我正在编写一个消耗资源的库,无论出于什么原因,API的设计方式是在不同的线程上引发事件,但是必须在主线程上完成API的调用.
假设我尝试使用的API定义为(我将省略事件定义)：
public sealed class DodgyService
{
public void MethodThatHasToBeCalledOnTheMainThread() { ... }
}
为了使用这个API,我在我的库中添加了一个名为Service(Yup,非常原始名称)的服务,它将创建一个新任务(当我指定一个从SynchronizationContext创建的TaskScheduler时,它将在主线程上运行).
这是我的实施：
public class Service
{
private readonly TaskFactory _taskFactory;
private readonly TaskScheduler _mainThreadScheduler;
public Service(TaskFactory taskFactory,TaskScheduler mainThreadScheduler)
{
_taskFactory = taskFactory;
_mainThreadScheduler = mainThreadScheduler;
}
// Assume this method can be called from any thread.
// In this sample is called by the main thread but most of the time
// the caller will be running on a background thread.
public Task ExecuteAsync(string taskName)
{
return _taskFactory.StartNew(
() => ReallyLongCallThatForWhateverStupidReasonHasToBeCalledOnMainThread(taskName),new CancellationToken(false),TaskCreationOptions.None,_mainThreadScheduler)
.ContinueWith(task => Trace.TraceInformation("ExecuteAsync has completed on \"{0}\"...",taskName));
}
private void ReallyLongCallThatForWhateverStupidReasonHasToBeCalledOnMainThread(string taskName)
{
Trace.TraceInformation("Starting \"{0}\" really long call...",taskName);
new DodgyService().MethodThatHasToBeCalledOnTheMainThread();
Trace.TraceInformation("Finished \"{0}\" really long call...",taskName);
}
}
现在,如果我执行我的服务调用(在主线程上)并尝试在主线程上等待应用程序进入死锁,因为主线程将等待已安排在主线程上执行的任务.
如何在不阻塞整个过程的情况下将这些调用编组到主线程上？
在某些时候,我想在创建新任务之前执行主线程的检测,但我不想破解它.
对于任何感兴趣的人,我得到了一个带有代码的虚拟here和一个展示该问题的WPF应用程序.
在btw上,库必须写在.net framework 4.0上
解决方法
从您的示例程序：
private void HandleClosed(object sender,EventArgs e)
{
var list = new[]
{
_service.ExecuteAsync("first task"),_service.ExecuteAsync("second task"),_service.ExecuteAsync("third task")
};
//uncommenting this line blocks all three prevIoUs activities as expected
//as it drives the current main thread to wait for other tasks waiting to be executed by the main thread.
//Task.WaitAll(list);
}
Task.WaitAll是一个阻塞调用,你不能在主线程上执行阻塞调用,否则你将导致死锁.你可以做什么(如果你使用的是Visual Studio 2012或更新版本)是使用NuGet软件包Microsoft.Bcl.Async,它为.Net 4.0提供异步/等待支持.
private async void HandleClosed(object sender,EventArgs e)
{
var list = new[]
{
_service.ExecuteAsync("first task"),_service.ExecuteAsync("third task")
};
//uncommenting this line blocks all three prevIoUs activities as expected
//as it drives the current main thread to wait for other tasks waiting to be executed by the main thread.
await TaskEx.WhenAll(list);
}
并且你的程序将不再死锁(在等待TaskEx.WhenAll(list)之后它也不会执行任何代码;但这是因为此代码在关闭过程中运行,当你等待它时,让关闭继续处理,如果它被放置在其他地方,就像点击事件,你会看到更多的正常行为).
另一种选择是拥有第二个“主线程”并将工作分配给它.通常当必须在“主”线程上运行某些东西实际上是说它们需要在“一个STA的Windows消息上运行,该对象最初是在”线程上创建的.这是一个如何实现的例子(取自here)
private void runBrowserThread(Uri url) {
var th = new Thread(() => {
var br = new WebBrowser();
br.DocumentCompleted += browser_DocumentCompleted;
br.Navigate(url);
Application.Run();
});
th.SetApartmentState(ApartmentState.STA);
th.Start();
}
void browser_DocumentCompleted(object sender,WebBrowserDocumentCompletedEventArgs e) {
var br = sender as WebBrowser;
if (br.Url == e.Url) {
Console.WriteLine("Natigated to {0}",e.Url);
Application.ExitThread(); // Stops the thread
}
}
