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Nitrox.Launcher/Models/Controls/RichTextBlock.cs

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+using System;
+using System.Collections.Generic;
+using System.Text.RegularExpressions;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Documents;
+using Avalonia.Media;
+
+namespace Nitrox.Launcher.Models.Controls;
+
+/// <summary>
+///     A basic Rich Textbox. Supports bold, italic, underline, colors and hyperlinks.
+/// </summary>
+/// <remarks>
+///     Tag legend:<br />
+///     [b][/b] - Bold <br />
+///     [i][/i] - Italicize <br />
+///     [u][/u] - Underline <br />
+///     [#colorHex][/#colorHex] - Change text color <br />
+///     [Flavor text](example.com) <br />
+/// </remarks>
+/// <example>
+///     [b]Text[/b] => <b>Text</b> <br />
+///     [i]Text[/i] => <i>Text</i> <br />
+///     [u]Text[/u] => <u>Text</u> <br />
+///     [#0000FF]Text[/#0000FF] => Text (with blue foreground) <br />
+///     <a href="https://example.com">Flavor text</a> <br />
+/// </example>
+public partial class RichTextBlock : TextBlock
+{
+    private static readonly TextDecorationCollection underlineTextDecoration = [new() { Location = TextDecorationLocation.Underline }];
+
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+    {
+        base.OnPropertyChanged(change);
+
+        if (change.Property == TextProperty)
+        {
+            Inlines?.Clear();
+            ParseTextAndAddInlines(Text ?? "", Inlines);
+            // If all text was just tags, set Text to empty. Otherwise, it will be displayed as fallback by Avalonia.
+            if (Inlines?.Count < 1)
+            {
+                Text = "";
+            }
+        }
+    }
+
+    [GeneratedRegex(@"\[\/?([^]]+)\](?:\(([^\)]*)\))?")]
+    private static partial Regex TagParserRegex { get; }
+
+    public static void ParseTextAndAddInlines(ReadOnlySpan<char> text, InlineCollection inlines)
+    {
+        if (inlines == null)
+        {
+            return;
+        }
+        Regex.ValueMatchEnumerator matchEnumerator = TagParserRegex.EnumerateMatches(text);
+        if (!matchEnumerator.MoveNext())
+        {
+            inlines.Add(new Run(text.ToString()));
+            return;
+        }
+
+        ValueMatch lastRange = default;
+        Dictionary<string, Action<Run, string>> activeTags = new(4);
+        do
+        {
+            ValueMatch range = matchEnumerator.Current;
+
+            // Handle text in-between previous and current tag.
+            ReadOnlySpan<char> textPart = text[(lastRange.Index + lastRange.Length)..range.Index];
+            if (!textPart.IsEmpty)
+            {
+                inlines.Add(CreateRunWithTags(textPart.ToString(), activeTags));
+            }
+
+            // Handle current tag (this tracks state of active tags at current text position)
+            ReadOnlySpan<char> match = text.Slice(range.Index, range.Length);
+            switch (match)
+            {
+                case ['[', '/', ..]:
+                    activeTags.Remove(match[2..^1].ToString());
+                    break;
+                case "[b]":
+                    activeTags["b"] = static (run, _) => run.FontWeight = FontWeight.Bold;
+                    break;
+                case "[u]":
+                    activeTags["u"] = static (run, _) => run.TextDecorations = underlineTextDecoration;
+                    break;
+                case "[i]":
+                    activeTags["i"] = static (run, _) => run.FontStyle = FontStyle.Italic;
+                    break;
+                case ['[', ..] when match.IndexOf("](", StringComparison.OrdinalIgnoreCase) > -1:
+                    TextBlock textBlock = new();
+                    textBlock.Classes.Add("link");
+                    textBlock.Text = match[1..match.IndexOfAny("]")].ToString();
+                    textBlock.Tag = match[(match.IndexOfAny("(")+1)..match.IndexOfAny(")")].ToString();
+                    inlines.Add(textBlock);
+                    break;
+                case ['[', '#', ..]:
+                    ReadOnlySpan<char> colorCode = match[1..match.IndexOfAny("]")];
+                    if (!Color.TryParse(colorCode, out Color _))
+                    {
+                        goto default;
+                    }
+                    activeTags[colorCode.ToString()] = static (run, tag) => run.Foreground = new SolidColorBrush(Color.Parse(tag));
+                    break;
+                default:
+                    // Unknown tag, let's handle as normal text (issue is likely due to input text not knowing about this RichTextBox format)
+                    inlines.Add(CreateRunWithTags(match.ToString(), activeTags));
+                    break;
+            }
+
+            lastRange = range;
+        } while (matchEnumerator.MoveNext());
+
+        // Handle any final text (after the last tag).
+        ReadOnlySpan<char> lastPart = text[(lastRange.Index + lastRange.Length)..];
+        if (!lastPart.IsEmpty)
+        {
+            inlines.Add(CreateRunWithTags(lastPart.ToString(), activeTags));
+        }
+    }
+
+    private static Run CreateRunWithTags(string text, Dictionary<string, Action<Run, string>> tags)
+    {
+        Run run = new(text);
+        KeyValuePair<string, Action<Run, string>>? lastColorTag = null;
+        foreach (KeyValuePair<string, Action<Run, string>> pair in tags)
+        {
+            switch (pair.Key)
+            {
+                case ['#', ..]:
+                    // Optimization: only the last color needs to be applied for the current run, ignore all others.
+                    lastColorTag = pair;
+                    break;
+                default:
+                    pair.Value(run, pair.Key);
+                    break;
+            }
+        }
+
+        lastColorTag?.Value(run, lastColorTag.Value.Key);
+        return run;
+    }
+
+    protected override Type StyleKeyOverride { get; } = typeof(TextBlock);
+}