multithreading - WinForms App 中的 F# 跨线程 UI 异常

Question

我在 F# 中开发一个简单的应用程序时遇到问题，该应用程序仅读取请求的 HTML 页面的长度。

似乎在开发 UI 应用程序时，VB.NET/C# 语言也会出现类似的错误。



但我对 F# 还是比较陌生，并没有真正想象在 F# 中完全解决此类问题的热度。

F#中的源代码:

http://pastebin.com/e6WM0Sjw

open System
open System.Net
open Microsoft.FSharp.Control.WebExtensions
open System.Windows.Forms

let form = new Form()
let text = new Label()
let button = new Button()

let urlList = [ "Microsoft.com", "http://www.microsoft.com/" 
                "MSDN", "http://msdn.microsoft.com/" 
                "Bing", "http://www.bing.com"
              ]

let fetchAsync(name, url:string) =
    async { 
        try 
            let uri = new System.Uri(url)
            let webClient = new WebClient()
            let! html = webClient.AsyncDownloadString(uri)
            text.Text <- String.Format("Read %d characters for %s", html.Length, name)
        with
            | ex -> printfn "%s" (ex.Message);
    }

let runAll() =
    urlList
    |> Seq.map fetchAsync
    |> Async.Parallel 
    |> Async.RunSynchronously
    |> ignore

form.Width  <- 400
form.Height <- 300
form.Visible <- true
form.Text <- "Test download tool"

text.Width <- 200
text.Height <- 50
text.Top <- 0
text.Left <- 0
form.Controls.Add(text)

button.Text <- "click me"
button.Top <- text.Height
button.Left <- 0
button.Click |> Event.add(fun sender -> runAll() |> ignore)
form.Controls.Add(button)

[<STAThread>]
do Application.Run(form)

此致，

谢谢!

Answer 1

您必须将线程上下文从 Async ThreadPool 切换到 UI 线程更新前 text.Text属性(property)。见 MSDN link对于 F# 异步特定的解释。

通过捕获 UI 上下文修改您的代码段后

let uiContext = System.Threading.SynchronizationContext()

放在您的 let form = new Form() 之后声明和更改 fetchAsync定义为

let fetchAsync(name, url:string) =
    async { 
        try 
            let uri = new System.Uri(url)
            let webClient = new WebClient()
            let! html = webClient.AsyncDownloadString(uri)
            do! Async.SwitchToContext(uiContext)
            text.Text <- text.Text + String.Format("Read {0} characters for {1}\n", html.Length, name)
        with
            | ex -> printfn "%s" (ex.Message);
    }

它可以正常工作。

更新:在与同事讨论调试器特性后，他强调需要干净地操作 UI 上下文，以下修改现在与运行方式无关:

open System
open System.Net
open Microsoft.FSharp.Control.WebExtensions
open System.Windows.Forms
open System.Threading

let form = new Form()
let text = new Label()
let button = new Button()

let urlList = [ "Microsoft.com", "http://www.microsoft.com/"
                "MSDN", "http://msdn.microsoft.com/"
                "Bing", "http://www.bing.com"
              ]

let fetchAsync(name, url:string, ctx) =
    async {
        try
            let uri = new System.Uri(url)
            let webClient = new WebClient()
            let! html = webClient.AsyncDownloadString(uri)
            do! Async.SwitchToContext ctx
            text.Text <- text.Text + sprintf "Read %d characters for %s\n" html.Length name
        with
            | ex -> printfn "%s" (ex.Message);
    }

let runAll() =
    let ctx = SynchronizationContext.Current
    text.Text <- String.Format("{0}\n", System.DateTime.Now)
    urlList
    |> Seq.map (fun(site, url) -> fetchAsync(site, url, ctx))
    |> Async.Parallel
    |> Async.Ignore
    |> Async.Start

form.Width  <- 400
form.Height <- 300
form.Visible <- true
form.Text <- "Test download tool"

text.Width <- 200
text.Height <- 100
text.Top <- 0
text.Left <- 0
form.Controls.Add(text)

button.Text <- "click me"
button.Top <- text.Height
button.Left <- 0
button.Click |> Event.add(fun sender -> runAll() |> ignore)
form.Controls.Add(button)

[<STAThread>]
do Application.Run(form)