using System.Diagnostics;

namespace ZymonicServices;

/// <summary>Call from a background timer. At most one callback may be waiting on the dispatcher.</summary>
public sealed class ZymonicDispatcherProbe
{
    private readonly Action<Action> _post;
    private int _pending;
    private int _generation;

    public ZymonicDispatcherProbe(Action<Action> post) => _post = post;

    // Keep an outstanding callback outstanding, but discard its timing after suspension or disabling.
    public void Invalidate() => Interlocked.Increment(ref _generation);

    public void Poll(ZymonicPerformanceRecorder recorder)
    {
        if (Interlocked.CompareExchange(ref _pending, 1, 0) == 0)
        {
            int generation = Volatile.Read(ref _generation);
            long started = Stopwatch.GetTimestamp();
            try
            {
                _post(() =>
                {
                    try
                    {
                        if (generation == Volatile.Read(ref _generation))
                        {
                            recorder.RecordElapsed("ui.dispatch", started);
                        }
                    }
                    finally
                    {
                        Volatile.Write(ref _pending, 0);
                    }
                });
            }
            catch
            {
                // A dispatcher can reject a post during shutdown. Diagnostics must not crash the app.
                Volatile.Write(ref _pending, 0);
            }
        }
    }
}