Dependency Injection with MVVM and Child Windows

Question

I am using MVVM Light and I'm currently using SimpleIoC that comes with the package. I'm getting a bit stuck with the dependency injection. I have a bunch of services that I want to use in my view models, however most windows are a List-Edit paradigm, i.e. one screen lists all of type Person and then you can Add or Edit a Person via a new screen.

When I was doing all code in the code behind my code for adding and editing a record was as follows:

View

private void btnEdit_Click(object sender, RoutedEventArgs e)
{
    _viewModel.Edit();
}

private void btnAdd_Click(object sender, RoutedEventArgs e)
{
    _viewModel.Add();
}

View Model

public void Add()
{
    var f = new TypeDetails();
    f.Show();
}
public void Edit()
{
   if (SelectedItem == null)
      return;
   var f = new TypeDetails(SelectedItem.Id);
   f.Show();
}

The constructor of TypeDetails is as follows:

public TypeDetails(int id = 0)
{
    InitializeComponent();
    _viewModel = new TypeDetailsViewModel(id);
    DataContext = _viewModel;
}

What would the best be to implement this type functionality with MVVM Light? I have been using the ViewModelLocator class for the List screens, however I cannot see a way to do this using the SimpleIoC. My way round so far has been to keep the constructor the same, which works fine until I need to inject dependencies into the TypeDetailsViewModel such as a service. With a service the constructor of TypeDetailsViewModel would be:

public TypeDetailsViewModel(ISomeService someService, int id = 0)
{
     ...
}

But that means in my view constructor I have to build these dependencies one at a time and manually inject them...

public TypeDetails(int id = 0)
{
    InitializeComponent();
    _viewModel = new TypeDetailsViewModel(SimpleIoC.Current.GetInstance<ISomeService>(),id);
    DataContext = _viewModel;
}

Is there a better way to do this?

score 5 · Accepted Answer · edited May 23 '17 at 11:44

First off I would look into the "RelayCommand" class which is part of MVVM Light. It will remove the need for events in your code behind. Start with that.
You should always favor "Constructor Injection" instead of the ServiceLocator (ex: SimpleIoC.Current.GetInstance())
Your ViewModel constructor should only be injecting services and not primitive types like "int". In your example "int id" should be the parameter of a method and not injected.

Ex: Instead, your TypeDetailsViewModel should look more like:

public TypeDetailsViewModel(ISomeService someService)
{
     TypeDetail GetDetailsCommand(int id)
     {
        ...
     }
}

Lastly, your Models should never have any reference to your ViewModels.
For your DataContext, you can use a ViewModelLocator (ViewModels in ViewModelLocator MVVM Light)
To hook up your View and ViewModel to use the GetDetailsCommand, you can use the EventToCommand behavior (http://msdn.microsoft.com/en-us/magazine/dn237302.aspx). Ex: The OnLoaded event on the View calls the GetDetailsCommand on your ViewModel.

I have been using RelayCommand, which has eliminated those events, I only had them for examples on how the code used to be. I am curious about where the DataContext is set for the Details screen, should the Ctor of the View call `SimpleIoC` and `GetInstance()`? With regards to the injection, how would the `GetDetails` method link up to the view in any way? Could you perhaps provide an expanded code sample? I love MVVM but I feel I'm missing something fundamental. — Ryan Amies, Aug 16 '13 at 21:46

Dependency Injection with MVVM and Child Windows

1 Answers1