如何使用TaskFactory.FromAsync和自定义异步过程创建任务

Question

我试图测试一些依赖于Task进行一些后台计算的类(从网络位置检索数据).该类获取一个未启动的Task实例,添加一个ContinueWith方法,然后在Task上调用Start.像这样的东西:

private void Search()
{
    Task<SearchResults> searchTask = m_searchProvider.GetSearchTask(m_searchCriteria);
    searchTask.ContinueWith(OnSearchTaskCompleted);
    searchTask.Start();
}

该类通过接口获取Task的实例,因此我能够注入我的测试所控制的Task实例.但是,我似乎无法创建一个我有足够控制权的人.

我不想在测试中引入线程,但仍希望使Task以异步方式运行,所以我试图做的是编写一个实现BeginInvoke/EndInvoke模式的类而不进行线程化,并使用TaskFactory.FromAsync方法创建任务.

想法是测试可以在启动任务的类上调用方法,然后当返回测试时,可以将结果数据提供给Async对象,该对象在保持同一线程的同时完成操作.

但是,当我尝试在该任务上调用Start时,我收到一条错误消息,指出"可能无法在具有空操作的任务上调用Start".不幸的是,谷歌对这条消息没什么帮助,所以我不确定我是否错误地实现了我的Async对象,或者错误地使用了TaskFactory.FromAsync.这是我的NonThreadedAsync类的代码和一个异常的测试:

public class NonThreadedAsync<TResult>
{
    private NonThreadedAsyncResult<TResult> m_asyncResult;

    public IAsyncResult BeginInvoke(
        AsyncCallback callback,
        object state)
    {
        m_asyncResult = new NonThreadedAsyncResult<TResult>(callback, state);
        return m_asyncResult;
    }

    public TResult EndInvoke(IAsyncResult asyncResult)
    {
        return m_asyncResult.GetResults();
    }

    public void Complete(TResult data)
    {
        m_asyncResult.CompleteAsync(data);
    }
}

public class NonThreadedAsyncResult<TResult> : IAsyncResult
{
    private readonly AsyncCallback m_asyncCallback;
    private readonly object m_state;
    private readonly ManualResetEvent m_waitHandle;
    private bool m_isCompleted;
    private TResult m_resultData;

    public NonThreadedAsyncResult(AsyncCallback asyncCallback, object state)
    {
        m_asyncCallback = asyncCallback;
        m_state = state;
        m_waitHandle = new ManualResetEvent(false);
        m_isCompleted = false;
    }

    public void CompleteAsync(TResult data)
    {
        m_resultData = data;
        m_isCompleted = true;
        m_waitHandle.Set();
        if (m_asyncCallback != null)
        {
            m_asyncCallback(this);
        }
    }

    public TResult GetResults()
    {
        if (!m_isCompleted)
        {
            m_waitHandle.WaitOne();
        }
        return m_resultData;
    }

    #region Implementation of IAsyncResult

    public bool IsCompleted
    {
        get { return m_isCompleted; }
    }

    public WaitHandle AsyncWaitHandle
    {
        get { return m_waitHandle; }
    }

    public object AsyncState
    {
        get { return m_state; }
    }

    public bool CompletedSynchronously
    {
        get { return false; }
    }

    #endregion
}

[TestClass]
public class NonThreadedAsyncTests
{
    [TestMethod]
    public void TaskFactoryFromAsync_CanStartReturnedTask()
    {
        NonThreadedAsync<int> async = new NonThreadedAsync<int>();

        Task<int> task = Task<int>.Factory.FromAsync(async.BeginInvoke, async.EndInvoke, null);

        task.Start();
    }
}

作为进一步的信息,如果我在调用Start()之前调试该测试,则任务实例将显示在Locals窗口中,如下所示:

Id = 1, Status = WaitingForActivation, Method = "{null}", Result = "{Not yet computed}"

但如果我展开它,可见属性中没有Method属性.

谁能看到我做错了什么？

[编辑:我还编写了一个测试,确认NonThreadedAsync类可以正确使用经典的Begin/End模式(至少,我对Begin/End模式的理解:))并通过:

[TestMethod]
public void NonThreadedAsync_ClassicAccessPattern()
{
    int result = 0;
    bool asyncCompleted = false;

    NonThreadedAsync<int> async = new NonThreadedAsync<int>();

    async.BeginInvoke(asyncResult =>
                            {
                                result = async.EndInvoke(asyncResult);
                                asyncCompleted = true;
                            },
                            null);

    Assert.IsFalse(asyncCompleted);
    Assert.AreEqual(0, result);

    async.Complete(54);

    Assert.IsTrue(asyncCompleted);
    Assert.AreEqual(54, result);
}

Answer 1

哦,我明白了.我们的API错误,因为它试图返回未启动的任务.从被测试的类中删除Start()可以解决问题.然而,在研究这个问题时,我发现我做了太多的事情来获得测试控制的异步任务.按照斯蒂芬Toub的帖子在这里,我们可以简单地用一个TaskCompletionSource:

[TestMethod]
public void TaskCompletionSource_WorksALotBetterThanMyOverEngineeredCustomStuff()
{
    int result = 0;

    TaskCompletionSource<int> tcs = new TaskCompletionSource<int>();

    Task<int> myTask = tcs.Task;

    // Pretend this is the class under test and that we've
    // passed in myTask
    myTask.ContinueWith(t => { result = t.Result; },
        TaskContinuationOptions.ExecuteSynchronously);

    Assert.AreEqual(0, result);

    tcs.SetResult(54);

    Assert.AreEqual(54, result);
}