Skip to main content

Simple F# REPL in WPF - part 2

In the first post I've managed to abstract the use of the F# interactive process behind an interface - it can now be used from an .net application:

Loading gist ....

The goal of the abstraction was to make it easy to use and I believe it has, but after reviewing the interface and implementation there will be responsiveness problems when I try and use it in an actual WPF UI.

I reckon this because the F# interactive process is running 'out of process' and the fact that any script I want to execute has to be written to the input stream and then interpreted & executed. This means there's going to be an appreciable delay in the responsiveness of the UI - a delay long enough to be observed by the user or at worst allow the user to enter another script before the previous has been executed and therefore screw up the output.

This can be observed in the console test harness as well, if I enter text before the initial setup has been completed it screws up an error - I've highlighted the text and the error it generated:
In a UI based application you typically disable controls when you want to prevent the user from entering text (data) when it's not applicable - using XAML framework properties like IsEnabled or IsReadOnly.

Exposing information to manipulate the XAML properties will be key to preventing user input at the wrong times during the engine execution cycle. Currently the interface doesn't expose any information about the current state of the F# interactive process - the name of the interface should have given me a clue, it contains the word 'engine'. Engines usually have a set of predefined states - stopped, starting, stated, idling etc.

I need to expose state of the REPL engine through the interface - this will allow me to enable\disable controls in any WPF UI. I've decided on the following states and these are exposed via an IObservable<state> property on the interface called 'State':

Loading gist ....

The observable property will pump when ever there is a state change in the engine and this will allow any UI to do what ever it wants when a state transition occurs. Implementing this in the engine is easy, in fact it improves the implementation because the control flow is easier to understand, shown below is the implementation of the Stop method:

Loading gist ....

Hooking this new property up in the console test harness is easy:
When running this produces the following results in the output window of visual studio:
The future WPF UI will disable the ability to execute a script while the state is not equal to 'Running'.

In the next post I'll actually get around to using the implementation in WPF :)

Labels:
Location: London N16, UK

Comments

Post a Comment

Popular posts from this blog

Showing a message box from a ViewModel in MVVM

I was doing a code review with a client last week for a WPF app using MVVM and they asked ' How can I show a message from the ViewModel? '. What follows is how I would (and have) solved the problem in the past. When I hear the words ' show a message... ' I instantly think you mean show a transient modal message box that requires the user input before continuing ' with something else ' - once the user has interacted with the message box it will disappear. The following solution only applies to this scenario. The first solution is the easiest but is very wrong from a separation perspective. It violates the ideas behind the Model-View-Controller pattern because it places View concerns inside the ViewModel - the ViewModel now knows about the type of the View and specifically it knows how to show a message box window: The second approach addresses this concern by introducing the idea of messaging\events between the ViewModel and the View. In the example below
5 comments
Read more

Implementing a busy indicator using a visual overlay in MVVM

This is a technique we use at work to lock the UI whilst some long running process is happening - preventing the user clicking on stuff whilst it's retrieving or rendering data. Now we could have done this by launching a child dialog window but that feels rather out of date and clumsy, we wanted a more modern pattern similar to the way <div> overlays are done on the web. Imagine we have the following simple WPF app and when 'Click' is pressed a busy waiting overlay is shown for the duration entered into the text box. What I'm interested in here is not the actual UI element of the busy indicator but how I go about getting this to show & hide from when using MVVM. The actual UI elements are the standard Busy Indicator coming from the WPF Toolkit : The XAML behind this window is very simple, the important part is the ViewHost. As you can see the ViewHost uses a ContentPresenter element which is bound to the view model, IMainViewModel, it contains 3 child v
Post a Comment
Read more

Custom AuthorizationHandler for SignalR Hubs

How to implement IAuthorizationRequirement for SignalR in Asp.Net Core v5.0 Been battling this for a couple of days, and eventually ended up raising an issue on Asp.Net Core gitHub  to find the answer. Wanting to do some custom authorization on a SignalR Hub when the client makes a connection (Hub is created) and when an endpoint (Hub method) is called:  I was assuming I could use the same Policy for both class & method attributes, but it ain't so - not because you can't, because you need the signatures to be different. Method implementation has a resource type of HubInnovationContext: I assumed class implementation would have a resource type of HubConnectionContext - client connects etc... This isn't the case, it's infact of type DefaultHttpContext . For me I don't even need that, it can be removed completely  from the inheritence signature and override implementation. Only other thing to note, and this could be a biggy, is the ordering of the statements in th
Post a Comment
Read more