取消任務中使用的取消令牌的正確方法是什么？

Question

我有創建取消令牌的代碼

public partial class CardsTabViewModel : BaseViewModel
{
   public CancellationTokenSource cts;

public async Task OnAppearing()
{
   cts = new CancellationTokenSource(); // << runs as part of OnAppearing()

使用它的代碼：

await GetCards(cts.Token);


public async Task GetCards(CancellationToken ct)
{
    while (!ct.IsCancellationRequested)
    {
        App.viewablePhrases = App.DB.GetViewablePhrases(Settings.Mode, Settings.Pts);
        await CheckAvailability();
    }
}

以及稍后在用戶離開運行上述代碼的屏幕時取消此取消令牌的代碼：

public void OnDisappearing()
{
   cts.Cancel();

關於取消，這是在任務中使用令牌時取消令牌的正確方法嗎？

我特別檢查了這個問題：

使用 IsCancellationRequested 屬性？

這讓我認為我沒有以正確的方式或可能導致異常的方式進行取消。

另外，在這種情況下，在我取消之后，我應該執行 cts.Dispose() 嗎？

Answer 1

一般來說，我在您的代碼中看到了 Cancel Token 的合理使用，但根據 Task Async Pattern，您的代碼可能不會立即取消。

while (!ct.IsCancellationRequested)
{
   App.viewablePhrases = App.DB.GetViewablePhrases(Settings.Mode, Settings.Pts);
   await CheckAvailability();   //Your Code could be blocked here, unable to cancel
}

為了立即響應，也應該取消阻塞代碼

await CheckAvailability(ct);   //Your blocking code in the loop also should be stoped

是否必須Dispose由你決定，如果被中斷的代碼中保留了很多內存資源，你應該這樣做。

Answer 2

CancellationTokenSource.Cancel()是開始取消的有效方式。

輪詢ct.IsCancellationRequested避免拋出OperationCanceledException 。 因為它是輪詢，所以它需要循環迭代才能完成，然后才能響應取消請求。

如果可以修改GetViewablePhrases()和CheckAvailability()以接受CancellationToken ，這可能會使取消響應更快，代價是拋出OperationCanceledException 。

“我應該做一個 cts.Dispose() 嗎？” 是不是那么簡單...

“始終盡快處理 IDisposables”

與其說是規則，不如說是指南。 Task本身是一次性的，但幾乎從未直接在代碼中處理過。

在某些情況下（當使用WaitHandle或取消回調處理程序時）處置cts將釋放資源/刪除 GC 根，否則只能由終結器釋放。 這些不適用於您的代碼，但將來可能適用。

在取消后添加對Dispose的調用將保證在未來的代碼版本中立即釋放這些資源。

但是，您必須在調用 dispose 之前等待使用cts的代碼完成，或者修改代碼以在處理后使用cts （或其令牌）處理ObjectDisposedException 。

Answer 3

我建議您查看 .net 類之一，以完全了解如何使用 CanncelationToken 處理等待方法，我選擇了 SeamaphoreSlim.cs

    public bool Wait(int millisecondsTimeout, CancellationToken cancellationToken)
    {
        CheckDispose();

        // Validate input
        if (millisecondsTimeout < -1)
        {
            throw new ArgumentOutOfRangeException(
                "totalMilliSeconds", millisecondsTimeout, GetResourceString("SemaphoreSlim_Wait_TimeoutWrong"));
        }

        cancellationToken.ThrowIfCancellationRequested();

        uint startTime = 0;
        if (millisecondsTimeout != Timeout.Infinite && millisecondsTimeout > 0)
        {
            startTime = TimeoutHelper.GetTime();
        }

        bool waitSuccessful = false;
        Task<bool> asyncWaitTask = null;
        bool lockTaken = false;

        //Register for cancellation outside of the main lock.
        //NOTE: Register/deregister inside the lock can deadlock as different lock acquisition orders could
        //      occur for (1)this.m_lockObj and (2)cts.internalLock
        CancellationTokenRegistration cancellationTokenRegistration = cancellationToken.InternalRegisterWithoutEC(s_cancellationTokenCanceledEventHandler, this);
        try
        {
            // Perf: first spin wait for the count to be positive, but only up to the first planned yield.
            //       This additional amount of spinwaiting in addition
            //       to Monitor.Enter()’s spinwaiting has shown measurable perf gains in test scenarios.
            //
            SpinWait spin = new SpinWait();
            while (m_currentCount == 0 && !spin.NextSpinWillYield)
            {
                spin.SpinOnce();
            }
            // entering the lock and incrementing waiters must not suffer a thread-abort, else we cannot
            // clean up m_waitCount correctly, which may lead to deadlock due to non-woken waiters.
            try { }
            finally
            {
                Monitor.Enter(m_lockObj, ref lockTaken);
                if (lockTaken)
                {
                    m_waitCount++;
                }
            }

            // If there are any async waiters, for fairness we'll get in line behind
            // then by translating our synchronous wait into an asynchronous one that we 
            // then block on (once we've released the lock).
            if (m_asyncHead != null)
            {
                Contract.Assert(m_asyncTail != null, "tail should not be null if head isn't");
                asyncWaitTask = WaitAsync(millisecondsTimeout, cancellationToken);
            }
                // There are no async waiters, so we can proceed with normal synchronous waiting.
            else
            {
                // If the count > 0 we are good to move on.
                // If not, then wait if we were given allowed some wait duration

                OperationCanceledException oce = null;

                if (m_currentCount == 0)
                {
                    if (millisecondsTimeout == 0)
                    {
                        return false;
                    }

                    // Prepare for the main wait...
                    // wait until the count become greater than zero or the timeout is expired
                    try
                    {
                        waitSuccessful = WaitUntilCountOrTimeout(millisecondsTimeout, startTime, cancellationToken);
                    }
                    catch (OperationCanceledException e) { oce = e; }
                }

                // Now try to acquire.  We prioritize acquisition over cancellation/timeout so that we don't
                // lose any counts when there are asynchronous waiters in the mix.  Asynchronous waiters
                // defer to synchronous waiters in priority, which means that if it's possible an asynchronous
                // waiter didn't get released because a synchronous waiter was present, we need to ensure
                // that synchronous waiter succeeds so that they have a chance to release.
                Contract.Assert(!waitSuccessful || m_currentCount > 0, 
                    "If the wait was successful, there should be count available.");
                if (m_currentCount > 0)
                {
                    waitSuccessful = true;
                    m_currentCount--;
                }
                else if (oce != null)
                {
                    throw oce;
                }

                // Exposing wait handle which is lazily initialized if needed
                if (m_waitHandle != null && m_currentCount == 0)
                {
                    m_waitHandle.Reset();
                }
            }
        }
        finally
        {
            // Release the lock
            if (lockTaken)
            {
                m_waitCount--;
                Monitor.Exit(m_lockObj);
            }

            // Unregister the cancellation callback.
            cancellationTokenRegistration.Dispose();
        }

        // If we had to fall back to asynchronous waiting, block on it
        // here now that we've released the lock, and return its
        // result when available.  Otherwise, this was a synchronous
        // wait, and whether we successfully acquired the semaphore is
        // stored in waitSuccessful.

        return (asyncWaitTask != null) ? asyncWaitTask.GetAwaiter().GetResult() : waitSuccessful;
    }

你也可以在這里查看全班， https://referencesource.microsoft.com/#mscorlib/system/threading/SemaphoreSlim.cs,6095d9030263f169