具有取消功能的長時間運行模式

Question

為了執行長時間運行（在此上下文中為搜索）操作，我將加載邏輯放入TPL任務中，因此在后台線程上調用了常規方法Search（） 。 Search（）操作可能足夠長，因此我需要能夠使用CancellationToken正確取消它。 但是Search（）操作直到完成才返回，因此我必須做一些邏輯才能實現便捷和（！）快速取消。

使用WaitHandle可以實現以下功能：

private void StartSearch() // UI thread
{
    CancellationTokenSource s = new CancellationTokenSource();
    Task.Factory.StartNew(() => StartSearchInternal(s.Token), s.Token)
}

private void StartSearchInternal(CancellationToken token) // Main Background Thread
{
    ManualResetEvent eHandle = new ManualResetEvent(false);
    Task.Factory.StartNew(() => Search(eHandle ), TaskScheduler.Default);
    WaitHandle.WaitAny(new [] { eHandle, token.WaitHandle });
    token.ThrowIfCancellationRequested();
}

private IEnumerable<object> Search(ManualResetEvent e) // Another Background thread
{
    try
    {
        // Real search call, i.e. to database, service, or AD, doesn't matter
        return RealSearch();
    }
    catch {} // Here, for simplicity of question, catch and eat all exceptions
    finally
    {
        try
        {
            e.Set();
        }
        catch {} 
    }
}

在我看來，這不是一個可以解決的優雅解決方案。

問：還有其他方法可以完成此任務嗎？

Answer 1

如果您可以控制StartSearchInternal()和Search(eHandle) ，那么您應該能夠在Search核心循環內使用ThrowIfCancellationRequested進行協作取消。

有關更多詳細信息，我強烈建議閱讀此文檔： “在.NET Framework 4中使用取消支持” 。

附帶說明，您可能應該在ViewModel類中的某個位置存儲對Task.Factory.StartNew(() => StartSearchInternal(s.Token), s.Token)返回的任務的引用。 您最有可能希望觀察其結果以及它可能引發的任何異常。 您可能要檢查Lucian Wischik的“異步重新進入以及處理它的模式” 。

Answer 2

這是我的評論，重構為包含代碼的答案。 它包含一些使用Task.Wait和異步模式的替代方法，其選擇取決於您是否從UI線程調用該方法。

O / P和其他答案中有一些注釋，其中包含有關異步行為的寶貴信息。 請閱讀這些內容，因為以下代碼具有許多“改進的機會”。

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace SearchAgent
{
    class CancellableSearchAgent
    {
        // Note: using 'object' is a bit icky - it would be better to define an interface or base class,
        // or at least restrict the type of object in some way, such as by making CancellableSearchAgent
        // a template CancellableSearchAgent<T> and replacing every copy of the text 'object' in this
        // answer with the character 'T', then make sure that the RealSearch() method return a collection
        // of objects of type T.
        private Task<IEnumerable<object>> _searchTask;
        private CancellationTokenSource _tokenSource;

        // You can use this property to check how the search is going.
        public TaskStatus SearchState
        {
            get { return _searchTask.Status; }
        }

        // When the search has run to completion, this will contain the result,
        // otherwise it will be null.
        public IEnumerable<object> SearchResult { get; private set; }

        // Create a new CancellableSearchAgent for each search.  The class encapsulates the 'workflow'
        // preventing issues with null members, re-using completed tasks, etc, etc.
        // You can add parameters, such as SQL statements as necessary.
        public CancellableSearchAgent()
        {
            _tokenSource = new CancellationTokenSource();
            _searchTask = Task<IEnumerable<object>>.Factory.StartNew(() => RealSearch(), TaskScheduler.Default);
        }

        // This method can be called from the UI without blocking.
        // Use this if the CancellableSearchAgent is part of your ViewModel (Presenter/Controller).
        public async void AwaitResultAsync()
        {
            SearchResult = await _searchTask;
        }

        // This method can be called from the ViewModel (Presenter/Controller), but will block the UI thread
        // if called directly from the View, making the UI unresponsive and unavailable for the user to
        // cancel the search.
        // Use this if CancellableSearchAgent is part of your Model.
        public IEnumerable<object> AwaitResult()
        {
            if (null == SearchResult)
            {
                try
                {
                    _searchTask.Wait(_tokenSource.Token);
                    SearchResult = _searchTask.Result;
                }
                catch (OperationCanceledException) { }
                catch (AggregateException)
                {
                    // You may want to handle other exceptions, thrown by the RealSearch() method here.
                    // You'll find them in the InnerException property.
                    throw;
                }
            }
            return SearchResult;
        }

        // This method can be called to cancel an ongoing search.
        public void CancelSearch()
        {
            _tokenSource.Cancel();
        }
    }
}