internal class OptionsCache<TOptions> where TOptions : class, new(){private readonly Func<TOptions> _createCache;private object _cacheLock = new object();private bool _cacheInitialized;private TOptions _options;private IEnumerable<IConfigureOptions<TOptions>> _setups;public OptionsCache(IEnumerable<IConfigureOptions<TOptions>> setups){_setups = setups;_createCache = CreateOptions;}private TOptions CreateOptions(){var result = new TOptions();if (_setups != null){foreach (var setup in _setups){setup.Configure(result);}}return result;}public virtual TOptions Value{get{return LazyInitializer.EnsureInitialized(ref _options,ref _cacheInitialized,ref _cacheLock,_createCache);}}}

    public class ConfigureOptions<TOptions> : IConfigureOptions<TOptions> where TOptions : class{public ConfigureOptions(Action<TOptions> action){if (action == null){throw new ArgumentNullException(nameof(action));}Action = action;}public Action<TOptions> Action { get; private set; }public virtual void Configure(TOptions options){if (options == null){throw new ArgumentNullException(nameof(options));}Action.Invoke(options);}}

    public class OptionsMonitor<TOptions> : IOptionsMonitor<TOptions> where TOptions : class, new(){private OptionsCache<TOptions> _optionsCache;private readonly IEnumerable<IConfigureOptions<TOptions>> _setups;private readonly IEnumerable<IOptionsChangeTokenSource<TOptions>> _sources;public OptionsMonitor(IEnumerable<IConfigureOptions<TOptions>> setups, IEnumerable<IOptionsChangeTokenSource<TOptions>> sources){_sources = sources;_setups = setups;_optionsCache = new OptionsCache<TOptions>(setups);}public TOptions CurrentValue{get{return _optionsCache.Value;}}public IDisposable OnChange(Action<TOptions> listener){var disposable = new ChangeTrackerDisposable();foreach (var source in _sources){Action<object> callback = null;IDisposable previousSubscription = null;callback = (s) =>{// 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.var token = source.GetChangeToken();// Recompute the options before calling the watchers_optionsCache = new OptionsCache<TOptions>(_setups);listener(_optionsCache.Value);// Remove the old callback after its been firedvar nextSubscription = token.RegisterChangeCallback(callback, s);disposable.Disposables.Add(nextSubscription);disposable.Disposables.Remove(previousSubscription);previousSubscription = nextSubscription;};previousSubscription = source.GetChangeToken().RegisterChangeCallback(callback, state: null);disposable.Disposables.Add(previousSubscription);}return disposable;}}