[Asp.net 5] Logging-日志系统的基本架构（上）

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本节主要介绍解决方案中的Microsoft.Framework.Logging.Abstractions、Microsoft.Framework.Logging俩个工程。

这俩个工程中所有类的关系如下图所示：

首先我们可以发现处于核心的是中间的四个接口：ILogger、ILoggerFactory、ILoggerProvider、ILogValues。

ILogger：记录日志的接口，所以写日志的类都该实现该接口，工程中有俩个类实现了该接口：Logger、Logger<T>
ILoggerFactory：创建ILogger的工厂。负责创建工厂的逻辑，但是一般不直接创建，而是调用内部ILoggerProvider去完成。
ILoggerProvider：能够直接创建ILogger实例，做为属性添加到ILoggerFactory中，ILogger的具体逻辑受控制ILoggerFactory。
ILogValues：在Logger的扩展方法中作为特殊类型的object传入，作为日志的数据源。

接口的定义源码如下：

    public interface ILogger
    {
        void Log(LogLevel logLevel, int eventId, object state, Exception exception, Func<object, Exception, string> formatter);

        bool IsEnabled(LogLevel logLevel);
        
        IDisposable BeginScopeImpl(object state);
    }

ILogger

    public interface ILoggerFactory
    {
        LogLevel MinimumLevel { get; set; }

        ILogger CreateLogger(string categoryName);

        void AddProvider(ILoggerProvider provider);
    }

ILoggerFactory

    public interface ILoggerProvider
    {
        ILogger CreateLogger(string name);
    }

ILoggerProvider

    public interface ILogValues
    {
        IEnumerable<KeyValuePair<string, object>> GetValues();
    }

ILogValues

ILogger以及实现类

ILogger接口、ILogger<TCategoryName>（没有任何定义）、Logger<T>这种泛型继承是否有相识之感，之前的博客文章中已经对于这种情况有所介绍（[Asp.net 5] Localization-resx资源文件的管理中IStringLocalizer 、IStringLocalizer<T> 、StringLocalizer<TResourceSource>是一致的）。实际Logger<T>中T是ILogger的实例子类，实际就是使用代理模式，内部包含ILogger实例，并且所有对外的方法都仅仅是内部ILogger实例的封装。

    public class Logger<T> : ILogger<T>
    {
        private readonly ILogger _logger;
        
        /// <summary>
        /// Creates a new <see cref="Logger{T}"/>.
        /// </summary>
        /// <param name="factory">The factory.</param>
        public Logger(ILoggerFactory factory)
        {
            _logger = factory.CreateLogger<T>();
        }

        IDisposable ILogger.BeginScopeImpl(object state)
        {
            return _logger.BeginScopeImpl(state);
        }

        bool ILogger.IsEnabled(LogLevel logLevel)
        {
            return _logger.IsEnabled(logLevel);
        }

        void ILogger.Log(LogLevel logLevel, int eventId, object state, Exception exception, Func<object, Exception, string> formatter)
        {
            _logger.Log(logLevel, eventId, state, exception, formatter);
        }
    }

Logger

Logger类同样实现了ILogger接口，也同样使用了代理模式，不过不同于Logger<T>泛型，Logger类有自己的内部逻辑。而是在内部封装了 ILogger[] _loggers对象。使得Logger更像LoggerManage。但是由于Logger同样实现Logger接口，所以Logger类是管理其它Logger类的代理。而Logger内部的_loggers是通过LoggerFactory对象封装的。当记录日志时，依次遍历内部的 _loggers对象，进行写日志操作。

    internal class Logger : ILogger
    {
        private readonly LoggerFactory _loggerFactory;
        private readonly string _name;
        private ILogger[] _loggers = new ILogger[0];

        public Logger(LoggerFactory loggerFactory, string name)
        {
            _loggerFactory = loggerFactory;
            _name = name;

            var providers = loggerFactory.GetProviders();
            _loggers = new ILogger[providers.Length];
            for (var index = 0; index != providers.Length; index++)
            {
                _loggers[index] = providers[index].CreateLogger(name);
            }
        }

