重新整理 .net core 实践篇—————配置系统之间谍[八]（文件监控）

前言

前文提及到了当我们的配置文件修改了，那么从 configurationRoot 在此读取会读取到新的数据，本文进行扩展，并从源码方面简单介绍一下，下面内容和前面几节息息相关。

正文

先看一下，如果文件修改，那么是否有一个回调函数，可以回调呢？

答案是有的:

IChangeToken IConfiguration.GetReloadToken()

这里演示一下:

IConfigurationBuilder builder = new ConfigurationBuilder();
builder.AddJsonFile(System.AppDomain.CurrentDomain.BaseDirectory + "/appsettings.json",optional:false,reloadOnChange: true);
var configurationRoot = builder.Build();

Console.WriteLine(configurationRoot["key1"]);
Console.WriteLine(configurationRoot["key2"]);

IChangeToken token = configurationRoot.GetReloadToken();

token.RegisterChangeCallback(state =>
{
	Console.WriteLine(configurationRoot["key1"]);
	Console.WriteLine(configurationRoot["key2"]);
},configurationRoot);

Console.ReadKey();

一开始的值是:

{
  "key1": "value1",
  "key2": "value2"
}

然后我进行了修改:

{
  "key1": "value1_change",
  "key2": "value2_change"
}

结果如下:

源码解读一下为什么这么做，因为在我们写代码中，这种监听场景比较常见,这里就以此为例。

如果下文如果感到有点不适，请先看一下这个:https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/change-tokens?view=aspnetcore-3.1

private readonly IList<IDisposable> _changeTokenRegistrations;
private ConfigurationReloadToken _changeToken = new ConfigurationReloadToken();
public ConfigurationRoot(IList<IConfigurationProvider> providers)
{
	if (providers == null)
	{
		throw new ArgumentNullException(nameof(providers));
	}

	_providers = providers;
	_changeTokenRegistrations = new List<IDisposable>(providers.Count);
	foreach (IConfigurationProvider p in providers)
	{
		p.Load();
		_changeTokenRegistrations.Add(ChangeToken.OnChange(() => p.GetReloadToken(), () => RaiseChanged()));
	}
}
private void RaiseChanged()
{
	ConfigurationReloadToken previousToken = Interlocked.Exchange(ref _changeToken, new ConfigurationReloadToken());
	previousToken.OnReload();
}

在ConfigurationRoot实例化的时候就为每一个provider 注册了监听事件，同时定义了回调事件。

然后看一下GetReloadToken:

/// <summary>
/// Returns a <see cref="IChangeToken"/> that can be used to observe when this configuration is reloaded.
/// </summary>
/// <returns>The <see cref="IChangeToken"/>.</returns>
public IChangeToken GetReloadToken() => _changeToken;

这里返回了ConfigurationReloadToken，也就是获取到监听对象,故而我们能够被回调。

https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/change-tokens?view=aspnetcore-3.1 中解释的比较详细故而不过多赘述。

那么就来看下json配置文件的Provider，看下其为啥能够这么监听。

public class JsonConfigurationProvider : FileConfigurationProvider
{
	/// <summary>
	/// Initializes a new instance with the specified source.
	/// </summary>
	/// <param name="source">The source settings.</param>
	public JsonConfigurationProvider(JsonConfigurationSource source) : base(source) { }

	/// <summary>
	/// Loads the JSON data from a stream.
	/// </summary>
	/// <param name="stream">The stream to read.</param>
	public override void Load(Stream stream)
	{
		try
		{
			Data = JsonConfigurationFileParser.Parse(stream);
		}
		catch (JsonException e)
		{
			throw new FormatException(SR.Error_JSONParseError, e);
		}
	}
}

上面的操作_changeTokenRegistrations.Add(ChangeToken.OnChange(() => p.GetReloadToken(), () => RaiseChanged())); 就能解释的通了，原理是利用文件系统的GetReloadToken()令牌，

只是在FileConfigurationProvider 上封装了一层转换。

简单看下:FileConfigurationProvider，下面值保留了Load部分。

public abstract class FileConfigurationProvider : ConfigurationProvider, IDisposable
{
	private void Load(bool reload)
	{
		IFileInfo file = Source.FileProvider?.GetFileInfo(Source.Path);
		if (file == null || !file.Exists)
		{
			if (Source.Optional || reload) // Always optional on reload
			{
				Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
			}
			else
			{
				var error = new StringBuilder($"The configuration file '{Source.Path}' was not found and is not optional.");
				if (!string.IsNullOrEmpty(file?.PhysicalPath))
				{
					error.Append($" The physical path is '{file.PhysicalPath}'.");
				}
				HandleException(ExceptionDispatchInfo.Capture(new FileNotFoundException(error.ToString())));
			}
		}
		else
		{
			// Always create new Data on reload to drop old keys
			if (reload)
			{
				Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
			}

