【异步编程】Part3:取消异步操做

时间 2019-11-29

标签异步编程 part3 取消异步繁體版

原文原文链接

在.Net和C#中运行异步代码至关简单，由于咱们有时候须要取消正在进行的异步操做，经过本文，能够掌握经过CancellationToken取消任务（包括non-cancellable任务）。git

早期

早期.Net 使用 BackgroundWorker 完成异步长时间运行操做。

能够使用CacnelAsync方法设置 CancellationPending = true

private void BackgroundLongRunningTask(object sender, DoWorkEventArgs e)
{
    BackgroundWorker worker = (BackgroundWorker)sender;

    for (int i = 1; i <= 10000; i++)
    {
        if (worker.CancellationPending == true)
        {
            e.Cancel = true;
            break;
        }
        
        // Do something
    }
}

View Code

已经再也不推荐这种方式来完成异步和长时间运行的操做，可是大部分概念在如今依旧能够使用。github

Task横空出世

Task表明一个异步操做， 该类表示一个异步不返回值的操做， 泛型版本Task<TResult>表示异步有返回值的操做。

可以使用async/await 语法糖代码去完成异步操做。

如下建立一个简单的长时间运行的操做：

/// <summary>
/// Compute a value for a long time.
/// </summary>
/// <returns>The value computed.</returns>
/// <param name="loop">Number of iterations to do.</param>
private static Task<decimal> LongRunningOperation(int loop)
{
    // Start a task and return it
    return Task.Run(() =>
    {
        decimal result = 0;

        // Loop for a defined number of iterations
        for (int i = 0; i < loop; i++)
        {
            // Do something that takes times like a Thread.Sleep in .NET Core 2.
            Thread.Sleep(10);
            result += i;
        }

        return result;
    });
}
// 这里咱们使用Thread.Sleep 模仿长时间运行的操做

View Code

简单异步调用代码：异步

public static async Task ExecuteTaskAsync()
{
    Console.WriteLine(nameof(ExecuteTaskAsync));
    Console.WriteLine("Result {0}", await LongRunningOperation(100));
    Console.WriteLine("Press enter to continue");
    Console.ReadLine();
}

敲黑板： C#取消异步操做分为

① 让代码可取消（Cancellable）

由于一些缘由，长时间运行的操做花费了冗长的时间（须要取消，避免占用资源）；或者不肯意再等待执行结果了

咱们会取消异步操做。

为完成目的须要在长时间运行的异步任务中传入CancellationToken:

/// <summary>
/// Compute a value for a long time.
/// </summary>
/// <returns>The value computed.</returns>
/// <param name="loop">Number of iterations to do.</param>
/// <param name="cancellationToken">The cancellation token.</param>
private static Task<decimal> LongRunningCancellableOperation(int loop, CancellationToken cancellationToken)
{
    Task<decimal> task = null;

    // Start a task and return it
    task = Task.Run(() =>
    {
        decimal result = 0;

        // Loop for a defined number of iterations
        for (int i = 0; i < loop; i++)
        {
            // Check if a cancellation is requested, if yes,
            // throw a TaskCanceledException.

            if (cancellationToken.IsCancellationRequested) throw new TaskCanceledException(task); // Do something that takes times like a Thread.Sleep in .NET Core 2.
            Thread.Sleep(10);
            result += i;
        }

        return result;
    });

    return task;
}

在长时间运行的操做中监测 IsCancellationRequested方法 (当前是否发生取消命令)，这里我倾向去包装一个 TaskCanceledException异常类（给上层方法调用者更多处理的可能性）；固然能够调用 ThrowIfCancellationRequested方法抛出 OperationCanceledException异常。

② 触发取消命令

CancellationToken结构体至关于打入在异步操做内部的楔子，随时等候后方发来的取消命令。

操纵以上CancellationToken状态的对象是 CancellationTokenSource，这个对象是取消操做的命令发布者。

默认的构造函数就支持了超时取消：

//  如下代码 利用 CancellationSource默认构造函数 完成超时取消
public static async Task ExecuteTaskWithTimeoutAsync(TimeSpan timeSpan)
{
    Console.WriteLine(nameof(ExecuteTaskWithTimeoutAsync));

    using (var cancellationTokenSource = new CancellationTokenSource(timeSpan))
    {
        try
        {
            var result = await LongRunningCancellableOperation(500, cancellationTokenSource.Token);
            Console.WriteLine("Result {0}", result);
        }
        catch (TaskCanceledException)
        {
            Console.WriteLine("Task was cancelled");
        }
    }
    Console.WriteLine("Press enter to continue");
    Console.ReadLine();
}

