停止异步操作以启动替换操作

Question

考虑一个异步操作，例如从数据存储中获取搜索结果。现在我希望可以取消当前挂起的搜索操作，开始新的搜索操作，这实际上取代了活动搜索，主要是因为搜索参数同时发生了变化。

我的搜索是通过 TPL (.NET 4.0) new Task 开始的，最终有 Continues，当然还有回调，在 UI-Context 上调用 UI-Methods。

所以，如果再次按下搜索按钮，当搜索测试正在运行时，我首先必须停止当前任务并等待它结束。如果我从 UI 线程执行此操作，我可能会遇到死锁，因为 UI 线程上的 Wait() 会阻塞它，因此可能的 Invoke() 将永远不会执行。

因此，在我当前的解决方案中，我启动了一个单独的任务，它等待正在运行的任务结束/中止，然后运行新的任务。这感觉有点笨重，我想知道是否没有更优雅的方式，因为我经常需要这种机制。

那么我是否可能缺少可用于这种场景的框架机制？或者更推荐的方式是什么？

Answer 1

我相信这就是您正在寻找的。我已经对代码进行了注释，因此大部分推理都嵌入在代码中。

基本上，让您的 continue 函数处理启动一个准备好的任务。

Task currentTask;
CancellationTokenSource cancelTokenSource;
CancellationToken cancelToken;
Task newTask;
CancellationTokenSource newCancelTokenSource;
CancellationToken newCancelToken;

public Form1()
{
    InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
    if(currentTask != null && currentTask.Status == TaskStatus.Running)
    {
        //Cancel the running task
        cancelTokenSource.Cancel();
        //Prepare a new Task to be triggered when the other cancels
        //You could store new tasks/tokens in a dictionary if you wanted,also
        //A new cancel token is always needed since the old stays cancelled
        newCancelTokenSource = new CancellationTokenSource();
        newCancelToken = newCancelTokenSource.Token;
        newTask = new Task(()=>LongRunningTask(), newCancelToken);

        //Continue that deals with both cancel and completion
        //There is a different way to deal with this below, also
        newTask.ContinueWith((previousTask)=>
        {
            if(previousTask.Status == TaskStatus.Cancelled)
            {
                label1.Text = "New Task Cancelled, Another New Starting";
                BeginNewTask();
            }
            else
                label1.Text = "New Task Ran To Completion";
        },
        //If cancelled token is passed, it will autoskip the continue
        new CancellationTokenSource().Token, TaskContinuationOptions.None,
        //This is to auto invoke the UI thread
        TaskScheduler.FromCurrentSynchronizationContext());
    }
    else
    {
        cancelTokenSource = new CancellationTokenSource();
        cancelToken = cancelTokenSource.Token;
        //Start a fresh task since none running
        currentTask = Task.Factory.StartNew(()=>LongRunningTask(), 
            cancelToken);

        //OnCancelContinue
        currentTask.ContinueWith((previousTask)=>
        {
            label1.Text = "First Task Cancelled, New Starting";
            BeginNewTask();
        },
        //If cancelled token is passed, it will autoskip the continue
        new CancellationTokenSource().Token, 
        TaskContinuationOptions.OnlyOnCancelled,
        //This is to auto invoke the UI thread
        TaskScheduler.FromCurrentSynchronizationContext());

        //OnCompleteContinue
        currentTask.ContinueWith((previousTask)=>
        {
            label1.Text = "First Task Ran To Completion";
        },
        //If cancelled token is passed, it will autoskip the continue
        new CancellationTokenSource().Token, 
        TaskContinuationOptions.OnlyOnRanToCompletion,
        //This is to auto invoke the UI thread
        TaskScheduler.FromCurrentSynchronizationContext());
    }
}

private void LongRunningTask()
{
     for(int i = 0; i < 60; i++)
     {
         if(cancelToken.IsCancellationRequested)
             cancelToken.ThrowIfCancellationRequested();
         Thread.Sleep(1000);
     }
}

private void BeginNewTask()
{
    //Since the old task is cancelled, reset it with the new one
    //Probably should do some error checks
    currentTask = newTask;
    cancelTokenSource = newCancelTokenSource;
    cancelToken = newCancelToken;
    //This is to make sure this task does not run on the UI thread
    currentTask.Start(TaskScheduler.Default);
}