Stateless vs Stateful ViewModel

Question

Here are two links which briefly describe the difference between the two: stateless, stateful

In short in the "Stateless" scenario we bind views directly to models, view models just expose the whole objects, not their properties, so we don't need any synchronization between models and view models.

In the "Stateful" scenario we make a copy of a model object and bind it to a view.

Are there any downsides in the "Stateless" scenario? Because it seems like it's a way to go by default. What stops us from implementing INotifyDataErrorInfo, INPC and all the stuff at the level of models?

Answer 1

What stops us from implementing INotifyDataErrorInfo, INPC and all the stuff at the level of models?

That how the model is described by the business and how it is shown in the UI can (and will) be different.

While simple scenarios might make it seem that models and UIs are exactly the same, reality is not so simple. ViewModel's primary purpose is to transform the business domain model into form that is easy to bind to UI.

Answer 2

Nearly ever user interface is going to require the ViewModel to have state that does not belong in the model so I believe that there is no such thing as a "stateless" ViewModel in MVVM.

Some examples:

• Enabled/Disabled state of user interface controls
• Selected items (listboxes, radio buttons, etc)
• Input data that has not yet been validated or committed to the model