------------------------------------------------------------------------------------------------------------

附①：高阶操做，完成手动取消：

天然咱们关注到 CancellationSource 的几个方法，要想在异步操做的时候手动取消操做，须要创建另外的线程等待手动取消操做的指令。

public static async Task ExecuteManuallyCancellableTaskAsync()
{
    Console.WriteLine(nameof(ExecuteManuallyCancellableTaskAsync));

    using (var cancellationTokenSource = new CancellationTokenSource())
    {
        // Creating a task to listen to keyboard key press
        var keyBoardTask = Task.Run(() =>
        {
            Console.WriteLine("Press enter to cancel");
            Console.ReadKey();

            // Cancel the task
 cancellationTokenSource.Cancel();
        });

        try
        {
            var longRunningTask = LongRunningCancellableOperation(500, cancellationTokenSource.Token);

            var result = await longRunningTask;
            Console.WriteLine("Result {0}", result);
            Console.WriteLine("Press enter to continue");
        }
        catch (TaskCanceledException)
        {
            Console.WriteLine("Task was cancelled");
        }

        await keyBoardTask;
    }
}
// 以上是一个控制台程序，异步接收控制台输入，发出取消命令。

附②：高阶操做，取消 non-Cancellable任务：

有时候，异步操做代码并不提供对 Cancellation的支持，也就是以上长时间运行的异步操做

LongRunningCancellableOperation(int loop, CancellationToken cancellationToken) 并不提供参数2的传入，至关于不容许打入楔子。

这时咱们怎样取消这样的non-Cancellable 任务？

可考虑利用 Task.WhenAny( params tasks) 操做曲线取消：

利用TaskCompletionSource 注册异步可取消任务
等待待non-cancellable 操做和以上创建的异步取消操做

private static async Task<decimal> LongRunningOperationWithCancellationTokenAsync(int loop, CancellationToken cancellationToken)
{
    // We create a TaskCompletionSource of decimal
    var taskCompletionSource = new TaskCompletionSource<decimal>();

    // Registering a lambda into the cancellationToken
    cancellationToken.Register(() =>
    {
        // We received a cancellation message, cancel the TaskCompletionSource.Task
        taskCompletionSource.TrySetCanceled();
    });

    var task = LongRunningOperation(loop);

    // Wait for the first task to finish among the two
    var completedTask = await Task.WhenAny(task, taskCompletionSource.Task);

    return await completedTask;
}

像上面代码同样执行取消命令：async

public static async Task CancelANonCancellableTaskAsync()
{
    Console.WriteLine(nameof(CancelANonCancellableTaskAsync));

    using (var cancellationTokenSource = new CancellationTokenSource())
    {
        // Listening to key press to cancel
        var keyBoardTask = Task.Run(() =>
        {
            Console.WriteLine("Press enter to cancel");
            Console.ReadKey();

            // Sending the cancellation message
            cancellationTokenSource.Cancel();
        });

        try
        {
            // Running the long running task
            var longRunningTask = LongRunningOperationWithCancellationTokenAsync(100, cancellationTokenSource.Token);
            var result = await longRunningTask;

            Console.WriteLine("Result {0}", result);
            Console.WriteLine("Press enter to continue");
        }
        catch (TaskCanceledException)
        {
            Console.WriteLine("Task was cancelled");
        }

        await keyBoardTask;
    }
}

总结：

大多数状况下，咱们不须要编写自定义可取消任务，由于咱们只须要使用现有API。但要知道它是如何在幕后工做老是好的。

参考资料：

https://johnthiriet.com/cancel-asynchronous-operation-in-csharp/

https://stackoverflow.com/questions/4238345/asynchronously-wait-for-taskt-to-complete-with-timeout

https://github.com/App-vNext/Polly/wiki/Timeout