如何序列化异步/等待？ [英] How to serialize async/await?

问题描述

让我们假设我有这个简单的片断：

Let's suppose I have this simple snippet:

async void button_Click(object sender, RoutedEventArgs e)
{
    await Task.Factory.StartNew(() =>
    {
        Console.WriteLine("start");
        Thread.Sleep(5000);
        Console.WriteLine("end");
    });
}

显然，每次我按下这个按钮一个新的任务甚至开始时previous任务仍然运行。我如何将推迟任何新的任务，直到所有previous任务已经完成？

Obviously, everytime I push that button a new task is started even when a previous task still runs. How would I postpone any new task until all previous tasks have finished?

更多的细节：

在上面的例子中，每一个新的任务是相同的前任务。然而，在原有范围内的任务序列事项：参数可能会改变（我可以模拟，它通过使用 DateTime.Now.Ticks ）。
任务应该他们是注册的顺序执行。特异性地，我的程序会跟一个串行设备。我以前有一个后台线程利用 BlockingCollection 做到了这一点。不过，这一次有一个严格的请求/响应协议，我想使用异步/的await，如果它是可能的。

In the example above, each new task is identical to the task before. However, in the original context the sequence of tasks matters: Parameters may change (I could "simulate" it by using DateTime.Now.Ticks). The tasks should be executed in the order they are "registered". Specificly, my program will talk to a serial device. I've done this before with a background thread utilizing a BlockingCollection. However, this time there's a strict request/response-protocol and I'd like to use async/await if it is possible.

可能的解决方法：

我可以想象创建任务并将它们存储在一个列表中。但我怎么就要求执行任务吗？或者我应该回到我之前已经使用了基于线程的解决方案？

I could imagine creating tasks and storing them in a list. But how would I execute the tasks with respect to the requirements? Or should I return to the thread-based solution I have used before?

推荐答案

您可以等待一个 SemaphoreSlim 异步和释放，一旦任务完成。不要忘了信号灯initialcount配置为 1 。

You could wait on a SemaphoreSlim asynchronously and release it once the job is done. Don't forget to configure the semaphore initialcount to 1.

private static SemaphoreSlim semaphore = new SemaphoreSlim(1);

private async static void DoSomethingAsync()
{
     await semaphore.WaitAsync();
     try
     {
        await Task.Factory.StartNew(() =>
        {
            Console.WriteLine("start");
            Thread.Sleep(5000);
            Console.WriteLine("end");
        });
     }
     finally
     {
        semaphore.Release();
     }
}

private static void Main(string[] args)
{
    DoSomethingAsync();
    DoSomethingAsync();
    Console.Read();
}

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