AsyncLock.IsTaken needed

[sjb-sjb]

A synchronous method is needed in AsyncLock to test whether or not the lock is currently taken. For example, we may have an AsyncCommand which CanExecute only when a shared async resource is not locked. We need to test whether the lock is taken, in order to calculate CanExecute:

    /// <summary>
    /// Returns true iff the lock is already taken. 
    /// </summary>
    public bool IsTaken {
        get {
            lock (this._mutex) {
                return this._taken;
            }
        }
    }

Ideally this property would be notified so that we can more easily call CanExecuteChanged.

[IanYates]

The first thing that comes to mind with notification is that you'd naturally have cross-thread issues (why else have a lock?).
I can see merit in the idea but there'd have to be warnings for people to not rely on it as a "this lock is available" because race conditions, yada yada :)

Nothing that can't be solved with a bit of documentation I suppose :)

[sjb-sjb]

I agree with your comment about the race condition. However I don't see any other way to notify a button that it needs to disable itself when the critical section has been entered.. If there is some other way to do it then please let me know. I do think we need to be able to notify the UI that a critical section is active / locked.

I also agree with the cross-thread comment, although I would say this is kind of a "normal" problem.

[StephenCleary]

The IsTaken is being considered as part of the new API. It won't ever notify, though; updating the UI is a completely disparate operation than serialization of access.

@sjb-sjb Enabling/disabling a control based on something outside the user's control seems like very odd UX to me. The user could easily click on a button, see it become disabled, and yet not have any response to their action. If you could describe your use case in more detail, maybe I can suggest an alternative approach?

[sjb-sjb]

Well, I am copying my sqlite database to and from the appdir so that the user can see their file in their documents directory. I have file open and file new commands that copy an existing or new file respectively. I want to disable file new while file open is in the critical section (which is pretty much all of file open) and vice versa.

Thanks for your comments.

[StephenCleary]

In that case, I'd just disable the New button while the Open command is in progress.

[sjb-sjb]

Well, yes, I agree. It's just a question of whether "in progress" is coming from the ViewModel or from the Model / service. I just thought the service (file service in this case) should be able to tell all users whether or not an operation is in progress / the service is busy.

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On May 30, 2016, at 10:15 AM, Stephen Cleary notifications@github.com wrote:

In that case, I'd just disable the New button while the Open command is in progress.

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[StephenCleary]

If your Model uses INotifyPropertyChanged, then there's no reason not to expose AsyncCommands / NotifyTask from your Model. Personally, I usually don't write my models like that, but if you do, they should work just fine.

[StephenCleary]

There appear to be more appropriate alternative solutions for this use case rather than exposing IsTaken.