SelectableTextBlock: A WPF TextBlock control with selectable text

SelectableTextBlock.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;

namespace S100debugger;

static class TextBlockExtension
{
	const int LinebreakLength = 2;

	public static TextPointer GetPositionFromIndex(this TextBlock textBlock, int index)
	{
		int curIndex = 0;
		int length;
		TextPointer pointer;

		pointer = textBlock.ContentStart;
		while (curIndex < index && pointer.CompareTo(textBlock.ContentEnd) < 0)
		{
			TextPointerContext context = pointer.GetPointerContext(LogicalDirection.Forward);
			switch (context)
			{
				case TextPointerContext.Text:
					length = pointer.GetTextRunLength(LogicalDirection.Forward);
					if (length + curIndex > index)
					{
						// found it in this run
						return pointer.GetPositionAtOffset(index - curIndex);
					}
					curIndex += length;
					break;

				case TextPointerContext.ElementStart:
					DependencyObject element = pointer.GetAdjacentElement(LogicalDirection.Forward);
					if (element is LineBreak)
						curIndex += LinebreakLength;
					break;
			}

			pointer = pointer.GetNextContextPosition(LogicalDirection.Forward);
		}
		return pointer;
	}

	public static int GetIndexFromPosition(this TextBlock textBlock, TextPointer position)
	{
		return new TextRange(textBlock.ContentStart, position).Text.Length;
	}
}

class SelectableTextBlock : TextBlock
{
	#region Constructor

	public SelectableTextBlock() : base()
	{
		m_selectionStart = ContentStart;
		m_selectionEnd = ContentStart;
		m_selectedRange = new(ContentStart, ContentStart);
	}

	#endregion


	#region Private Fields & Properties

	readonly TextRange m_selectedRange;

	TextPointer m_selectionStart;
	TextPointer m_selectionEnd;
	Point m_startPoint;
	bool m_isSelecting = false;

	#endregion


	#region Public Properties & Events

	public string SelectedText
	{
		get => m_selectedRange.Text;
		set
		{
			m_selectedRange.Text = value;
			m_selectionStart = m_selectedRange.Start;
			m_selectionEnd = m_selectedRange.End;
			SetSelected(m_selectedRange);
		}
	}

	public TextRange SelectedRange => m_selectedRange;

	public int SelectionStart
	{
		get => new TextRange(ContentStart, m_selectedRange.Start).Text.Length;
		set
		{
			int length = SelectionLength;
			SetNormal(m_selectedRange);
			m_selectionStart = this.GetPositionFromIndex(value);
			m_selectionEnd = this.GetPositionFromIndex(value + length);
			m_selectedRange.Select(m_selectionStart, m_selectionEnd);
			SetSelected(m_selectedRange);
		}
	}

	public int SelectionLength
	{
		get => m_selectedRange.Text.Length;
		set
		{
			int index = SelectionStart;
			SetNormal(m_selectedRange);
			m_selectionEnd = this.GetPositionFromIndex(value + index);
			m_selectionStart = m_selectedRange.Start;
			m_selectedRange.Select(m_selectionStart, m_selectionEnd);
			SetSelected(m_selectedRange);
		}
	}

	// Set this property to 1/2 the character width (obviously simplest
	// if using a fixed-pitch font). It ensures the first character
	// remains in the selection.
	public double CharWidthAdjustment { get; set; } = 0;

	public Brush SelectedBackground { get; set; } = SystemColors.HighlightBrush;
	public Brush SelectedForeground { get; set; } = SystemColors.HighlightTextBrush;

	public event EventHandler? SelectionChanging;
	public event EventHandler? SelectionChanged;

	#endregion


	#region Private Methods

	private void SetSelected(TextRange range)
	{
		range.ApplyPropertyValue(ForegroundProperty, SelectedForeground);
		range.ApplyPropertyValue(BackgroundProperty, SelectedBackground);
	}

	private void SetNormal(TextRange range)
	{
		range.ApplyPropertyValue(ForegroundProperty, Foreground);
		range.ApplyPropertyValue(BackgroundProperty, Background);
	}

	private void EndSelection()
	{
		if (m_isSelecting)
		{
			m_isSelecting = false;
			SelectionChanged?.Invoke(this, EventArgs.Empty);
		}
	}

	#endregion


	#region Protected Method Overrides

	protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
	{
		base.OnMouseLeftButtonDown(e);
		SetNormal(m_selectedRange);
		m_isSelecting = true;
		m_startPoint = e.GetPosition(this);
		m_selectionStart = GetPositionFromPoint(m_startPoint, true);
		m_selectionEnd = m_selectionStart;
		m_selectedRange.Select(m_selectionStart, m_selectionEnd);
		SelectionChanging?.Invoke(this, EventArgs.Empty);
	}

	protected override void OnMouseMove(MouseEventArgs e)
	{
		base.OnMouseMove(e);
		if (!m_isSelecting)
			return;

		if (e.LeftButton != MouseButtonState.Pressed)
		{
			EndSelection();
			return;
		}

		TextPointer end = GetPositionFromPoint(e.GetPosition(this), true);
		if (m_selectionEnd.CompareTo(end) == 0)
			return;
		SetNormal(m_selectedRange);

		// Adjust start depending on selection direction
		Point start = m_startPoint;
		if (m_selectionStart.CompareTo(end) > 0)
			start.X += CharWidthAdjustment;
		else
			start.X -= CharWidthAdjustment;
		m_selectionStart = GetPositionFromPoint(start, true);
		m_selectionEnd = end;
		m_selectedRange.Select(m_selectionStart, m_selectionEnd);
		SetSelected(m_selectedRange);
		SelectionChanging?.Invoke(this, EventArgs.Empty);
	}

	protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
	{
		base.OnMouseLeftButtonUp(e);
		EndSelection();
	}

	protected override void OnMouseLeave(MouseEventArgs e)
	{
		base.OnMouseLeave(e);
		EndSelection();
	}

	#endregion
}

class TextBlockRunBuilder
{
	readonly List<Inline> m_range = new();
	readonly StringBuilder m_sb = new();

	public void Append(string text) => m_sb.Append(text);
	public void Append(char ch) => m_sb.Append(ch);
	public void Append(char ch, int count) => m_sb.Append(ch, count);
	public void AppendLine() => m_sb.AppendLine();

	public void Append(char ch, Brush foreground, Brush? background = null)
	{
		Append(ch.ToString(), foreground, background);
	}

	public void Append(string text, Brush foreground, Brush? background = null)
	{
		if (m_sb.Length > 0)
		{
			m_range.Add(new Run(m_sb.ToString()));
			m_sb.Clear();
		}
		Run run = new(text);
		run.Foreground = foreground;
		if (background != null)
			run.Background = background;
		m_range.Add(run);
	}

	public void SetTextBlock(TextBlock block)
	{
		if (m_sb.Length > 0)
		{
			m_range.Add(new Run(m_sb.ToString()));
			m_sb.Clear();
		}
		block.Inlines.Clear();
		block.Inlines.AddRange(m_range);
		m_range.Clear();
	}
}