c# - 没有 UI 线程的任务同步

Question

在下面的代码中，我想同步任务列表结果的报告。这现在正在工作，因为 task.Result 会阻塞直到任务完成。但是，任务 id = 3 需要很长时间才能完成并阻止所有其他已完成的任务报告其状态。

我认为我可以通过将报告 (Console.Write) 移动到 .ContinueWith 指令中来做到这一点，但我没有 UI 线程，所以如何让 TaskScheduler 同步 .ContinueWith 任务？

我现在拥有的:

static void Main(string[] args)
{
    Console.WriteLine("Starting on {0}", Thread.CurrentThread.ManagedThreadId);

    var tasks = new List<Task<int>>();

    for (var i = 0; i < 10; i++)
    {
        var num = i;
        var t = Task<int>.Factory.StartNew(() =>
        {
           if (num == 3)
           {
               Thread.Sleep(20000);
           }
           Thread.Sleep(new Random(num).Next(1000, 5000));
           Console.WriteLine("Done {0} on {1}", num, Thread.CurrentThread.ManagedThreadId);
           return num;
        });
        tasks.Add(t);
    }

    foreach (var task in tasks)
    {
        Console.WriteLine("Completed {0} on {1}", task.Result, Thread.CurrentThread.ManagedThreadId);
    }

    Console.WriteLine("End of Main");
    Console.ReadKey();
}

我想转移到这个或类似的东西，但我需要 Console.Write("Completed...") 全部发生在同一个线程上:

static void Main(string[] args)
{
    Console.WriteLine("Starting on {0}", Thread.CurrentThread.ManagedThreadId);

    for (var i = 0; i < 10; i++)
    {
        var num = i;
        Task<int>.Factory.StartNew(() =>
        {
           if (num == 3)
           {
               Thread.Sleep(20000);
           }
           Thread.Sleep(new Random(num).Next(1000, 10000));
           Console.WriteLine("Done {0} on {1}", num, Thread.CurrentThread.ManagedThreadId);
           return num;
       }).ContinueWith(value =>
       {
           Console.WriteLine("Completed {0} on {1}", value.Result, Thread.CurrentThread.ManagedThreadId);
       } 

     /* need syncronization context */);
    }

    Console.WriteLine("End of Main");
    Console.ReadKey();
}

-- 解决方案--在获得一些评论并阅读了一些解决方案之后，这是完成我想要的完整解决方案。这里的目标是尽可能快地处理几个长时间运行的任务，然后一次处理每个任务的结果。

static void Main(string[] args)
{
    Console.WriteLine("Starting on {0}", Thread.CurrentThread.ManagedThreadId);

    var results = new BlockingCollection<int>();

    Task.Factory.StartNew(() =>
    {
        while (!results.IsCompleted)
        {
            try
            {
                var x = results.Take();
                Console.WriteLine("Completed {0} on {1}", x, Thread.CurrentThread.ManagedThreadId);
            }
            catch (InvalidOperationException)
            {
            }
        }
        Console.WriteLine("\r\nNo more items to take.");
    });

    var tasks = new List<Task>();

    for (var i = 0; i < 10; i++)
    {
        var num = i;
        var t = Task.Factory.StartNew(() =>
        {
            if (num == 3)
            {
                Thread.Sleep(20000);
            }
            Thread.Sleep(new Random(num).Next(1000, 10000));
            Console.WriteLine("Done {0} on {1}", num, Thread.CurrentThread.ManagedThreadId);
            results.Add(num);
        });

        tasks.Add(t);
    }

    Task.Factory.ContinueWhenAll(tasks.ToArray(), _ => results.CompleteAdding());

    Console.WriteLine("End of Main");
    Console.ReadKey();
}

Answer 1

您必须创建某种类型的编写器任务，但是请记住，即使此 任务也可以重新安排到另一个 native 或托管线程上!在 TPL 中使用默认调度程序，您无法控制哪个托管线程接收工作。

public class ConcurrentConsole
{
    private static BlockingCollection<string> output
        = new BlockingCollection<string>();

    public static Task CreateWriterTask(CancellationToken token)
    {
        return new Task(
            () =>
            {
                while (!token.IsCancellationRequested)
                {
                    string nextLine = output.Take(token);
                    Console.WriteLine(nextLine);
                }
            },
            token);
    }

    public static void WriteLine(Func<string> writeLine)
    {
        output.Add(writeLine());
    }
}

当我将您的代码切换为使用它时，我收到了以下输出:

End of Main
Done 1 on 6
Completed 1 on 6
Done 5 on 9
Completed 5 on 9
Done 0 on 4
Completed 0 on 4
Done 2 on 5
Completed 2 on 13
Done 7 on 10
Completed 7 on 10
Done 4 on 8
Completed 4 on 5
Done 9 on 12
Completed 9 on 9
Done 6 on 6
Completed 6 on 5
Done 8 on 11
Completed 8 on 4
Done 3 on 7
Completed 3 on 7

即使您的代码将 () => String.Format("Completed {0} on {1}"... 发送到 ConcurrentConsole.WriteLine，确保ManagedThreadId 将在 ConcurrentConsole 任务上获取，它仍然会改变它运行的线程。尽管与执行任务相比可变性较小。