asynchronous - 等待取消异步工作流

Question

Cancel CancellationTokenSource 的成员对象“传达取消请求”，我认为这意味着它是触发并忘记并且不会等到取消完成(例如，所有异常处理程序都已运行)。这很好，但我需要等到一个未完成的异步完全取消后再创建另一个异步。有没有简单的方法来实现这一点？

Answer 1

我认为没有任何直接的方法可以使用 F# 异步库中的标准库函数来做到这一点。最近的操作我们Async.TryCancelled当工作流(实际上)取消时运行回调，但是必须手动将消息从回调发送到启动工作流的代码。

使用事件和我编写的 F# 异步扩展(也包含在 FSharpX 包中)的扩展相对容易解决 - 扩展是 GuardedAwaitObservable可用于等待事件的发生(可以通过某些操作立即触发)。

以下Async.StartCancellable方法采用异步工作流并返回 Async<Async<unit>> .当您在外部工作流程上绑定(bind)时，它会启动参数(如 Async.StartChild )，当您在返回的内部工作流程上绑定(bind)时，它会取消计算并等待直到实际取消:

open System.Threading

module Async = 
  /// Returns an asynchronous workflow 'Async<Async<unit>>'. When called
  /// using 'let!', it starts the workflow provided as an argument and returns
  /// a token that can be used to cancel the started work - this is an
  /// (asynchronously) blocking operation that waits until the workflow
  /// is actually cancelled 
  let StartCancellable work = async {
    let cts = new CancellationTokenSource()
    // Creates an event used for notification
    let evt = new Event<_>()
    // Wrap the workflow with TryCancelled and notify when cancelled
    Async.Start(Async.TryCancelled(work, ignore >> evt.Trigger), cts.Token)
    // Return a workflow that waits for 'evt' and triggers 'Cancel'
    // after it attaches the event handler (to avoid missing event occurrence)
    let waitForCancel = Async.GuardedAwaitObservable evt.Publish cts.Cancel
    return async.TryFinally(waitForCancel, cts.Dispose) }

编辑 将结果包装在 TryFinally 中处置 CancellationTokenSource正如乔恩所建议的那样。我认为这应该足以确保它被正确处理。

这是使用该方法的示例。 loop函数是我用于测试的简单工作流程。其余代码启动它，等待 5.5 秒然后取消它:

/// Sample workflow that repeatedly starts and stops long running operation
let loop = async {
  for i in 0 .. 9999 do
    printfn "Starting: %d" i
    do! Async.Sleep(1000)
    printfn "Done: %d" i }

// Start the 'loop' workflow, wait for 5.5 seconds and then
// cancel it and wait until it finishes current operation  
async { let! cancelToken = Async.StartCancellable(loop)
        printfn "started"
        do! Async.Sleep(5500)
        printfn "cancelling"
        do! cancelToken
        printfn "done" }
|> Async.Start

为完整起见，具有 FSharpX 必要定义的示例为 here on F# snippets .