WinForms App中的F#跨线程UI异常

Question

我在使用F#开发一个简单的应用程序时遇到问题,它只读取所请求的HTML页面的长度.

当您开发UI应用程序时,似乎这样的错误对于VB.NET/C#语言也是类似的.

但我对F#很陌生,并且真的没想象会在F#中解决这个问题.

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)

最好的祝福,

谢谢!

Answer 1

您必须Async ThreadPool在更新text.Text属性之前将线程上下文切换到UI线程.有关F#Async特定说明,请参阅MSDN链接.

通过捕获UI上下文修改您的代码段后

let uiContext = System.Threading.SynchronizationContext()

在您的let form = new Form()陈述之后放置并将fetchAsync定义更改为

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);
    }

它没有任何问题.

更新:在与一位强调需要干净地操纵UI上下文的同事讨论调试器特性之后,以下修改现在对于运行方式是不可知的:

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)