CODEWITHSUNDEEP
wpfdesign-patternsmvvm
Dean Kuga
Dean Kuga

Reputation: 12119

Is a reference in a View to a ViewModel (which is its DataContext) in any way detrimental to MVVM pattern?

Let's say a ViewModel publishes an Event and a View which is using that VM as its DataContext subscribes to that Event by obtaining a reference to the VM by casting its DataContext to VM instance:

vm = DataContext as MainViewModel;
if (vm == null) return;
vm.SomeUIRelatedNotice += DoSomethingUIRelated;

The way I see this, the VM ramains decoupled and its testability is in no way affected, and the View already has a reference to the VM so I see no problem with this at all but I want to hear from the MVVM purists if they think this is bad design with negative impact on the MVVM pattern and if so why?

Upvotes: 6

Views: 272

Answers (2)

Adi Lester
Adi Lester

Reputation: 25201

As long as the view model is not performing any view logic itself, I don't consider this to be breaking the MVVM pattern.

It looks like in your case the view model is trying to interact and direct the view somehow, so you should probably know that there are other ways to achieve such behavior which are considered to be more pure MVVM:

Upvotes: 3

Viv
Viv

Reputation: 17380

View knowing a VM is not wrong at all and like you mentioned is perfectly fine.

In such cases, the question I ask myself is

"Would doing this X action make my VM's break testability? or hinder the scope of testability in my business logic?"

If answer is "Yes" - You're kind-of heading down the wrong path. If "No" go right ahead. In my opinion that's all there is to MVVM. It helps you unit test logic and classes which we couldnt before with things like MVC and tight coupling.

As you arent unit-testing view related stuff to a large extent, this wouldn't affect your MVVM "true-ness" in any way.

Side-Note

While I would stick with my above opinion. One thing is, have you considered using a Messenger Simple Tutorial of MVVM Light's Messenger / EventAggregator pattern. With a Messenger pattern, your VM would be just publishing the Message's and you're Views could essentially just subscribe to these messages without even having to refer to the VM (make the message type custom strong if you want uniqueness). This way you do not have to do the whole casting from DataContext and can also unit-test the Message publishing from the VM perfectly fine.

Upvotes: 2

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