Add support for fake input in asciicast recordings

* Fixes a bug with `SelectionPrompt` and page size.
* Allow `IAnsiConsoleInput` to return `null`.

resources/scripts/Generator/Commands/AsciiCast/AsciiCastOut.cs

@@ -0,0 +1,95 @@
+using System;
+using System.Globalization;
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using Spectre.Console;
+
+namespace Generator.Commands
+{
+    public class AsciiCastOut : IAnsiConsoleOutput
+    {
+        private sealed class AsciiCastWriter : TextWriter
+        {
+            private readonly TextWriter _wrappedTextWriter;
+            private readonly StringBuilder _builder = new StringBuilder();
+            private int? _firstTick;
+
+            public AsciiCastWriter(TextWriter wrappedTextWriter)
+            {
+                _wrappedTextWriter = wrappedTextWriter;
+            }
+
+            public override void Write(string value)
+            {
+                if (value == null)
+                {
+                    return;
+                }
+
+                Append(value);
+                _wrappedTextWriter.Write(value);
+                base.Write(value);
+            }
+
+            public override Encoding Encoding => _wrappedTextWriter.Encoding;
+
+            private void Append(string value)
+            {
+                var tick = 0m;
+                if (_firstTick.HasValue)
+                {
+                    tick = Environment.TickCount - _firstTick.Value;
+                }
+                else
+                {
+                    _firstTick = Environment.TickCount;
+                }
+
+                tick /= 1000m;
+
+                _builder.Append('[')
+                    .AppendFormat(CultureInfo.InvariantCulture, "{0}", tick)
+                    .Append(", \"o\", \"").Append(JsonEncodedText.Encode(value)).AppendLine("\"]");
+            }
+
+            public string GetJsonAndClearBuffer()
+            {
+                var json = _builder.ToString();
+
+                // reset the buffer and also reset the first tick count
+                _builder.Clear();
+                _firstTick = null;
+                return json;
+            }
+        }
+
+        private readonly IAnsiConsoleOutput _wrappedAnsiConsole;
+        private readonly AsciiCastWriter _asciiCastWriter;
+
+        public AsciiCastOut(IAnsiConsoleOutput wrappedAnsiConsole)
+        {
+            _wrappedAnsiConsole = wrappedAnsiConsole ?? throw new ArgumentNullException(nameof(wrappedAnsiConsole));
+            _asciiCastWriter = new AsciiCastWriter(_wrappedAnsiConsole.Writer);
+        }
+
+        public TextWriter Writer => _asciiCastWriter;
+
+        public bool IsTerminal => _wrappedAnsiConsole.IsTerminal;
+
+        public int Width => _wrappedAnsiConsole.Width;
+
+        public int Height => _wrappedAnsiConsole.Height;
+
+        public void SetEncoding(Encoding encoding)
+        {
+            _wrappedAnsiConsole.SetEncoding(encoding);
+        }
+
+        public string GetCastJson(string title, int? width = null, int? height = null)
+        {
+            var header = $"{{\"version\": 2, \"width\": {width ?? _wrappedAnsiConsole.Width}, \"height\": {height ?? _wrappedAnsiConsole.Height}, \"title\": \"{JsonEncodedText.Encode(title)}\", \"env\": {{\"TERM\": \"Spectre.Console\"}}}}";
+            return $"{header}{Environment.NewLine}{_asciiCastWriter.GetJsonAndClearBuffer()}{Environment.NewLine}";
+        }
+    }
+}