public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
  WebHost.CreateDefaultBuilder(args)
  .ConfigureAppConfiguration(option =>
   {
   option.AddJsonFile("appsettings.json",optional:true,reloadOnChange:true);
   })
  .UseStartup<Startup>();

 public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder,string path,bool optional,bool reloadOnChange)
 {
 return builder.AddJsonFile((IFileProvider) null, path, optional, reloadOnChange);
 }

 public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder,IFileProvider provider,string path,bool optional,bool reloadOnChange)
 {
  return builder.AddJsonFile((Action<JsonConfigurationSource>) (s =>
  {
  s.FileProvider = provider;
  s.Path = path;
  s.Optional = optional;
  s.ReloadOnChange = reloadOnChange;
  s.ResolveFileProvider();
  }));
 }

 public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder, Action<JsonConfigurationSource> configureSource)
 {
  return builder.Add<JsonConfigurationSource>(configureSource);
 }

 public static IConfigurationBuilder Add<TSource>(this IConfigurationBuilder builder,Action<TSource> configureSource)where TSource : IConfigurationSource, new()
 {
  TSource source = new TSource();
  if (configureSource != null)
  configureSource(source);
  return builder.Add((IConfigurationSource) source);
 }

public class ConfigurationBuilder : IConfigurationBuilder
 {
  public IList<IConfigurationSource> Sources { get; } = new List<IConfigurationSource>();

  public IConfigurationBuilder Add(IConfigurationSource source)
  {
   Sources.Add(source);
   return this;
  }

  public IConfigurationRoot Build()
  {
   var providers = new List<IConfigurationProvider>();
   foreach (var source in Sources)
   {
    var provider = source.Build(this);
    providers.Add(provider);
   }
   return new ConfigurationRoot(providers);
  }
 }

public static void Main(string[] args)
  {
   CreateWebHostBuilder(args).Build().Run();
  }

public class JsonConfigurationSource : FileConfigurationSource
 {
  public override IConfigurationProvider Build(IConfigurationBuilder builder)
  {
   EnsureDefaults(builder);
   return new JsonConfigurationProvider(this);
  }
 }

 public void EnsureDefaults(IConfigurationBuilder builder)
  {
   FileProvider = FileProvider "htmlcode">

 public class JsonConfigurationProvider : FileConfigurationProvider
 {
  
  public JsonConfigurationProvider(JsonConfigurationSource source) : base(source) { }

  
  public override void Load(Stream stream)
  {
   try {
    Data = JsonConfigurationFileParser.Parse(stream);
   } catch (JsonReaderException e)
   {
    throw new FormatException(Resources.Error_JSONParseError, e);
   }
  }
 }


看不出什么的代码,事出反常必有妖~~
看看base的构造函数.


 public FileConfigurationProvider(FileConfigurationSource source)
  {
   Source = source;

   if (Source.ReloadOnChange && Source.FileProvider != null)
   {
    _changeTokenRegistration = ChangeToken.OnChange(
     () => Source.FileProvider.Watch(Source.Path),
     () => {
      Thread.Sleep(Source.ReloadDelay);
      Load(reload: true);
     });
   }
  }


真是个天才,问题就在这个构造函数里,它构造函数调用了一个 ChangeToken.OnChange 方法,这是实现ReloadOnChange的关键,如果你点到这里还没有关掉,恭喜,好戏开始了.
ReloadOnChange
Talk is cheap. Show me the code (屁话少说,放 码 过来).


 public static class ChangeToken
 {
  public static ChangeTokenRegistration<Action> OnChange(Func<IChangeToken> changeTokenProducer, Action changeTokenConsumer)
  {
   return new ChangeTokenRegistration<Action>(changeTokenProducer, callback => callback(), changeTokenConsumer);
  }
 }


OnChange方法里,先不管什么func,action,就看看这两个参数的名称,producer,consumer,生产者,消费者,不知道看到这个关键词想到的是什么,反正我想到的是小学时学习食物链时的:snake:与:rat:.
那么我们来看看这里的:snake:是什么,:rat:又是什么,还得回到 FileConfigurationProvider 的构造函数.
可以看到生产者:rat:是:


() => Source.FileProvider.Watch(Source.Path)


消费者:snake:是:


() => {
 Thread.Sleep(Source.ReloadDelay);
 Load(reload: true);
}


我们想一下,一旦有一条:rat:跑出来,就立马被:snake:吃了,
那我们这里也一样,一旦有FileProvider.Watch返回了什么东西,就会发生Load()事件来重新加载数据.
:snake:与:rat:好理解,可是代码就没那么好理解了,我们通过 OnChange 的第一个参数 Func<IChangeToken> changeTokenProducer 方法知道,这里的:rat:,其实是 IChangeToken .
IChangeToken


 public interface IChangeToken
 {
  bool HasChanged { get; }

  bool ActiveChangeCallbacks { get; }

  IDisposable RegisterChangeCallback(Action<object> callback, object state);
 }


IChangeToken的重点在于里面有个RegisterChangeCallback方法,:snake:吃:rat:的这件事,就发生在这回调方法里面.
我们来做个:snake:吃:rat:的实验.
实验1


 static void Main()
  {
   //定义一个C:\Users\liuzh\MyBox\TestSpace目录的FileProvider
   var phyFileProvider = new PhysicalFileProvider("C:\\Users\\liuzh\\MyBox\\TestSpace");

