模式与能力取消长时间运行的操作 [英] Pattern for long-running operation with cancellation ability

问题描述

为了进行长时间运行（让它在这种情况下搜索）操作，我把装载逻辑的TPL任务里面，所以一般的方法搜索（）被称为后台线程。 搜索（）操作可够长的了，所以我需要正确地取消它的能力，使用的的CancellationToken 。但搜索（）操作没有返回，直到完成它，所以我必须做一些逻辑，以实现方便和（！）快速取消。

In order to perform long-running (let it be search in this context) operation, I put the loading logic inside a TPL task, so the general method Search() is called on background thread. Search() operation can be long enough, so I need the ability to cancel it properly, using the CancellationToken. But the Search() operation did not return until it finished, so I have to do some logic in order to implement convenient and (!) fast cancellation.

使用的的的WaitHandle的 我可以实现这样的：

Using WaitHandle's I can implement something like this:

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 {} 
    }
}

这是在我看来，事实并非如此优雅的解决方案，可以作出

It's seems to me that this is not so elegant solution, that can be made.

问：是否有此任务的任何其他方法？

Q: Is there any other approaches for this task?

推荐答案

这是我的意见重构到包含代码的答案。它包含几个选择使用Task.Wait和异步模式，其选择将取决于你是否调用从UI线程的方法。

This is my comment refactored into an answer containing code. It contains a couple of alternatives for using Task.Wait and the async pattern, the choice of which will depend on whether you call the method from the UI thread.

有一些意见的O / P和包含有关异步行为的有价值的信息，其他的答案。 。请仔细阅读这些如下面的代码有很多改进的机会

There are several comments to the O/P and other answers that contain valuable information regarding asynchronous behaviours. Please read these as the code below has many 'opportunities for improvement'.

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();
        }
    }
}

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