feat: mark current line in editor (#51)

Use WindowsText color to mark the line for the caret.
Define the resource color LineNumberCurrent.

Project/Resources/Mode.xsd

@@ -47,6 +47,7 @@
       <xsd:element name="CaretLine" type="EnvironmentEntry" minOccurs="0" maxOccurs="1" />
 
       <xsd:element name="LineNumbers" type="EnvironmentEntry" minOccurs="0" maxOccurs="1" />
+      <xsd:element name="LineNumberSelected" type="EnvironmentEntry" minOccurs="0" maxOccurs="1" />
 
       <xsd:element name="FoldLine" type="EnvironmentEntry" minOccurs="0" maxOccurs="1" />
       <xsd:element name="FoldMarker" type="EnvironmentEntry" minOccurs="0" maxOccurs="1" />

Project/Src/Document/HighlightingStrategy/DefaultHighlightingStrategy.cs

@@ -65,6 +65,7 @@ namespace ICSharpCode.TextEditor.Document
                 ["CaretMarker"] = new HighlightColor(nameof(SystemColors.MenuBar), bold: false, italic: false),
                 ["CaretLine"] = new HighlightBackground(nameof(SystemColors.ControlLight), nameof(SystemColors.Window), bold: false, italic: false),
                 ["LineNumbers"] = new HighlightBackground(nameof(SystemColors.GrayText), nameof(SystemColors.Window), bold: false, italic: false),
+                ["LineNumberSelected"] = new HighlightBackground(nameof(SystemColors.WindowText), nameof(SystemColors.Window), bold: true, italic: false),
                 ["FoldLine"] = new HighlightColor(nameof(SystemColors.ControlDark), bold: false, italic: false),
                 ["FoldMarker"] = new HighlightColor(nameof(SystemColors.WindowText), nameof(SystemColors.Window), bold: false, italic: false),
                 ["SelectedFoldLine"] = new HighlightColor(nameof(SystemColors.WindowText), bold: false, italic: false),

Project/Src/Gui/AbstractMargin.cs

@@ -21,6 +21,8 @@ namespace ICSharpCode.TextEditor
     /// </summary>
     public abstract class AbstractMargin
     {
+        private int _caretLine = -1;
+
         protected Rectangle drawingPosition = new Rectangle(x: 0, y: 0, width: 0, height: 0);
 
         protected TextArea textArea;
@@ -42,6 +44,8 @@ namespace ICSharpCode.TextEditor
 
         public ITextEditorProperties TextEditorProperties => textArea.Document.TextEditorProperties;
 
+        public bool MarkSelectedLine { get; set; } = true;
+
         public virtual Cursor Cursor { get; set; } = Cursors.Default;
 
         public virtual int Width => -1;
@@ -63,6 +67,17 @@ namespace ICSharpCode.TextEditor
             MouseLeave?.Invoke(this, e);
         }
 
+        public virtual void SelectedLineChanged(int line)
+        {
+            if (!IsVisible || _caretLine == line || !MarkSelectedLine)
+            {
+                return;
+            }
+
+            _caretLine = line;
+            textArea.Invalidate();
+        }
+
         public virtual void Paint(Graphics g, Rectangle rect)
         {
             Painted?.Invoke(this, g, rect);

Project/Src/Gui/GutterMargin.cs

@@ -57,11 +57,13 @@ namespace ICSharpCode.TextEditor
                 return;
 
             var lineNumberPainterColor = textArea.Document.HighlightingStrategy.GetColorFor("LineNumbers");
+            var lineNumberCurrentPainterColor = textArea.Document.HighlightingStrategy.GetColorFor("LineNumberSelected");
             var fontHeight = textArea.TextView.FontHeight;
             var fillBrush = textArea.Enabled
                 ? BrushRegistry.GetBrush(lineNumberPainterColor.BackgroundColor)
                 : SystemBrushes.InactiveBorder;
             var drawBrush = BrushRegistry.GetBrush(lineNumberPainterColor.Color);
+            Brush currentLineBrush = BrushRegistry.GetBrush(lineNumberCurrentPainterColor.Color);
 
             for (var y = 0; y < (drawingPosition.Height + textArea.TextView.VisibleLineDrawingRemainder)/fontHeight + 1; ++y)
             {
@@ -71,12 +73,17 @@ namespace ICSharpCode.TextEditor
                 {
                     g.FillRectangle(fillBrush, backgroundRectangle);
                     var curLine = textArea.Document.GetFirstLogicalLine(textArea.Document.GetVisibleLine(textArea.TextView.FirstVisibleLine) + y);
+                    bool isCurrentLine = curLine == textArea.Caret.Line && MarkSelectedLine;
+                    Brush lineBrush = isCurrentLine ? currentLineBrush : drawBrush;
+                    Font font = isCurrentLine
+                        ? lineNumberCurrentPainterColor.GetFont(TextEditorProperties.FontContainer)
+                        : lineNumberPainterColor.GetFont(TextEditorProperties.FontContainer);
 
                     if (curLine < textArea.Document.TotalNumberOfLines)
                         g.DrawString(
                             (curLine + 1).ToString(),
-                            lineNumberPainterColor.GetFont(TextEditorProperties.FontContainer),
-                            drawBrush,
+                            font,
+                            lineBrush,
                             backgroundRectangle,
                             numberStringFormat);
                 }