索引

笔记

异步

async 表达式

Async<’T>类型：

Async.Start 异步执行

Async.Parallel seq<Async<’T>> 并行执行

Async.RunSynchronously同步执行，并获取返回值，通常在Async.Parallel之后调用

异步中的异常处理：

未捕捉的异常将中断进程

1.可以在async表达式中使用try with表达式

2.使用Async.Catch延迟处理异常

例如：

open System.IO
let filePath = ""
let processBytes data:byte[] =
    data

async {
    printfn "Processing file [%s]" (Path.GetFileName(filePath))
    use fileStream = new FileStream(filePath, FileMode.Open)
    let bytesToRead = int fileStream.Length
    let! data = fileStream.AsyncRead(bytesToRead)
    printfn
        "Opened [%s], read [%d] bytes"
        (Path.GetFileName(filePath))
        data.Length
    let data' = processBytes data
    use resultFile = new FileStream(filePath + ".results", FileMode.Create)
    do! resultFile.AsyncWrite(data', 0, data'.Length)
    printfn "Finished processing file [%s]" <| Path.GetFileName(filePath)
}
|> Async.Catch
|> Async.RunSynchronously
|> function
| Choice1Of2 result -> printfn "%A" result
| Choice2Of2 (ex:exn) -> printfn "%s" ex.Message

3.Async.StartWithContinuations

Async.StartWithContinuations(
    superAwesomeAsyncTask,
    (fun result -> printfn "Task completed with result %A" result),
    (fun exn    -> printfn "Task threw an exception with Message: %s" exn.Message),
    (fun oce    -> printfn "Task was cancelled. Message: %s" oce.Message)
)

异步中的取消：

不同于Thread.Abort()，调用取消不会立即终止线程，它会在下一次调用let!、do!等操作时停止执行剩余代码，执行取消处理代码。CancelDefaultToken用来跟踪是否执行取消处理,Async.CancelDefaultToken()取消最后一个启动的任务

例：取消最后一个启动的任务

open System
open System.Threading
let cancelableTask =
    async {
        printfn "Waiting 10 seconds..."
        for i = 1 to 10 do
            printfn "%d..." i
            do! Async.Sleep(1000)
        printfn "Finished!"
    }
// Callback used when the operation is canceled
let cancelHandler (ex : OperationCanceledException) =
    printfn "The task has been canceled."
Async.TryCancelled(cancelableTask, cancelHandler)
|> Async.Start
// ...
Async.CancelDefaultToken()

如何取消所有异步任务：

open System.Threading
let computation = Async.TryCancelled(cancelableTask, cancelHandler)
let cancellationSource = new CancellationTokenSource()
Async.Start(computation, cancellationSource.Token)
// ...
cancellationSource.Cancel()

Async.Sleep:

Async.Sleep不同于Thread.Sleep，它不阻塞工作线程，sleep的线程被其他工作线程重用，当sleep结束，将唤醒另一个线程来完成剩余工作

自定义异步扩展方法:

使用Async.FromBeginEnd方法来自定义异步扩展方法

open System
open System.IO
type System.IO.Directory with
    /// Retrieve all files under a path asynchronously
    static member AsyncGetFiles(path : string, searchPattern : string) =
        let dele = new Func<string * string, string[]>(Directory.GetFiles)
        Async.FromBeginEnd(
            (path, searchPattern),
            dele.BeginInvoke,
            dele.EndInvoke)

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