In the last post I discussed an asynchronous version of the
ManualResetEvent and as promised this time we will be looking at an
asynchronous version of the
AutoResetEvent. I’m using Stephen Toubs post
as reference and we will be building a version that is functional in style that maps straight into asynchronous work flows without and conversion
What is an AutoResetEvent?
AutoResetEvent can be described as a turnstile mechanism, it lets a single waiting person through before re-latching
waiting for the next signal. This is opposed to a
ManualResetEvent which functions like an ordinary gate. Calling Set opens
the gate, allowing any number of threads that are waiting to be let through. Calling Reset closes the gate.
First of all here is the shape of the type that we will be building:
type AsyncAutoResetEvent = new : ?reusethread:bool -> AsyncAutoResetEvent member Set : unit -> unit member WaitAsync : unit -> Async<bool>
Fairly simple: implied constructor,
Thinking about this logically we may need the following items:
- A queue mechanism to store asynchronous waiters -
let mutable awaits = Queue<_>().
- A way of knowing if a signal has been made in the absence of any waiters -
let mutable signalled = false.
- We can also declare a short-circuit asynchronous workflow for the situation that
Set()is called before
let completed = async.Return true. This will save us constructing an
AsyncResultCell<_>and going though the rest of the asynchronous mechanism.
Also notice that an optional parameter called
reusethread is defined, we use the
? prefix when defining it to make it
optional. We then make use of the
defaultArg function to give it a default value of false if a one is not passed in. This
will be used in the
Set operation to determine if the code will run on the same thread or a thread in the ThreadPool.
open System open System.Threading open System.Collections.Generic type AsyncAutoResetEvent(?reusethread) = let mutable awaits = Queue<_>() let mutable signalled = false let completed = async.Return true let reuseThread = defaultArg reusethread false
The first step is to use a locking construct to control access to the mutable queue
awaits. Inside this lock we
check to see if
signalled is true and if so we reset it to false and return our pre-built
completed asynchronous workflow. If
signalled is false then we create a new
AsyncResultCell<_> and add it to the queue then return the
AsyncResult to the caller.
member x.WaitAsync() = lock awaits (fun () -> if signalled then signalled <- false completed else let are = AsyncResultCell<_>() awaits.Enqueue are are.AsyncResult)
We first declare a function called
getWaiter(), we use this function to return an option type
that is either
Some AsyncResultCell<bool> or
None. We use the lock function to control access to the mutable queue
lock awaits. Once
inside the lock we use pattern matching to capture
- The first pattern match
(x,_)checks if there are any waiters (
awaits.Count > 0) and then dequeues an
AsyncResultCell<bool>from the queue and returns it within an option type:
Some <| awaits.Dequeue().
- The second pattern match
signalledis set to false before setting its value to true. This causes next
WaitAsync()caller to get the short-circuited value
completed. This means that an
AsyncResultCell<bool>does not need to be created and go though the whole async mechanism. We then return
Noneas there is no waiter to be notified.
- The final pattern match
(_,_)is used when there are no waiting callers and
signalledhas already being set, there is simply nothing to do in this situation so we return
We use the
getWaiter() function via pattern match. If we have a result i.e. Some AsyncResultCell
AsyncOK(true) to indicate a completion. Notice that we also pass in the
reuseThread boolean that was declared as part of the
reuseThread is true then the notification to the waiter happens synchronously use this with care! Personally I would stick
with the default of false to ensure that the operation is completed via the thread pool, unless you have a performance critical reason and the
waiting code that executes is very fast.
member x.Set() = let getWaiter()= lock awaits (fun () -> match (awaits.Count, signalled) with | (x,_) when x > 0 -> Some <| awaits.Dequeue() | (_,y) when not y -> signalled <- true;None | (_,_) -> None) match getWaiter() with | Some a -> a.RegisterResult(AsyncOk(true), reuseThread) | None _ -> ()
The reason for using the
getWaiter() function is to separate the locking function away from the notification, if
was called within the lock and
reuseThread was true then the awaiting function would be called synchronously within the lock which
would not be a very good situation to be in.
So there we have it, I could take this series further and convert the other primitives that Stephen Toub describes but there should be enough information in these two posts to set you on your way. If anyone would like me to complete the series then let me know. I may well finish them off and post them on GitHub in the future, time permitting.
Thanks for tuning in, until next time…