3.1,自定义日志框架

进行本章的学习时，请使用 learn 分支。

https://github.com/whuanle/maomi/tree/learn

本节示例项目在 Demo2.MyLogger.Console 中。

创建控制台项目后，添加 Microsoft.Extensions.Logging.Console 引用。

创建 MyLoggerOptions ，存储日志配置：

public class MyLoggerOptions
{
	/// <summary>
	/// 最小日志等级
	/// </summary>
	public LogLevel DefaultLevel { get; set; } = LogLevel.Debug;
}

创建自定义日志记录器：

/// <summary>
/// 自定义的日志记录器
/// </summary>
public class MyConsoleLogger : ILogger
{
	// 日志名称
	private readonly string _name;
	private readonly MyLoggerOptions _options;

	public MyConsoleLogger(string name, MyLoggerOptions options)
	{
		_name = name;
		_options = options;
	}

	public IDisposable? BeginScope<TState>(TState state) where TState : notnull => default!;

	// 判断是否启用日志等级
	public bool IsEnabled(LogLevel logLevel)
	{
		return logLevel >= _options.DefaultLevel;
	}

	// 记录日志，formatter 由框架提供的字符串格式化器
	public void Log<TState>(
		LogLevel logLevel,
		EventId eventId,
		TState state,
		Exception? exception,
		Func<TState, Exception?, string> formatter)
	{
		if (!IsEnabled(logLevel))
		{
			return;
		}
		if (logLevel == LogLevel.Critical)
		{
			System.Console.ForegroundColor = System.ConsoleColor.Red;
			System.Console.WriteLine($"[{logLevel}] {_name} {formatter(state, exception)} {exception}");
			System.Console.ResetColor();
		}
		else if (logLevel == LogLevel.Error)
		{
			System.Console.ForegroundColor = System.ConsoleColor.DarkRed;
			System.Console.WriteLine($"[{logLevel}] {_name} {formatter(state, exception)} {exception}");
			System.Console.ResetColor();
		}
		else
		{
			System.Console.WriteLine($"[{logLevel}] {_name} {formatter(state, exception)} {exception}");
		}
	}
}

创建自定义日志提供器：

[ProviderAlias("MyConsole")]
public sealed class MyLoggerProvider : ILoggerProvider
{
	private MyLoggerOptions _options;
	private readonly ConcurrentDictionary<string, MyConsoleLogger> _loggers =
		new(StringComparer.OrdinalIgnoreCase);

	public MyLoggerProvider(MyLoggerOptions options)
	{
		_options = options;
	}

	public ILogger CreateLogger(string categoryName) =>
		_loggers.GetOrAdd(categoryName, name => new MyConsoleLogger(name, _options));

	public void Dispose()
	{
		_loggers.Clear();
	}
}

编写扩展函数，注入自定义日志提供器：

public static class MyLoggerExtensions
{
	public static ILoggingBuilder AddMyConsoleLogger(
		this ILoggingBuilder builder, Action<MyLoggerOptions> action)
	{
		MyLoggerOptions options = new();
		if (action != null)
		{
			action.Invoke(options);
		}

		builder.AddConfiguration();
		builder.Services.TryAddEnumerable(
			ServiceDescriptor.Singleton<ILoggerProvider>(new MyLoggerProvider(options)));
		return builder;
	}
}

最后使用 Microsoft.Extensions.Logging 中的 LoggerFactory，构建日志工厂，从中生成 ILogger 对象，最后打印日志：

static void Main(string[] args)
{
	using ILoggerFactory factory = LoggerFactory.Create(builder =>
	{
		builder.AddConsole();
		builder.AddMyConsoleLogger(options =>
		{
			options.DefaultLevel = LogLevel.Debug;
		});
	});
	ILogger logger1 = factory.CreateLogger("Program");
	ILogger logger2 = factory.CreateLogger<Program>();

	logger1.LogError(new Exception("报错了"), message: "Hello World! Logging is {Description}.", args: "error");
	logger2.LogError(new Exception("报错了"), message: "Hello World! Logging is {Description}.", args: "error");
}