Async.AwaitEvent<'Del,'T> 方法 (F#)

创建一个异步计算，该异步计算通过将一个处理程序添加到 CLI 事件来等待对此事件的单个调用。 完成或取消计算后，将从此事件中移除该处理程序。

命名空间/模块路径： Microsoft.FSharp.Control

程序集：FSharp.Core（在 FSharp.Core.dll 中）

// Signature:
static member AwaitEvent : IEvent<'Del,'T> * ?(unit -> unit) -> Async<'T> (requires delegate)

// Usage:
Async.AwaitEvent (event)
Async.AwaitEvent (event, cancelAction = cancelAction)

参数

• event
  类型：IEvent<'Del,'T>

  要处理一次的事件。

• cancelAction
  类型：(unit -> unit)

  发出取消时要执行而不是取消的可选函数。

返回值

一个等待调用事件的异步计算。

备注

计算将在等待事件时响应取消。 如果发生取消并指定了 cancelAction，则会执行取消，并且计算将继续等待事件。 如果未指定 cancelAction，则取消将导致计算立即取消。

示例

以下代码示例演示如何使用Async.AwaitEvent 来设置响应文件更改事件的文件操作。 在这种情况下，等待该事件可以防止试图在文件处于锁定状态时对文件进行访问。

open System.Windows.Forms
open System.IO

let filename = "longoutput.dat"
if File.Exists(filename) then
    File.Delete(filename)
let watcher = new FileSystemWatcher(Path = Directory.GetCurrentDirectory(),
                                    NotifyFilter = NotifyFilters.LastWrite,
                                    Filter = filename)
watcher.Changed.Add(fun args -> printfn "The file %s is changed." args.Name)
watcher.EnableRaisingEvents <- true

let testFile = File.CreateText("Test.txt")
testFile.WriteLine("Testing...")
testFile.Close()

let form = new Form(Text = "Test Form")
let buttonSpacing = 5
let button1 = new Button(Text = "Start")
let button2 = new Button(Text = "Start Invalid", Top = button1.Height + buttonSpacing)
let button3 = new Button(Text = "Cancel", Top = 2 * (button1.Height + buttonSpacing))
let label1 = new Label(Text = "", Width = 200, Top = 3 * (button1.Height + buttonSpacing))
let label2 = new Label(Text = "", Width = 200, Top = 4 * (button1.Height + buttonSpacing))
form.Controls.AddRange [| button1; button2; button3; label1 |]
form.Controls.Add(button1)

let bufferData = Array.zeroCreate<byte> 100000000

let async1 filename =
     async {
       printfn "Creating file %s." filename
       use outputFile = File.Create(filename)
       printfn "Attempting to write to file %s." filename
       do! outputFile.AsyncWrite(bufferData) 
     }

let async2 filename =
     async {
       printfn "Waiting for file system watcher notification."
       // If you omit the call to AwaitEvent, an exception is thrown that indicates that the
       // file is locked.
       let! args = Async.AwaitEvent(watcher.Changed)
       printfn "Attempting to open and read file %s." filename
       use inputFile = File.OpenRead(filename)
       let! buffer = inputFile.AsyncRead(100000000)
       printfn "Successfully read file %s." filename
       return buffer
     }   

button1.Click.Add(fun _ ->
                  // Start these as tasks simultaneously.
                  Async.StartAsTask(async1 filename) |> ignore
                  Async.StartAsTask(async2 filename) |> ignore
                  ())
button2.Click.Add(fun _ ->
                  Async.StartAsTask(async1 filename) |> ignore
                  Async.StartAsTask(async2 "longoutputX.dat")  |> ignore
                  ())
Application.Run(form)

示例输出

平台

Windows 8，Windows 7，Windows server 2012中，Windows server 2008 R2

版本信息

F#核心库版本

支持:2.0，4.0，可移植

请参见

参考

Control.Async 类 (F#)

Microsoft.FSharp.Control 命名空间 (F#)