acdream/src/AcDream.App/Rendering/HostQuiescenceGate.cs

namespace AcDream.App.Rendering;

/// <summary>
/// Single logical admission gate for callbacks entering the application host.
/// Physical event removal remains important for ownership, but closing this
/// gate first makes a still-attached native callback inert while teardown
/// converges.
/// </summary>
internal sealed class HostQuiescenceGate
{
    private readonly object _sync = new();
    private bool _accepting = true;

    public bool IsAccepting
    {
        get
        {
            lock (_sync)
                return _accepting;
        }
    }

    internal bool IsEnteredByCurrentThread => Monitor.IsEntered(_sync);

    /// <summary>
    /// Closes admission after any callback already executing on another
    /// thread has returned. Monitor reentrancy lets the native Closing
    /// callback close its own gate without waiting on itself.
    /// </summary>
    public void StopAccepting()
    {
        lock (_sync)
            _accepting = false;
    }

    public void Invoke(Action callback)
    {
        ArgumentNullException.ThrowIfNull(callback);
        lock (_sync)
        {
            if (_accepting)
                callback();
        }
    }

    public void Invoke<T>(Action<T> callback, T value)
    {
        ArgumentNullException.ThrowIfNull(callback);
        lock (_sync)
        {
            if (_accepting)
                callback(value);
        }
    }
}