)
+                                {
+                                    break;
+                                }
+
+                                currentBreakPosition = nextBreakPosition;
+                            }
+
+                            measuredLength = currentBreakPosition;
+                        }
+                    }
+
+                    collapsedLength += measuredLength;
+
+                    var shapedTextCharacters = new List<ShapedTextCharacters>(shapedTextRuns.Count);
+
+                    if (collapsedLength > 0)
+                    {
+                        var splitResult = TextFormatterImpl.SplitShapedRuns(shapedTextRuns, collapsedLength);
+
+                        shapedTextCharacters.AddRange(splitResult.First);
+
+                        TextLineImpl.SortRuns(shapedTextCharacters);
+                    }
+
+                    shapedTextCharacters.Add(shapedSymbol);
+
+                    return shapedTextCharacters;
+                }
+
+                availableWidth -= currentRun.Size.Width;
+
+                collapsedLength += currentRun.GlyphRun.Characters.Length;
+
+                runIndex++;
+            }
+
+            return null;
+        }
+    }
+}

src/Avalonia.Visuals/Media/TextFormatting/TextLayout.cs

@@ -41,7 +41,7 @@ public TextLayout(
             IBrush? foreground,
             TextAlignment textAlignment = TextAlignment.Left,
             TextWrapping textWrapping = TextWrapping.NoWrap,
-            TextTrimming textTrimming = TextTrimming.None,
+            TextTrimming? textTrimming = null,
             TextDecorationCollection? textDecorations = null,
             FlowDirection flowDirection = FlowDirection.LeftToRight,
             double maxWidth = double.PositiveInfinity,
@@ -58,7 +58,7 @@ public TextLayout(
                 CreateTextParagraphProperties(typeface, fontSize, foreground, textAlignment, textWrapping,
                     textDecorations, flowDirection, lineHeight);
 
-            _textTrimming = textTrimming;
+            _textTrimming = textTrimming ?? TextTrimming.None;
 
             _textStyleOverrides = textStyleOverrides;
 
@@ -641,14 +641,7 @@ private IReadOnlyList<TextLine> CreateTextLines()
         /// <returns>The <see cref="TextCollapsingProperties"/>.</returns>
         private TextCollapsingProperties GetCollapsingProperties(double width)
         {
-            return _textTrimming switch
-            {
-                TextTrimming.CharacterEllipsis => new TextTrailingCharacterEllipsis(width,
-                    _paragraphProperties.DefaultTextRunProperties),
-                TextTrimming.WordEllipsis => new TextTrailingWordEllipsis(width,
-                    _paragraphProperties.DefaultTextRunProperties),
-                _ => throw new ArgumentOutOfRangeException(),
-            };
+            return _textTrimming.CreateCollapsingProperties(new TextCollapsingCreateInfo(width, _paragraphProperties.DefaultTextRunProperties));
         }
     }
 }