How many views, viewmodels and models do I need?

Question

I'm trying to develop a WPF application - actually it's more a tool - using the MVVM pattern. I've read several articles, watched videos, posted questions but somehow I feel that my approach to or understanding of this MVVM thing is not "the right one".

I'll start from the UI. In a first stage the UI shall display the following:

• Group box with
  • text box for the username
  • text box for the password (let's ignore the PasswordBox for the start)
• Group box with
  • combo box for a list of project names available for this user (will be retrieved from a webservice)
  • button "Login" for logging in to the selected project
  • button "Logout" for logging out from the current project

So I would identify the following data that need to be handled:

• A string for the username
• A string for the password
• An ObservableCollection<Project> for the list of projects
• A Project object representing the active project
• A boolean if logged in to a project or not

My worries, I have no idea how I should structure or design this following MVVM. How many views, viewmodels and models should I use here? Of course, the application will grow but not this much. Let's stick to the above.

Answer 1

There is no right or wrong answer to this

Think of Views, ViewModels, and Models as cut off points. They allow you to modularize your application versus taking a non-reusable monolithic approach.

In general, ViewModels to Views is 1:M however reality is that most of the time they are 1:1 relationship. The ViewModel and View are where the bulk of work resides, with the Model acting as nothing more than a POCO which implements INotifyPropertyChanged for binding needs.

In your example I would use a single View backed by a single ViewModel and multiple Models as is needed (Project, UserCredentials, etc...). You may have services which perform the actual login effort however you can perform that work in the ViewModel.