			static Stream OpenRead(IFileInfo fileInfo)
			{
				if (fileInfo.PhysicalPath != null)
				{
					// The default physical file info assumes asynchronous IO which results in unnecessary overhead
					// especally since the configuration system is synchronous. This uses the same settings
					// and disables async IO.
					return new FileStream(
						fileInfo.PhysicalPath,
						FileMode.Open,
						FileAccess.Read,
						FileShare.ReadWrite,
						bufferSize: 1,
						FileOptions.SequentialScan);
				}

				return fileInfo.CreateReadStream();
			}

			using Stream stream = OpenRead(file);
			try
			{
				Load(stream);
			}
			catch (Exception e)
			{
				HandleException(ExceptionDispatchInfo.Capture(e));
			}
		}
		// REVIEW: Should we raise this in the base as well / instead?
		OnReload();
	}

	/// <summary>
	/// Loads the contents of the file at <see cref="Path"/>.
	/// </summary>
	/// <exception cref="FileNotFoundException">If Optional is <c>false</c> on the source and a
	/// file does not exist at specified Path.</exception>
	public override void Load()
	{
		Load(reload: false);
	}
}

看下上面的load，上面的load就是读取文件，然后交由JsonConfigurationProvider 的load调用 Data = JsonConfigurationFileParser.Parse(stream);转换为字典。

这就是为上文中的ConfigurationRoot 要调用一下load了。

上文的ConfigurationRoot 调用Load 部分。

foreach (IConfigurationProvider p in providers)
{
	p.Load();
	_changeTokenRegistrations.Add(ChangeToken.OnChange(() => p.GetReloadToken(), () => RaiseChanged()));
}

这就回到了对前面系列中的内存字典操作了,而FileConfigurationProvider 又继承ConfigurationProvider。

ConfigurationProvider 代码如下，主要是实现IConfigurationProvider接口,很多不同的文件配置都会用到这个，比如说ini文件、xml文件等等都会先转换为字典，然后继承ConfigurationProvider:

public abstract class ConfigurationProvider : IConfigurationProvider
{
	private ConfigurationReloadToken _reloadToken = new ConfigurationReloadToken();

	/// <summary>
	/// Initializes a new <see cref="IConfigurationProvider"/>
	/// </summary>
	protected ConfigurationProvider()
	{
		Data = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
	}

	/// <summary>
	/// The configuration key value pairs for this provider.
	/// </summary>
	protected IDictionary<string, string> Data { get; set; }

	/// <summary>
	/// Attempts to find a value with the given key, returns true if one is found, false otherwise.
	/// </summary>
	/// <param name="key">The key to lookup.</param>
	/// <param name="value">The value found at key if one is found.</param>
	/// <returns>True if key has a value, false otherwise.</returns>
	public virtual bool TryGet(string key, out string value)
		=> Data.TryGetValue(key, out value);

	/// <summary>
	/// Sets a value for a given key.
	/// </summary>
	/// <param name="key">The configuration key to set.</param>
	/// <param name="value">The value to set.</param>
	public virtual void Set(string key, string value)
		=> Data[key] = value;

	/// <summary>
	/// Loads (or reloads) the data for this provider.
	/// </summary>
	public virtual void Load()
	{ }

	/// <summary>
	/// Returns the list of keys that this provider has.
	/// </summary>
	/// <param name="earlierKeys">The earlier keys that other providers contain.</param>
	/// <param name="parentPath">The path for the parent IConfiguration.</param>
	/// <returns>The list of keys for this provider.</returns>
	public virtual IEnumerable<string> GetChildKeys(
		IEnumerable<string> earlierKeys,
		string parentPath)
	{
		string prefix = parentPath == null ? string.Empty : parentPath + ConfigurationPath.KeyDelimiter;

		return Data
			.Where(kv => kv.Key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
			.Select(kv => Segment(kv.Key, prefix.Length))
			.Concat(earlierKeys)
			.OrderBy(k => k, ConfigurationKeyComparer.Instance);
	}

	private static string Segment(string key, int prefixLength)
	{
		int indexOf = key.IndexOf(ConfigurationPath.KeyDelimiter, prefixLength, StringComparison.OrdinalIgnoreCase);
		return indexOf < 0 ? key.Substring(prefixLength) : key.Substring(prefixLength, indexOf - prefixLength);
	}

	/// <summary>
	/// Returns a <see cref="IChangeToken"/> that can be used to listen when this provider is reloaded.
	/// </summary>
	/// <returns>The <see cref="IChangeToken"/>.</returns>
	public IChangeToken GetReloadToken()
	{
		return _reloadToken;
	}

