MVVM WPF ViewModels for Adding New Entity

Question

My concept for MVVM in WPF is that we have a ViewModel for every Model in your application. This means that if we have Customer class (entity) then we will have CustomerViewModel. The CustomerViewModel will have all the properties which are necessary to represent a customer. The CustomerView usercontrol will be responsible for creating the UI for the Customer model.

Now, let's say that we are adding a new customer. So, we have a form which consists of FirstName, LastName etc. Do we need a ViewModel for this scenario. I mean all I have to do is to take all the input values from TextBox and create a Customer object and then save it to the database. Why should I bother creating a ViewModel for this scenario?

Answer 1

First of all, that is not the main purpose of MVVM, to "mirror" everything. The View should provide the means for a user input, and certainly not process calls to any of the database layers. The ViewModel should be a GUI-agnostic application backbone, and it definetly should handle the creating of customers.

That said, what you should do, is have a ViewModel which represents a workspace for handling customers, and not just a customer ViewModel. If you really want to save on a few objects being created, add to that workspace the possibility to create and add a new customer (not CustomerViewModel). That way, you can have a View of the workspace which has elements for each relevant/required property of the customer, and by invoking some command added to that workspace ViewModel, you could get the current values filled in those (data bound to ViewModel) View elements directly to the customer model.

Consider if you could probably drop the specific customer (and other Model) ViewModels if you refactor things a bit, it would be good practice to keep things from adhering blindly to a certain pattern without explicit cause.

Answer 2

Let's pretend for a second that there is no business model. The only thing you have is a view. If you were to model just that view, without any knowledge of what the data means elsewhere in the system, that is a ViewModel.

The goal of a ViewModel is to, well, model the view it backs. This is a different goal than modeling the idea of a customer in your business domain. To say you will have have one ViewModel per business entity, then, is to say you will have one view per business entity, which leads to run-of-the-mill data-focused UI.

In your particular case, think about the customer edit view. It has fields that correspond to a customer's properties, and thus seems like a natural fit for binding to a customer directly. However, where on the customer object is the "Submit" action modeled? Where is the "Cancel" action modeled? Where is it modeled that field X is an enumerated value selected from a list?

How about the password? If persisted as a binary hashed value, does the view know how to hash its text? What if the system has password strength requirements?

The ViewModel is the mortar between the business model and the UI. It takes concerns from one and translates them into terms of the other. It is the point at which all the above issues are addressed. To say a ViewModel isn't necessary is to ignore its necessity.

Answer 3

You don't need to have a separate ViewModel for Add, you only need a single ViewModel which should do Edit and Add scenarios. If you can delete the record from the edit page than that ViewModel should also have the ability to delete. Your ViewModel should reflect the functionality your View exposes regardless of data.

I think you should reconsider having a single ViewModel for each model. I like to think of ViewModels as normalizing behavior inserted of data. By having a ViewModel for each Model class you will run into architecture issues sooner or later. I look at the application from top down overview, what is my UI trying to accomplish and from there I will get to the ViewModel and eventually I will get to my DataFactory and how the ViewModel maps down to data is almost always not 1 to 1 except for the most simplistic Views. If you try to map 1 to 1 you will have bad UI or your data will not be normalized very well.

The Stack we have here is:

1. View
2. ViewModel (Controls everything the user can do in the view, wraps properties from our POCO's)
3. DataFactory (Maps our POCO's to Entity Framework objects and CRUD)
4. POCO's (Business Logic, all rules and validation)
5. Entity Framework (Model, Data Access)

One note here is that ViewModel contains properties from multiple POCO's!

We inject the DataFactory through StructureMap and Unit test with xUnit along with Moq.

To answer you second question I would create a separate view only view to drop in as a user control. But you should still have a CRUD ViewModel in you app that encapsulate all that functionality in a user friendly way.

Thanks.

Answer 4

One reason for this VM abstraction is for testability. Another reason why you want a ViewModel is because it's basically a Data Transfer Object that might be a combination of fields from multiple models in a single container that is more relevant to your current view. Yet another reason to have VM is to take advantage of WPF two ways binding capabilities.

Using your regular model (plain POCO), you can update the View when your model change, but since your model does not implement dependency properties (most likely), you won't be able to take advantage of updating your model when the value in WPF control changes. Which mean you have to manual add a handler and do the copy this value from this control back to the model kind of thing.