运行“异步"后台线程上的方法 [英] Run "async" method on a background thread

问题描述

我正在尝试从普通方法运行异步"方法:

I'm trying to run an "async" method from an ordinary method:

public string Prop
{
    get { return _prop; }
    set
    {
        _prop = value;
        RaisePropertyChanged();
    }
}

private async Task<string> GetSomething()
{
    return await new Task<string>( () => {
        Thread.Sleep(2000);
        return "hello world";
    });
}

public void Activate()
{
    GetSomething.ContinueWith(task => Prop = task.Result).Start();
    // ^ exception here
}

抛出的异常是:

可能不会在继续任务上调用开始.

Start may not be called on a continuation task.

那到底是什么意思?如何简单地在后台线程上运行异步方法，然后将结果分配回UI线程?

What does that mean, anyway? How can I simply run my async method on a background thread, dispatch the result back to the UI thread?

修改

还尝试了 Task.Wait ，但等待一直没有结束:

Also tried Task.Wait, but the waiting never ends:

public void Activate()
{
    Task.Factory.StartNew<string>( () => {
        var task = GetSomething();
        task.Wait();

        // ^ stuck here

        return task.Result;
    }).ContinueWith(task => {
        Prop = task.Result;
    }, TaskScheduler.FromCurrentSynchronizationContext());
    GetSomething.ContinueWith(task => Prop = task.Result).Start();
}

推荐答案

要具体解决您的示例，

public void Activate()
{
    Task.Factory.StartNew(() =>
    {
        //executes in thread pool.
        return GetSomething(); // returns a Task.
    }) // returns a Task<Task>.
    .Unwrap() // "unwraps" the outer task, returning a proxy
              // for the inner one returned by GetSomething().
    .ContinueWith(task =>
    {
        // executes in UI thread.
        Prop = task.Result;
    }, TaskScheduler.FromCurrentSynchronizationContext());
}

这行得通，但是已经过时了.

This will work, but it's old-school.

在后台线程上运行某些东西并分派回UI线程的现代方法是使用 Task.Run()， async 和 await :

The modern way to run something on a background thread and dispatch back to UI thread is to use Task.Run(), async, and await:

async void Activate()
{
    Prop = await Task.Run(() => GetSomething());
}

Task.Run 将在线程池线程中启动某些内容.当您 await 时，它会自动返回到启动它的执行上下文中.在这种情况下，您的UI线程.

Task.Run will start something in a thread pool thread. When you await something, it automatically comes back in on the execution context which started it. In this case, your UI thread.

通常，您永远不需要调用 Start().首选 async 方法， Task.Run 和 Task.Factory.StartNew -所有这些方法都会自动启动任务.当父项完成时，使用 await 或 ContinueWith 创建的继续也会自动开始.

You should generally never need to call Start(). Prefer async methods, Task.Run, and Task.Factory.StartNew -- all of which start the tasks automatically. Continuations created with await or ContinueWith are also started automatically when their parent completes.

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