ASP.NET Core 6 實作 Logger

前言

ASP.NET Core 已有許多的 Logging Provider ，
如果自已實作一個 Color Console 的 Logger 要如何做呢?

實作

需要一個 Config 物件來記錄設定值，Logger 物件來寫 Log，
Logger Provider 物件負責建立 Logger 及 Extensions 物件來簡化註冊 Logger。

1.建立 CustomLoggerConfiguration

public class CustomLoggerConfiguration
{
    public int EventId { get; set; }
    public Dictionary<LogLevel, ConsoleColor> LogLevels { get; set; } = new()
    {
        [LogLevel.Information] = ConsoleColor.Yellow
    };
}

2.建立 Logger 物件

public class CustomLogger : ILogger
{
    private static object _lock = new object();
    private readonly string _name;
    private readonly Func<CustomLoggerConfiguration> _getCurrentConfig;

    public CustomLogger(string name,
        Func<CustomLoggerConfiguration> getCurrentConfig) =>
            (_name, _getCurrentConfig) = (name, getCurrentConfig);

    public IDisposable BeginScope<TState>(TState state) => default!;

    public bool IsEnabled(LogLevel logLevel) => 
        _getCurrentConfig().LogLevels.ContainsKey(logLevel);

    public void Log<TState>(LogLevel logLevel,
        EventId eventId,
        TState state,
        Exception exception,
        Func<TState, Exception, string> formatter
        )
    {
        if (!IsEnabled(logLevel))
        {
            return;
        }
        lock (_lock)
        {
            var config = _getCurrentConfig();
            if (config.EventId == 0 ||
                config.EventId == eventId.Id)
            {
                var color = Console.ForegroundColor;
                Console.ForegroundColor = config.LogLevels[logLevel];
                Console.Write($"{logLevel} - ");
                Console.Write($"{eventId.Id} - {_name} -");
                Console.WriteLine($"{formatter(state, exception)}");
                Console.ForegroundColor = color;
            }
        }
    }
}

註: Console 並不是 Thread-Safe 物件，所以這裡使用 lock
註: Config 有設定，才會啟用 Log

3.建立 Logger Provider

public class CustomLoggerProvider :
    ILoggerProvider
{
    private readonly IDisposable _onChangeToken;
    private CustomLoggerConfiguration _currentConfig;
    private readonly ConcurrentDictionary<string, CustomLogger> _loggers =
        new(StringComparer.OrdinalIgnoreCase);

    public CustomLoggerProvider(IOptionsMonitor<CustomLoggerConfiguration> config)
    {
        _currentConfig = config.CurrentValue;
        _onChangeToken = config.OnChange(updatedConfig => _currentConfig = updatedConfig);
    }

    private CustomLoggerConfiguration GetCurrentConfig() => _currentConfig;

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

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

4.建立 Extensions 物件來簡化註冊 Logger

public static class CustomLoggerExtensions
{
    public static ILoggingBuilder AddCustomLogger(this ILoggingBuilder builder)
    {
        builder.AddConfiguration();

        builder.Services.TryAddEnumerable(
            ServiceDescriptor.Singleton<ILoggerProvider, CustomLoggerProvider>());

        LoggerProviderOptions.RegisterProviderOptions
            <CustomLoggerConfiguration, CustomLoggerProvider>(builder.Services);

        return builder;
    }

    public static ILoggingBuilder AddCustomLogger(this ILoggingBuilder builder,
        Action<CustomLoggerConfiguration> configure)
    {
        builder.AddCustomLogger();
        builder.Services.Configure(configure);
        return builder;
    }
}

5.在 Program.cs 中使用 CustomLogger

//Program.cs
builder.Logging.ClearProviders();
builder.Logging.AddCustomLogger();

6.在 appsettings.json 中設定 LogLevel 要顯示的 Color

{
  "Logging": {
    "LogLevel": {
      "Default": "Trace",
      "Microsoft": "Warning"
    },
    "CustomLogger": {
      "LogLevels": {
        "Trace": "White",
        "Debug": "Blue",
        "Information": "Green",
        "Warning": "DarkYellow",
        "Error": "Red",
        "Critical": "Cyan"
      }
    }
  }
}

7.在 HomeControll 中依各 LogLevel 寫出 Log

logger.LogTrace("Log Trace");
logger.LogDebug("Log Debue");
logger.LogInformation(" Log Info");
logger.LogWarning("Log Warning");
logger.LogError("Log Error");
logger.LogCritical("Log Critical");

註: 程式運行中，修改 appsettings.json 中 Debug 為 DarkBlue ，重整頁面，Log 就會套用新的設定值

參考資源

Logging in .NET Core and ASP.NET Core
Implement a custom logging provider in .NET
ConsoleColor Enum