通过按到达顺序浏览消息来扫描消息，直至提供的函数返回 Some 值为止。 其他消息保留在队列中。
命名空间/模块路径： Microsoft.FSharp.Control
程序集：FSharp.Core（在 FSharp.Core.dll 中）
// Signature:
member this.TryScan : ('Msg -> Async<'T> option) * ?int -> Async<'T option>
// Usage:
mailboxProcessor.TryScan (scanner)
mailboxProcessor.TryScan (scanner, timeout = timeout)
参数
scanner
类型：'Msg -> Async<'T> option用于在跳过消息时返回 None 、 在处理消息并将其从队列中移除时返回 Some 的函数。
timeout
类型：int可选超时（以毫秒为单位）。 默认值为 -1，它对应于 Infinite。
返回值
scanner 脱离读取消息生成的异步计算（Async 对象）。
备注
如果超时时间已过，None 返回。 此方法可在代理的主体内使用。 对于每个代理至多只有一个并发读取器可处于活动状态，因此 Receive、TryReceive、Scan 或者 TryScan 中至多只有一个处于活动状态。 scanner 函数的正文在其执行期间被锁定，但会在执行异步工作流之前解锁。
示例
下面的代码示例演示如何使用 TryScan 方法。 此示例是作业提交代理。 有三个代理：启动每个工作的一个调用 runAgent，另一个名为的 inprogressAgent表示所有正在运行的工作，并且，completeAgent表示一个调用工作完成的通知。 如果没有匹配的工作，TryScan 用于 cancelJob 功能找到工作将会移除，或将失败。
open System
open System.Threading
let random = System.Random()
// Generates mock jobs by using Async.Sleep.
let createJob(id:int, source:CancellationTokenSource) =
let job = async {
// Let the time be a random number between 1 and 10000.
// The mock computed result is a floating point value.
let time = random.Next(10000)
let result = random.NextDouble()
let count = ref 1
while (!count <= 100 && not source.IsCancellationRequested) do
do! Async.Sleep(time / 100)
count := !count + 1
return result
}
id, job, source
type Result = double
// A Job consists of a job ID, a computation that produces a single result,
// and a cancellation token source object that can be used to cancel the job.
type Job = int * Async<Result> * CancellationTokenSource
type Message = int * Result
let context = System.Threading.SynchronizationContext.Current
// This agent processes when jobs are completed.
let completeAgent = MailboxProcessor<Message>.Start(fun inbox ->
let rec loop n =
async {
let! (id, result) = inbox.Receive()
printfn "The result of job #%d is %f" id result
do! loop (n + 1)
}
loop (0))
// inprogressAgent maintains a queue of in-progress jobs that can be
// scanned to remove canceled jobs. It never runs its processor function,
// so we set it to do nothing.
let inprogressAgent = new MailboxProcessor<Job>(fun _ -> async { () })
// This agent starts each job in the order in which it is received.
let runAgent = MailboxProcessor<Job>.Start(fun inbox ->
let rec loop n =
async {
let! (id, job, source) = inbox.Receive()
printfn "Starting job #%d" id
// Post to the in-progress queue.
inprogressAgent.Post(id, job, source)
// Start the job.
Async.StartWithContinuations(job,
(fun result -> completeAgent.Post(id, result)),
(fun _ -> ()),
(fun cancelException -> printfn "Canceled job #%d" id),
source.Token)
do! loop (n + 1)
}
loop (0))
for id in 1 .. 10 do
let source = new CancellationTokenSource()
runAgent.Post(createJob(id, source))
let cancelJob(cancelId) =
Async.RunSynchronously(
inprogressAgent.TryScan((fun (jobId, result, source) ->
let action =
async {
printfn "Canceling job #%d" cancelId
source.Cancel()
return cancelId
}
// Return Some(async) if the job ID matches.
if (jobId = cancelId) then
Some(action)
else
None), 1000))
printfn "Specify a job by number to cancel it, then press Enter."
let mutable finished = false
while not finished do
let input = System.Console.ReadLine()
let a = ref 0
if (Int32.TryParse(input, a) = true) then
match cancelJob(!a) with
| Some id -> printfn "A job was canceled: job #%d" id
| None -> printfn "Job not found."
else
printfn "Terminating."
finished <- true
下面是为示例部分。
平台
Windows 8，Windows 7，Windows Server 2012，Windows Server 2008 R2
版本信息
F# 核心库版本
支持：2.0，4.0，可移植