Skip to main content

Listening to a single value from an Rx observable sequence

I have an observable sequence and I want to observe only a defined number of values, the number is immaterial, the important part is the fact I don't want to be notified of any further values.

How can this can be achieved?

The answer is easy, use the Rx extension method Take, it'll return the required number of values only. A simple example:

   1:  Observable.FromEventPattern<RoutedEventArgs>(clickButton, "Click")
   2:      .Take(1)
   3:      .ObserveOn(Scheduler.CurrentThread)
   4:      .Subscribe(vt => Debug.WriteLine("{0}: Click called...", ++count));

When the button is clicked in the UI it produces a line in the output window in visual studio:
So using the Take method I can limit the number of observations irrespective of the number of times I click the 'Click Me!' button:

   1:  Observable.FromEventPattern<RoutedEventArgs>(clickButton, "Click")
   2:      .Take(1)
   3:      .ObserveOn(Scheduler.CurrentThread)
   4:      .Subscribe(vt => Debug.WriteLine("{0}: Click called...", ++count));
I was then wondering, and this is the reason for the post:

What's happened to the under lying subscription, has it been disposed?

The seems very similar to a previous post about Rx subscribers and what happens to subscriptions when the OnCompleted is invoked by the disposing the under lying stream. I suspect the answer is 'I don't have anything to worry about, it will all be disposed automatically'.

Lets find out by assigning the subscription to a variable and when the form is closed check the internals of the subscription - yes I am mixing metaphors for .Net event handling ;)
Okay, wasn't expecting that! The under lying composite disposable hasn't been disposed as I was expecting...

I was really expecting it to be automatically disposed at this point, I wonder if adding an OnCompleted implementation for the subscription makes any difference:
So now the subscription is being disposed automatically...

WTF! - How does defining an OnCompleted method mean it can be disposed? Surely what defines it as being eligible for disposing is we're using the Take method with a value of 1.

Now you could say:

'Why worry, I'm sure Rx will take care of it...'

I say anything I create\instantiate which is disposable I am responsible for make sure it is cleaned-up, and this is especially important if you have transient scopes which are regularly being created and destroyed.

Also, the fact I'm using the FromEventPattern extension is not important, the behaviour of the Take method is.


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

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

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