   //让这个Provider开始监听这个目录下的所有文件
   var changeToken = phyFileProvider.Watch("*.*");

   //注册"老鼠被蛇吃"); }, new object());

   //添加一个文件到目录
   AddFileToPath();

   Console.ReadKey();

  }

  static void AddFileToPath()
  {
   Console.WriteLine("老鼠出洞了");
   File.Create("C:\\Users\\liuzh\\MyBox\\TestSpace\\老鼠出洞了.txt").Dispose();
  }


这是运行结果

可以看到,一旦在监听的目录下创建文件,立即触发了执行回调函数,但是如果我们继续手动地更改(复制)监听目录中的文件,回调函数就不再执行了.
这是因为changeToken监听到文件变更并触发回调函数后,这个changeToken的使命也就完成了,要想保持一直监听,那么我们就在在回调函数中重新获取token,并给新的token的回调函数注册通用的事件,这样就能保持一直监听下去了.
这也就是ChangeToken.Onchange所作的事情,我们看一下源码.


 public static class ChangeToken
 {
  public static ChangeTokenRegistration<Action> OnChange(Func<IChangeToken> changeTokenProducer, Action changeTokenConsumer)
  {
   return new ChangeTokenRegistration<Action>(changeTokenProducer, callback => callback(), changeTokenConsumer);
  }
 }
 public class ChangeTokenRegistration<TAction>
 {
  private readonly Func<IChangeToken> _changeTokenProducer;
  private readonly Action<TAction> _changeTokenConsumer;
  private readonly TAction _state;

  public ChangeTokenRegistration(Func<IChangeToken> changeTokenProducer, Action<TAction> changeTokenConsumer, TAction state)
  {
   _changeTokenProducer = changeTokenProducer;
   _changeTokenConsumer = changeTokenConsumer;
   _state = state;

   var token = changeTokenProducer();

   RegisterChangeTokenCallback(token);
  }

  private void RegisterChangeTokenCallback(IChangeToken token)
  {
   token.RegisterChangeCallback(_ => OnChangeTokenFired(), this);
  }

  private void OnChangeTokenFired()
  {
   var token = _changeTokenProducer();

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


简单来说,就是给token注册了一个 OnChangeTokenFired 的回调函数,仔细看看 OnChangeTokenFired 里做了什么,总体来说三步.
1.获取一个新的token.

2.调用消费者进行消费.

3.给新获取的token再次注册一个OnChangeTokenFired的回调函数.


如此周而复始~~
实验2
既然知道了OnChange的工作方式,那么我们把实验1的代码修改一下.


  static void Main()
  {
   var phyFileProvider = new PhysicalFileProvider("C:\\Users\\liuzh\\MyBox\\TestSpace");
   ChangeToken.OnChange(() => phyFileProvider.Watch("*.*"),
    () => { Console.WriteLine("老鼠被蛇吃"); });
   Console.ReadKey();
  }


执行效果看一下

可以看到,只要被监控的目录发生了文件变化,不管是新建文件,还是修改了文件内的内容,都会触发回调函数,其实JsonConfig中,这个回调函数就是Load(),它负责重新加载数据,可也就是为什么Asp .net core中如果把ReloadOnchang设置为true后,Json的配置一旦更新,配置就会自动重载.
PhysicalFilesWatcher
那么,为什么文件一旦变化,就会触发ChangeToken的回调函数呢"htmlcode">

 public PhysicalFilesWatcher(string root,FileSystemWatcher fileSystemWatcher,bool pollForChanges,ExclusionFilters filters)
 {
  this._root = root;
  this._fileWatcher = fileSystemWatcher;
  this._fileWatcher.IncludeSubdirectories = true;
  this._fileWatcher.Created += new FileSystemEventHandler(this.OnChanged);
  this._fileWatcher.Changed += new FileSystemEventHandler(this.OnChanged);
  this._fileWatcher.Renamed += new RenamedEventHandler(this.OnRenamed);
  this._fileWatcher.Deleted += new FileSystemEventHandler(this.OnChanged);
  this._fileWatcher.Error += new ErrorEventHandler(this.OnError);
  this.PollForChanges = pollForChanges;
  this._filters = filters;
  this.PollingChangeTokens = new ConcurrentDictionary<IPollingChangeToken, IPollingChangeToken>();
  this._timerFactory = (Func<Timer>) (() => NonCapturingTimer.Create(new TimerCallback(PhysicalFilesWatcher.RaiseChangeEvents), (object) this.PollingChangeTokens, TimeSpan.Zero, PhysicalFilesWatcher.DefaultPollingInterval));
 }


如果你和我一样,对源码感兴趣,可以从官方的 aspnet/Extensions 中下载源码研究: https://github.com/aspnet/Extensions
在下一篇文章中,我会讲解如何自定义一个以Mysql为数据源的ConfigureSoure,并实现自动更新功能,同时还会整理Configure相关类的UML类图,有兴趣的可以关注我以便第一时间收到下篇文章.
本文章涉及的代码地址: https://github.com/liuzhenyulive/MiniConfiguration
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。