Add BepInEx specific log file listener

BepInEx/BepInEx.csproj

@@ -59,6 +59,7 @@
     <Compile Include="ConsoleUtil\SafeConsole.cs" />
     <Compile Include="Bootstrap\Chainloader.cs" />
     <Compile Include="Contract\BaseUnityPlugin.cs" />
+    <Compile Include="Logging\DiskLogListener.cs" />
     <Compile Include="Logging\LogEventArgs.cs" />
     <Compile Include="Logging\Logger.cs" />
     <Compile Include="Logging\LogLevel.cs" />

BepInEx/Bootstrap/Chainloader.cs

@@ -62,6 +62,7 @@ namespace BepInEx.Bootstrap
 
 
 			Logger.Listeners.Add(new UnityLogListener());
+			Logger.Listeners.Add(new DiskLogListener());
 
 			if (!TraceLogSource.IsListening)
 				Logger.Sources.Add(TraceLogSource.CreateSource());

BepInEx/Logging/DiskLogListener.cs

@@ -0,0 +1,69 @@
+using System;
+using System.IO;
+using System.Text;
+using System.Threading;
+using BepInEx.Configuration;
+
+namespace BepInEx.Logging
+{
+	/// <summary>
+	/// Logs entries using Unity specific outputs.
+	/// </summary>
+	public class DiskLogListener : ILogListener
+	{
+		protected LogLevel DisplayedLogLevel = (LogLevel)Enum.Parse(typeof(LogLevel), ConfigConsoleDisplayedLevel.Value, true);
+
+		protected TextWriter LogWriter = TextWriter.Synchronized(new StreamWriter(Path.Combine(Paths.BepInExRootPath, "LogOutput.log"), ConfigAppendLog.Value, Encoding.UTF8));
+
+		protected Timer FlushTimer;
+
+		public DiskLogListener()
+		{
+			FlushTimer = new Timer(o =>
+			{
+				LogWriter?.Flush();
+			}, null, 2000, 2000);
+		}
+
+		public void LogEvent(object sender, LogEventArgs eventArgs)
+		{
+			if (!ConfigWriteUnityLog.Value && eventArgs.Source is UnityLogSource)
+				return;
+
+			if (eventArgs.Level.GetHighestLevel() > DisplayedLogLevel)
+				return;
+
+			LogWriter.WriteLine($"[{eventArgs.Level,-7}:{((ILogSource)sender).SourceName,10}] {eventArgs.Data}");
+		}
+
+		public void Dispose()
+		{
+			FlushTimer.Dispose();
+			LogWriter.Flush();
+			LogWriter.Dispose();
+		}
+
+		~DiskLogListener()
+		{
+			Dispose();
+		}
+
+		private static readonly ConfigWrapper<string> ConfigConsoleDisplayedLevel = ConfigFile.CoreConfig.Wrap(
+			"Logging.Disk",
+			"DisplayedLogLevel",
+			"Only displays the specified log level and above in the console output.",
+			"Info");
+
+		private static readonly ConfigWrapper<bool> ConfigWriteUnityLog = ConfigFile.CoreConfig.Wrap(
+			"Logging.Disk",
+			"WriteUnityLog",
+			"Include unity log messages in log file output.",
+			false);
+
+		private static readonly ConfigWrapper<bool> ConfigAppendLog = ConfigFile.CoreConfig.Wrap(
+			"Logging.Disk",
+			"AppendLog",
+			"Appends to the log file instead of overwriting, on game startup.",
+			false);
+	}
+}