Question

I’m developing Windows Phone 7 application, and heard that MVVM pattern is useful. I read many articles and questions on this site, but all I get is the explanation of the view model as a bridge between view and model. If model is a Customer class, then the view model is thought to be a collection of customers, used for data binding. This is a very simple usage!

If I want to load customers from network, show it on the view, then store it to local database, should I place all that logic code in the view model ?

Answer 1

No you should not, read about Separation of Concerns. You want only one reason for each of your classes to change, the view-model should only be responsible for translating model information to be presented on the view.

A separate service implementation should be responsible for obtaining the model instances from your data store.

For example:

public class MyViewModel
{
    private readonly ICustomerRetrieval _retrievalService;

    public MyViewModel(ICustomerRetrieval retrievalService)
    {
        _retrievalService = retrievalService;
    }

    public IEnumerable<CustomerViewModel> Customers
    {
        get { return _retrievalService.GetAllCustomers().Select(AsViewModel);
    }

    private CustomerViewModel AsViewModel(CustomerModel cust)
    {
        return new CustomerViewModel(cust);
    }
}

You can see the service is responsible for locating the customer models, while the view-model is merely responsible for translating these into something usable from the UI.