Introduce TextBlock.LineSpacing (#13243)

* Introduce TextBlock.LineSpacing

* Introduce TextBlock.LineSpacing

src/Avalonia.Base/Media/TextFormatting/TextLineImpl.cs

@@ -1233,6 +1233,7 @@ namespace Avalonia.Media.TextFormatting
 
             var height = descent - ascent + lineGap;
             var lineHeight = _paragraphProperties.LineHeight;
+            var lineSpacing = _paragraphProperties.LineSpacing;
 
             var bounds = new Rect();
 
@@ -1339,7 +1340,7 @@ namespace Avalonia.Media.TextFormatting
             return new TextLineMetrics
             {
                 HasOverflowed = hasOverflowed,
-                Height = height,
+                Height = height + lineSpacing,
                 Extent = bounds.Height,
                 NewlineLength = newLineLength,
                 Start = start,

src/Avalonia.Base/Media/TextFormatting/TextParagraphProperties.cs

@@ -21,6 +21,11 @@
         /// </summary>
         public abstract double LineHeight { get; }
 
+        /// <summary>
+        /// Paragraph's line spacing
+        /// </summary>
+        internal double LineSpacing { get; set; }
+
         /// <summary>
         /// Indicates the first line of the paragraph.
         /// </summary>

src/Avalonia.Controls/TextBlock.cs

@@ -83,6 +83,16 @@ namespace Avalonia.Controls
                 validate: IsValidLineHeight,
                 inherits: true);
 
+        /// <summary>
+        /// Defines the <see cref="LineSpacing"/> property.
+        /// </summary>
+        public static readonly AttachedProperty<double> LineSpacingProperty =
+            AvaloniaProperty.RegisterAttached<TextBlock, Control, double>(
+                nameof(LineSpacing),
+                0,
+                validate: IsValidLineSpacing,
+                inherits: true);
+
         /// <summary>
         /// Defines the <see cref="LetterSpacing"/> property.
         /// </summary>
@@ -265,6 +275,15 @@ namespace Avalonia.Controls
             set => SetValue(LineHeightProperty, value);
         }
 
+        /// <summary>
+        /// Gets or sets the extra distance of each line to the next line.
+        /// </summary>
+        public double LineSpacing
+        {
+            get => GetValue(LineSpacingProperty);
+            set => SetValue(LineSpacingProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the letter spacing.
         /// </summary>
@@ -620,7 +639,10 @@ namespace Avalonia.Controls
                 Foreground);
 
             var paragraphProperties = new GenericTextParagraphProperties(FlowDirection, TextAlignment, true, false,
-                defaultProperties, TextWrapping, LineHeight, 0, LetterSpacing);
+                defaultProperties, TextWrapping, LineHeight, 0, LetterSpacing)
+            {
+                LineSpacing = LineSpacing
+            };
 
             ITextSource textSource;
 
@@ -800,6 +822,8 @@ namespace Avalonia.Controls
 
         private static bool IsValidLineHeight(double lineHeight) => double.IsNaN(lineHeight) || lineHeight > 0;
 
+        private static bool IsValidLineSpacing(double lineSpacing) => !double.IsNaN(lineSpacing) && !double.IsInfinity(lineSpacing);
+
         private void OnInlinesChanged(InlineCollection? oldValue, InlineCollection? newValue)
         {
             if (oldValue is not null)