Question

I’m learning about MVVM and one of the things I don’t get is how the model and the view model are supposed to communicate. I also don’t understand whether they are separate classes, composite classes, or whether the ModelView is supposed to inherit from the model.

I need to get some data from a web service, so I think the model should be responsible with it and make the appropriate web service calls. But since these requests really originate in the view as a result of the user wanting to see some information it means the ModelView has to forward that request to the model somehow, then provide an asynchronous notification mechanism so that the view isn’t stuck while the model asynchronously retrieves the data. To summarize, suppose we have the following use case:

View: ComboBox –> bound to List in ModelView. Model view is connected to Model in (?????) way. The data that will populate the list can be retrieved by a web service call. How does this scenario work?

Answer 1

Let’s make your scenario a bit more complicated: user clicks a button and then a list needs to be populated with a data that comes from a web service.

In this scenario:

• View’s list is bound to ObservableCollection in ViewModel.
• View’s button is bound to a command in ViewModel.
• ViewModel’s command’s handler will fire an asynchronous request to a web service and subscribe a listener that will handle a response.
• The response handler will populate the observable collection with the items from the response once it’s received. Since View is bound to the ObservableCollection it will automagically update its list on the screen.

Now whether you encapsulate a web service call in an another class (e.g. Model) or leave it in ViewModel highly depends on your own preferences (e.g. having it in a model allows to to easily test ViewModel by injecting a mock rather than having a web service running, etc.)