Attention MVVM Purists: Should a ViewModel access localized resources?

Question

I struggle to understand the purpose of the ViewModel sometimes, especially with respect to what it should and shouldn't do.

So... I find myself wanting to expose a display string from my ViewModel. Ok, actually it's a Date string, where I want to display something like "Unknown" if the date isn't set. Really I want a solution to cover the general case, so I don't want a discussion about using null and fallback values. It could equally be a status message like "Awaiting Dispatch" or "Launch Aborted".

So the quesion is, how should the ViewModel expose display strings to the View. The App will need to be localized, so we can't have hard coded strings in the ViewModel.

Is it ok to access the app's resources from the ViewModel, and return the display string?

As an aside, should I be using a resx string resource, or a Xaml resource dictionary? I'm leaning toward the Xaml. Advantages/Disadvantages?

Thanks, Mark

Answer 1

As mentioned in the other answers, this is clearly the view's responsibility. The view model should expose the data property (a status enum or something similar) for the view to inspect in order to determine the particulars of the display message. From a purist's perspective, generating display properties from the VM or code behind is right out.

I'm surprised to hear no suggestions of datatriggers, though. Use them in your TextBlock to bind to the exposed status enum value and update the text appropriately. This question compares their performance against binding to a converter and suggests that triggers will perform better in this scenario. I haven't done any testing myself, so I'm not sure, but it seems intuitively reasonable to me, provided you aren't listening for an outrageous number of different enum values. I also think it's the more appropriate method in this case, and I'll cite this question asking for a comparison of VC vs. DT use cases, though I don't think it's terribly contentious. To put it another way, if one holds that a VM is a value converter on steroids (a statement which I'm very leery of and will not agree or disagree with at the moment), then this argument holds: a VM shouldn't be exposing display properties; a VM's purpose and abilities are a super-set of those of a VC; therefore, a VC should not be exposing display properties.

I'm doing my best to give the purist's answer (or what I think it is, at least), but there are certainly easier ways to code it than writing a lot of datatriggers, especially if you have many different values to address.

Regarding your resource question, have a look at this MSDN article on resource files, which is, in certain ways, easier to work with than a resx file. Which approach you should use depends on your use cases.

Answer 2

What you are doing is taking data that is inside the system and converting it to a format that makes it more useful and clear to the user. This is the responsibility of the view. It would be the responsibility of the view to format a valid date into the correct culture and it has the same responsibility for any text that you wish to display.

In the same respect the status examples you give would probably be stored as an enum in the viewmodel (since that would make it easier to apply business logic to them, which is the role of the viewmodel) and it would be the responsibility of the view to display the values in a way that works for the user in all respects. This would include size, colour, font, position and culture.

That being said though I have been known to put display logic in my viewmodel (concatenating firstname and surname for example) simply because it is easier. I've even been known to use code-behind (shock, horror!) where it suits my purpose. There is a sliding scale between purity and pragmatism, and it is up to you where you sit on that line.