07_generate_a_viewModel_interface.md

Generate a ViewModel Interface

To support some unit testing scenarios, it can be necessary to generate an interface for your viewModel. You can do this manually, or you can use MvvmGen's ViewModelGenerateInterface attribute.

This attribute will generate an interface for your ViewModel class that you decorated already with the ViewModel attribute. The code snippet below shows a ViewModel that uses the attribute.

[ViewModelGenerateInterface]
[ViewModel]
public partial class EmployeeViewModel
{
  [Property] private string _firstName;
}

In the generated code below you can see that an IEmployeeViewModel interface is created and the generated EmployeeViewModel class implements this interface.

partial class EmployeeViewModel : global::MvvmGen.ViewModels.ViewModelBase, IEmployeeViewModel
{
     ...
}

public interface IEmployeeViewModel
{
    string FirstName { get; set; }
}

If you want, you can specify the name of the interface by using the attribute's InterfaceName property like this: [ViewModelGenerateInterface(InterfaceName="ICustomName")].

Interfaces and Factories

When you also generate a factory for your ViewModel, the factory will automatically use the generated interface as a return type for its Create method. Let's take the ViewModel that you see below as an example:

[ViewModelGenerateInterface]
[ViewModelGenerateFactory]
[ViewModel]
public partial class EmployeeViewModel
{
    [Property] private string _firstName;
}

Below you can see the generated code. Note that the factory uses now the generated IEmployeeViewModel interface as a return type for its create method.

partial class EmployeeViewModel : global::MvvmGen.ViewModels.ViewModelBase, IEmployeeViewModel
{
    ...
}

public interface IEmployeeViewModel
{
    string FirstName { get; set; }
}

public interface IEmployeeViewModelFactory : IViewModelFactory<IEmployeeViewModel> { }

public class EmployeeViewModelFactory : IEmployeeViewModelFactory
{
    public EmployeeViewModelFactory()
    {
    }

    public IEmployeeViewModel Create() => new EmployeeViewModel();
}

In case you want to generate an interface but still want a concrete return type from your factory, you can use the factory attribute's ReturnType property like you see it in the code snippet below:

[ViewModelGenerateInterface]
[ViewModelGenerateFactory(ReturnType = typeof(EmployeeViewModel))]
[ViewModel]
public partial class EmployeeViewModel
{
    [Property] private string _firstName;
}

Now you learned about generating interfaces for ViewModels. If you want to learn how interfaces and factories are used, take a look at the EmployeeManager application that you find in the MvvmGen Samples repository.

Next, let's look at how to set up dependency injection.