f# - 等待邮箱处理器

Question

是否可以等待邮箱处理器，以下代码在 F# 交互式中工作，但有没有办法在应用程序或单元测试中等待它？

[<TestMethod>]
member this.TestMailboxProcessor() =
    let mailboxProcessor = MailboxProcessor<string>.Start(fun inbox ->
        async {
            while true do
            let! msg = inbox.Receive()
            printfn "agent got message %s" msg // too late, UnitTest exits
        }
    )

    mailboxProcessor.Post "ping"
    Console.WriteLine "message posted" // I see this in the console
    Assert.IsTrue(true)

Answer 1

在这种情况下这是不可能的，但您可以定义您的消息类型以包含 AsyncReplyChannel<'t> ，然后允许您使用 MailboxProcessor.PostAndReply而不是邮政。通过这种方式，调用代码可以(同步或异步)等待响应值，或者至少是处理完成的指示。

您修改后的源代码可能如下所示:

[<TestMethod>]
member this.TestMailboxProcessor() =
    let mailboxProcessor =
        MailboxProcessor<string * AsyncReplyChannel<unit>>.Start(fun inbox ->
            async {
                while true do
                let! msg, replyChannel = inbox.Receive()
                printfn "agent got message %s" msg
                (* 
                  Reply takes a value of the generic param of
                  AsyncReplyChannel<'t>, in this case just a unit
                *)
                replyChannel.Reply()
            }
        )

    (*
      You can't create an AsyncReplyChannel<'t> value, but this does it for you.
      Also always, always use timeouts when awaiting message replies. 
    *)
    mailboxProcessor.PostAndReply(
        (fun replyChannel -> "ping", replyChannel),
        timeout = 1000)
    (* This gets printed only after the message has been posted and processed *)
    Console.WriteLine "message posted"
    Assert.IsTrue(true)

不过，MailboxProcessors 是一个有点棘手的话题，所以请确保您始终使用超时，否则如果您的代码出现错误，或异常终止消息循环，您的代码将永远挂起。在测试中不好，在生产中更糟。