InactivityWatcher.cs

// Csproj: <PackageReference Include="Microsoft.Windows.CsWin32" Version="0.1.588-beta" PrivateAssets="all" />
// NativeMethods.txt: GetLastInputInfo

using System;
using System.Threading;
using Windows.Win32;
using Windows.Win32.UI.KeyboardAndMouseInput;

public sealed class InactivityWatcher : IDisposable
{
    private readonly TimeSpan inactivityDuration;
    private readonly Timer timer;
    private bool stopOrStartWasCalled;

    public event EventHandler Inactive;

    public InactivityWatcher(TimeSpan inactivityDuration)
    {
        this.inactivityDuration = inactivityDuration;
        timer = new(OnTimerCallback);
    }

    public void Dispose() => timer.Dispose();

    public void Restart()
    {
        stopOrStartWasCalled = true;
        timer.Change(dueTime: inactivityDuration, period: Timeout.InfiniteTimeSpan);
    }

    public void Stop()
    {
        stopOrStartWasCalled = true;
        timer.Change(dueTime: Timeout.InfiniteTimeSpan, period: Timeout.InfiniteTimeSpan);
    }

    private unsafe void OnTimerCallback(object state)
    {
        // Workaround for https://github.com/microsoft/win32metadata/issues/683#issuecomment-931662592
        var lastInputInfo = new LASTINPUTINFO { cbSize = (uint)sizeof(LASTINPUTINFO) };
        if (!PInvoke.GetLastInputInfo(&lastInputInfo))
        {
            Restart();
            return;
        }

        var timeSinceLastInputEvent = TimeSpan.FromMilliseconds(Environment.TickCount - lastInputInfo.dwTime);
        if (timeSinceLastInputEvent < inactivityDuration)
        {
            timer.Change(dueTime: inactivityDuration - timeSinceLastInputEvent, period: Timeout.InfiniteTimeSpan);
            return;
        }

        stopOrStartWasCalled = false;
        try
        {
            Inactive.Invoke(this, EventArgs.Empty);
        }
        finally
        {
            if (!stopOrStartWasCalled) Restart();
        }
    }
}