        public void Log(LogLevel logLevel, int eventId, object state, Exception exception, Func<object, Exception, string> formatter)
        {
            if (logLevel >= _loggerFactory.MinimumLevel)
            {
                foreach (var logger in _loggers)
                {
                    logger.Log(logLevel, eventId, state, exception, formatter);
                }
            }
        }

        public bool IsEnabled(LogLevel logLevel)
        {
            if (logLevel < _loggerFactory.MinimumLevel)
            {
                return false;
            }
            foreach (var logger in _loggers)
            {
                if (logger.IsEnabled(logLevel))
                {
                    return true;
                }
            }
            return false;
        }

        public IDisposable BeginScopeImpl(object state)
        {
            var loggers = _loggers;
            var scope = new Scope(loggers.Length);
            for (var index = 0; index != loggers.Length; index++)
            {
                scope.SetDisposable(index, loggers[index].BeginScopeImpl(state));
            }
            return scope;
        }

        internal void AddProvider(ILoggerProvider provider)
        {
            var logger = provider.CreateLogger(_name);
            _loggers = _loggers.Concat(new[] { logger }).ToArray();
        }

        private class Scope : IDisposable
        {
            private bool _isDisposed;

            private IDisposable _disposable0;
            private IDisposable _disposable1;
            private readonly IDisposable[] _disposable;

            public Scope(int count)
            {
                if (count > 2)
                {
                    _disposable = new IDisposable[count - 2];
                }
            }

            public void SetDisposable(int index, IDisposable disposable)
            {
                if (index == 0)
                {
                    _disposable0 = disposable;
                }
                else if (index == 1)
                {
                    _disposable1 = disposable;
                }
                else
                {
                    _disposable[index - 2] = disposable;
                }
            }

            protected virtual void Dispose(bool disposing)
            {
                if (!_isDisposed)
                {
                    if (disposing)
                    {
                        if (_disposable0 != null)
                        {
                            _disposable0.Dispose();
                        }
                        if (_disposable1 != null)
                        {
                            _disposable1.Dispose();
                        }
                        if (_disposable != null)
                        {
                            var count = _disposable.Length;
                            for (var index = 0; index != count; ++index)
                            {
                                if (_disposable[index] != null)
                                {
                                    _disposable[index].Dispose();
                                }
                            }
                        }
                    }

                    _isDisposed = true;
                }
            }

            // This code added to correctly implement the disposable pattern.
            public void Dispose()
            {
                // Do not change this code. Put cleanup code in Dispose(bool disposing) above.
                Dispose(true);
                // TODO: tell GC not to call its finalizer when the above finalizer is overridden.
                // GC.SuppressFinalize(this);
            }

            internal void Add(IDisposable disposable)
            {
                throw new NotImplementedException();
            }
        }
    }

Logger

ILoggerFactory以及实现类

ILoggerFactory主要有俩个作用，添加新的ILoggerProvider，创建ILogger。而ILoggerFactory的唯一实现类LoggerFactory，简单实现了上面两个功能，并且在LoggerFactory内部额外维护_loggers副本，该处loggers都是Logger类型（非泛型），只是categoryName不同。所以通过LoggerFactory创建的Logger如下图所示：

    public class LoggerFactory : ILoggerFactory
    {
        private readonly Dictionary<string, Logger> _loggers = new Dictionary<string, Logger>(StringComparer.Ordinal);
        private ILoggerProvider[] _providers = new ILoggerProvider[0];
        private readonly object _sync = new object();

        public ILogger CreateLogger(string categoryName)
        {
            Logger logger;
            lock (_sync)
            {
                if (!_loggers.TryGetValue(categoryName, out logger))
                {
                    logger = new Logger(this, categoryName);
                    _loggers[categoryName] = logger;
                }
            }
            return logger;
        }

        public LogLevel MinimumLevel { get; set; } = LogLevel.Verbose;

        public void AddProvider(ILoggerProvider provider)
        {
            lock (_sync)
            {
                _providers = _providers.Concat(new[] { provider }).ToArray();
                foreach (var logger in _loggers)
                {
                    logger.Value.AddProvider(provider);
                }
            }
        }

        internal ILoggerProvider[] GetProviders()
        {
            return _providers;
        }
    }