等待任务完成而不阻止UI线程

Question

我有一个相当复杂的WPF应用程序(很像VS2013)IDocuments并且ITools停靠在应用程序的主shell中.Tools当主窗口关闭时,其中一个需要安全关闭,以避免进入"坏"状态.所以我使用Caliburn Micro的public override void CanClose(Action<bool> callback)方法来执行一些数据库更新等.我遇到的问题是这个方法中的所有更新代码都使用MongoDB Driver 2.0,这个东西是async.一些代码; 目前我正在尝试表演

public override void CanClose(Action<bool> callback)
{
    if (BackTestCollection.Any(bt => bt.TestStatus == TestStatus.Running))
    {
        using (ManualResetEventSlim tareDownCompleted = new ManualResetEventSlim(false))
        {
            // Update running test.
            Task.Run(async () =>
                {
                    StatusMessage = "Stopping running backtest...";
                    await SaveBackTestEventsAsync(SelectedBackTest);
                    Log.Trace(String.Format(
                        "Shutdown requested: saved backtest \"{0}\" with events",
                        SelectedBackTest.Name));

                    this.source = new CancellationTokenSource();
                    this.token = this.source.Token;
                    var filter = Builders<BsonDocument>.Filter.Eq(
                        BackTestFields.ID, DocIdSerializer.Write(SelectedBackTest.Id));
                    var update = Builders<BsonDocument>.Update.Set(BackTestFields.STATUS, TestStatus.Cancelled);
                    IMongoDatabase database = client.GetDatabase(Constants.DatabaseMappings[Database.Backtests]);
                    await MongoDataService.UpdateAsync<BsonDocument>(
                        database, Constants.Backtests, filter, update, token);
                    Log.Trace(String.Format(
                        "Shutdown requested: updated backtest \"{0}\" status to \"Cancelled\"",
                        SelectedBackTest.Name));
                }).ContinueWith(ant =>
                    {
                        StatusMessage = "Disposing backtest engine...";
                        if (engine != null)
                            engine.Dispose();
                        Log.Trace("Shutdown requested: disposed backtest engine successfully");
                        callback(true);
                        tareDownCompleted.Set();
                    });
            tareDownCompleted.Wait();
        }
    }
}

现在,首先我没有这个ManualResetEventSlim,这显然会CanClose在我在后台[thread-pool]线程上更新数据库之前返回调用者.为了防止返回,直到我完成更新,我试图阻止返回,但这冻结了UI线程并防止发生任何事情.

如何在不过早返回调用者的情况下运行清理代码？

谢谢你的时间.

---

注意,我无法OnClose使用异步签名覆盖该方法,因为调用代码不会等待它(我无法控制它).

Answer 1

我认为你没有多少选择阻止回报.但是,尽管UI线程被锁定,您的更新仍应继续运行.我不会使用ManualResetEventSlim,而是使用简单的wait()和没有延续的单个任务.原因是默认情况下,Task.Run会阻止子任务(您的继续)附加到父项,因此您的继续可能没有时间在窗口关闭之前完成,请参阅此文章.

public override void CanClose(Action<bool> callback)
{
    if (BackTestCollection.Any(bt => bt.TestStatus == TestStatus.Running))
    {
        // Update running test.
        var cleanupTask = Task.Run(async () =>
        {
            Application.Current.Dispatcher.Invoke(DispatcherPriority.Background, new Action(delegate { StatusMessage.Text = "Stopping running backtest..."; }));
            await SaveBackTestEventsAsync(SelectedBackTest);

            // other cleanup  tasks
            // No continuation

            Application.Current.Dispatcher.Invoke(DispatcherPriority.Background, new Action(delegate { StatusMessage.Text = "Disposing backtest engine..."; }));
            if (engine != null)
                engine.Dispose();
            Log.Trace("Shutdown requested: disposed backtest engine successfully");
            callback(true);
        });
        while (!cleanupTask.IsCompleted)
        {
            Application.Current.Dispatcher.Invoke(DispatcherPriority.Background, new Action(delegate { }));
        }
    }
}

如果确实需要使用continuation,也可以将TaskFactory.StartNew与TaskCreationOptions.AttachedToParent一起使用.

编辑:我已经将我的答案与@Saeb Amini结合起来,这有点像黑客攻击,但你保留了一些UI响应能力.

编辑2:这是解决方案的示例演示(使用新的WPF项目测试):

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    protected override void OnClosing(CancelEventArgs e)
    {
        var dispatcher = Application.Current.Dispatcher;
        var cleanupTask = Task.Run(
            async () =>
            {
                dispatcher.Invoke(DispatcherPriority.Background, new Action(delegate {StatusMessage.Text = "Stopping running backtest..."; }));
                await Task.Delay(2000);
                dispatcher.Invoke(DispatcherPriority.Background, new Action(delegate { StatusMessage.Text = "Disposing backtest engine..."; }));
                await Task.Delay(2000);
            });

        while (!cleanupTask.IsCompleted)
        {
            dispatcher.Invoke(DispatcherPriority.Background, new Action(delegate { }));
        }
    }
}

Answer 2

您可以使用类似于 WinForm 的东西，Application.DoEvents但对于 WPF，它涉及使用标志、触发您的任务，而不是 Wait为它执行任务，而是在循环中不断处理 UI 消息，直到您的任务完成并设置标志。例如：

if (BackTestCollection.Any(bt => bt.TestStatus == TestStatus.Running))
{
    bool done = false;
    // Update running test.
    Task.Run(async () =>
    {
        StatusMessage = "Stopping running backtest...";
        await SaveBackTestEventsAsync(SelectedBackTest);
        Log.Trace(String.Format(
            "Shutdown requested: saved backtest \"{0}\" with events",
            SelectedBackTest.Name));

        this.source = new CancellationTokenSource();
        this.token = this.source.Token;
        var filter = Builders<BsonDocument>.Filter.Eq(
            BackTestFields.ID, DocIdSerializer.Write(SelectedBackTest.Id));
        var update = Builders<BsonDocument>.Update.Set(BackTestFields.STATUS, TestStatus.Cancelled);
        IMongoDatabase database = client.GetDatabase(Constants.DatabaseMappings[Database.Backtests]);
        await MongoDataService.UpdateAsync<BsonDocument>(
            database, Constants.Backtests, filter, update, token);
        Log.Trace(String.Format(
            "Shutdown requested: updated backtest \"{0}\" status to \"Cancelled\"",
            SelectedBackTest.Name));
        StatusMessage = "Disposing backtest engine...";
        if (engine != null)
            engine.Dispose();
        Log.Trace("Shutdown requested: disposed backtest engine successfully");
        callback(true);
        done = true;
    });

    while (!done)
    {
        Application.Current.Dispatcher.Invoke(DispatcherPriority.Background,
                                new Action(delegate { }));
    }
}

这有点 hacky，但考虑到您的情况并且无法控制调用代码，这可能是您保持响应式 UI 而不立即返回给调用者的唯一选择。