	/// <summary>
	/// Triggers the reload change token and creates a new one.
	/// </summary>
	protected void OnReload()
	{
		ConfigurationReloadToken previousToken = Interlocked.Exchange(ref _reloadToken, new ConfigurationReloadToken());
		previousToken.OnReload();
	}

	/// <summary>
	/// Generates a string representing this provider name and relevant details.
	/// </summary>
	/// <returns> The configuration name. </returns>
	public override string ToString() => $"{GetType().Name}";
}

上述就是这个框架实现文件配置和文件监控的大致原理了。

这里再梳理一遍，使用JsonConfigurationFileParser.Parse 将steam流转换成字典，利用ChangeToken 对文件进行监听，如果有修改从加载即可。

好了，看完原理后，我们发现是ChangeToken的监听机制。那么问题来了，如果你看过上述ChangeToken的链接，你会发现RegisterChangeCallback只会调用一次。

原理很简单，因为这是令牌机制的，令牌过期了，那么这个RegisterChangeCallback自然调用一次，因为过期只有一次。

我们可以无限套娃方式:

static void Main(string[] args)
{
	IConfigurationBuilder builder = new ConfigurationBuilder();
	// builder.AddJsonFile(System.AppDomain.CurrentDomain.BaseDirectory + "/appsettings.dev.json", optional: false, reloadOnChange: true);
	builder.AddJsonFile(System.AppDomain.CurrentDomain.BaseDirectory + "/appsettings.json",optional:false,reloadOnChange: true);
	var configurationRoot = builder.Build();

	Console.WriteLine(configurationRoot["key1"]);
	Console.WriteLine(configurationRoot["key2"]);

	Register(configurationRoot);

	Console.ReadKey();
}

public  static void Register(IConfigurationRoot configurationRoot)
{
	IChangeToken token = configurationRoot.GetReloadToken();

	token.RegisterChangeCallback(state =>
	{
		Console.WriteLine(configurationRoot["key1"]);
		Console.WriteLine(configurationRoot["key2"]);
		Register(configurationRoot);
	}, configurationRoot);
}

也可以这么做，利用ChangeToken 本身的方法:

static void Main(string[] args)
{
	IConfigurationBuilder builder = new ConfigurationBuilder();
	// builder.AddJsonFile(System.AppDomain.CurrentDomain.BaseDirectory + "/appsettings.dev.json", optional: false, reloadOnChange: true);
	builder.AddJsonFile(System.AppDomain.CurrentDomain.BaseDirectory + "/appsettings.json",optional:false,reloadOnChange: true);
	var configurationRoot = builder.Build();

	Console.WriteLine(configurationRoot["key1"]);
	Console.WriteLine(configurationRoot["key2"]);

	ChangeToken.OnChange(configurationRoot.GetReloadToken, () =>
	{
		Console.WriteLine(configurationRoot["key1"]);
		Console.WriteLine(configurationRoot["key2"]);
	});

	Console.ReadKey();
}

这里OnChange的原理也是套娃，我把关键代码贴一下。

public static class ChangeToken
{
	/// <summary>
	/// Registers the <paramref name="changeTokenConsumer"/> action to be called whenever the token produced changes.
	/// </summary>
	/// <param name="changeTokenProducer">Produces the change token.</param>
	/// <param name="changeTokenConsumer">Action called when the token changes.</param>
	/// <returns></returns>
	public static IDisposable OnChange(Func<IChangeToken> changeTokenProducer, Action changeTokenConsumer)
	{
		if (changeTokenProducer == null)
		{
			throw new ArgumentNullException(nameof(changeTokenProducer));
		}
		if (changeTokenConsumer == null)
		{
			throw new ArgumentNullException(nameof(changeTokenConsumer));
		}

		return new ChangeTokenRegistration<Action>(changeTokenProducer, callback => callback(), changeTokenConsumer);
	}
	private class ChangeTokenRegistration<TState> : IDisposable
	{
		   private void OnChangeTokenFired()
		{
			// The order here is important. We need to take the token and then apply our changes BEFORE
			// registering. This prevents us from possible having two change updates to process concurrently.
			//
			// If the token changes after we take the token, then we'll process the update immediately upon
			// registering the callback.
			IChangeToken token = _changeTokenProducer();

			try
			{
				_changeTokenConsumer(_state);
			}
			finally
			{
				// We always want to ensure the callback is registered
				RegisterChangeTokenCallback(token);
			}
		}

		private void RegisterChangeTokenCallback(IChangeToken token)
		{
			IDisposable registraton = token.RegisterChangeCallback(s => ((ChangeTokenRegistration<TState>)s).OnChangeTokenFired(), this);

			SetDisposable(registraton);
		}
	}
}

同样是套娃工程，SetDisposable是关键，比我们自己写要好，回收机制利用到位，有兴趣可以看下。

结

以上只是个人整理，如有错误，望请指出，谢谢。

下一节配置系统之变色龙（环境配置）。