using System;
using System.Threading;

namespace MediaBrowser.Common.Configuration
{
    /// <summary>
    /// Marks the flow of control that is applying an invalidation published by another instance, so the
    /// reactions to it can tell a remote write apart from a local one.
    /// </summary>
    /// <remarks>
    /// Applying an invalidation raises the same update events a local save raises, because the in-process
    /// consumers have to re-read the configuration either way. Some of those consumers answer an update by
    /// writing, and that write must neither repeat what the publishing instance already did nor fan back
    /// out over the bus. The flag rides the execution context, so it reaches the queued and asynchronous
    /// event handlers as well as the synchronous ones.
    /// </remarks>
    public static class ConfigurationInvalidationContext
    {
        private static readonly AsyncLocal<bool> _applyingRemoteInvalidation = new();

        /// <summary>
        /// Gets a value indicating whether the current flow of control is applying an invalidation
        /// published by another instance rather than handling a local save.
        /// </summary>
        public static bool IsApplyingRemoteInvalidation => _applyingRemoteInvalidation.Value;

        /// <summary>
        /// Marks the current flow of control as applying a remote invalidation until the returned scope is
        /// disposed.
        /// </summary>
        /// <returns>The scope to dispose once the invalidation has been applied.</returns>
        public static IDisposable BeginApply() => new ApplyScope();

        private sealed class ApplyScope : IDisposable
        {
            private readonly bool _previous;
            private bool _disposed;

            public ApplyScope()
            {
                _previous = _applyingRemoteInvalidation.Value;
                _applyingRemoteInvalidation.Value = true;
            }

            public void Dispose()
            {
                if (_disposed)
                {
                    return;
                }

                _disposed = true;
                _applyingRemoteInvalidation.Value = _previous;
            }
        }
    }
}