cursor-lang/CursorLang.Settings.Tests/Infrastructure/Sta.cs

using System.Windows;
using System.Windows.Threading;

namespace CursorLang.Settings.Tests.Infrastructure;

/// <summary>
/// The user interface thread for the tests.
/// </summary>
/// <remarks>
/// Half of the application — windows, the dispatcher and the timers on it —
/// only works on an STA thread with a message queue, while tests run on a pool
/// thread. The thread is therefore started once for the whole run: a process
/// may hold only one <see cref="Application"/>, and recreating it between tests
/// is not possible.
///
/// The queue on that thread is pumped for real, so timers fire on their own:
/// the test only has to await the consequences through <see cref="WaitFor"/>.
/// </remarks>
internal static class Sta
{
    private static readonly Lock Gate = new();

    private static readonly TimeSpan DefaultTimeout = TimeSpan.FromSeconds(5);

    internal static Dispatcher Dispatcher
    {
        get
        {
            lock (Gate)
            {
                return field ??= Start();
            }
        }
    }

    /// <summary>Runs an action on the interface thread and waits for it to finish.</summary>
    internal static void Run(Action action) => Dispatcher.Invoke(action);

    /// <summary>The same for an action that returns a result.</summary>
    internal static TResult Run<TResult>(Func<TResult> action) => Dispatcher.Invoke(action);

    /// <summary>
    /// Runs an action on a separate STA thread and waits for it to finish.
    /// </summary>
    /// <remarks>
    /// Needed where the foreignness of the thread is the point: kernel objects
    /// such as a mutex let their own owner in again, so a "second instance" of
    /// the application on the same thread does not count as second.
    /// </remarks>
    internal static void RunApart(Action action)
    {
        Exception? failure = null;

        var thread = new Thread(() =>
        {
            try
            {
                action();
            }
            catch (Exception e)
            {
                failure = e;
            }
        })
        {
            IsBackground = true,
            Name = "CursorLang.Tests apart",
        };

        thread.SetApartmentState(ApartmentState.STA);
        thread.Start();
        thread.Join();

        if (failure is not null)
        {
            throw new InvalidOperationException("The action on the separate thread failed", failure);
        }
    }

    /// <summary>
    /// Waits until the dispatcher queue drains: calls deferred through
    /// <c>BeginInvoke</c> have run by that time.
    /// </summary>
    internal static void Drain() =>
        Dispatcher.Invoke(static () => { }, DispatcherPriority.ApplicationIdle);

    /// <summary>
    /// Waits for a condition without getting in the way of the timers.
    /// </summary>
    /// <remarks>
    /// Waiting is allowed from any thread, including the interface thread
    /// itself: there a plain wait would stop the message queue, and with it
    /// everything being waited for. On the interface thread the queue therefore
    /// keeps being pumped by a nested loop.
    /// </remarks>
    internal static void WaitFor(Func<bool> condition, string because, TimeSpan? timeout = null)
    {
        DateTime deadline = DateTime.UtcNow + (timeout ?? DefaultTimeout);

        while (!condition())
        {
            Assert.True(DateTime.UtcNow < deadline, $"Waited in vain: {because}");
            Idle(TimeSpan.FromMilliseconds(5));
        }
    }

    /// <summary>
    /// Waits for the given time while still pumping the queue: that is how
    /// "nothing happened during this time" is verified.
    /// </summary>
    internal static void Pause(TimeSpan duration)
    {
        Idle(duration);
        Drain();
    }

    // A wait during which the dispatcher queue gets its chance to run
    private static void Idle(TimeSpan duration)
    {
        if (Dispatcher.CheckAccess())
        {
            var frame = new DispatcherFrame();
            var timer = new DispatcherTimer(
                duration,
                DispatcherPriority.Background,
                (_, _) => frame.Continue = false,
                Dispatcher);

            try
            {
                Dispatcher.PushFrame(frame);
            }
            finally
            {
                timer.Stop();
            }

            return;
        }

        Thread.Sleep(duration);
    }

    private static Dispatcher Start()
    {
        var ready = new TaskCompletionSource<Dispatcher>();

        var thread = new Thread(() =>
        {
            Dispatcher dispatcher = Dispatcher.CurrentDispatcher;

            // The application has no reason to shut down after its windows:
            // tests open and close them by the dozen
            var application = new Application { ShutdownMode = ShutdownMode.OnExplicitShutdown };

            // The shared style dictionary is merged by App.xaml, which the tests
            // do not have. Without it the settings window still builds, but not
            // the way the user will see it
            application.Resources.MergedDictionaries.Add(new ResourceDictionary
            {
                Source = new Uri(
                    "pack://application:,,,/CursorLang.Settings;component/Themes/Controls.xaml",
                    UriKind.Absolute),
            });

            ready.SetResult(dispatcher);
            Dispatcher.Run();
        })
        {
            IsBackground = true,
            Name = "CursorLang.Tests UI",
        };

        thread.SetApartmentState(ApartmentState.STA);
        thread.Start();

        return ready.Task.GetAwaiter().GetResult();
    }
}