Skip to main content

MVVM anti-pattern: Using UI Message Bus to communicate between View Models & Services

This anti-pattern is probably more applicable to large MVVM based applications, and it might even just be a pet dislike of mine. I better clarify what I mean, this pattern is known by different terms, it's also known as Event Broker (PRISM). It shouldn't be confused with an enterprise message bus, what I'm talking about is a UI layer applicable pattern, which is used to communicate internally by the UI implementation. The UI might well still use an enterprise message bus to communicate with disparate backend systems.

Shown below is a really simple interface & implementation of a message bus, this service would normally be a singleton instance shared between all view models & other services:
Conceptually as you can see all it does is connect a publisher to a consumer on a particular topic. Now this a very powerful concept and when used well. It provides a clean way to communicate between disparate view models & services in the UI.

For me the ability to communicate this way, cross-cutting any structure you have for the view models, services & controllers seems wrong - when you've gone to the effort to separate out the classes and logically grouped them this all seems to be undone by using a message bus.

In larger MVVM applications you're more likely to have a controller sitting behind your main view model to orchestrate service communication and interaction between view models & services, and this is probably why I see this as an anti-pattern because it can destroy this structure. It almost could be considered another way to do the Service Locator pattern.

Another downside (just as important IMO) comes when you're debugging or re-factoring a code base - how difficult can it be to find where a message came from or which subscriber is causing an issue, especially when the call stack has been lost because the subscriber is handling the message on another thread.


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