04_inject_services.md

Inject Services

In this article you learn everything about injecting services into your ViewModel:

1. Inject a Service
2. Define a Custom Property Name
3. Set the Access Modifier

Inject a Service

Usually you add constructor parameters to your ViewModel class, so that you can pass dependencies to it. This keeps your ViewModel loosely coupled and testable. Also for this very typical scenario, MvvmGen has a feature. Let’s say you have an EmployeeDataProvider implementation like below that you use to save an Employee. The Save method could save an Employee to a database, to a file or to any other place.

public interface IEmployeeDataProvider
{
  void Save(Employee employee);
}

public class EmployeeDataProvider : IEmployeeDataProvider
{
  public void Save(Employee employee)
  {
    // TODO: Save employee to a file, a db, a REST API etc. 
  }
}

The ViewModel would depend on the IEmployeeDataProvider interface. So, in a unit test, you mock this interface with a test implementation that does not access the real database, and in your app, you use the real EmployeeDataProvider. Now, to inject a service like an IEmployeeDataProvider instance into your ViewModel, you use MvvmGen’s Inject attribute on your ViewModel class. With the Inject attribute, you define the service type that you want to inject, in this case an IEmployeeDataProvider:

[Inject(typeof(IEmployeeDataProvider))]
[ViewModel(ModelType = typeof(Employee))]
public partial class EmployeeViewModel
{ ... }

Now the generated class that you see in the next code snippet has an IEmployeeDataProvider constructor parameter. This constructor parameter is used to initialize a protected EmployeeDataProvider property that you can use in your code.

partial class EmployeeViewModel : ViewModelBase
{
  public EmployeeViewModel(MyWpfApp.Data.IEmployeeDataProvider employeeDataProvider)
  {
    this.EmployeeDataProvider = employeeDataProvider;
    ...
  }
  ...

  protected MyWpfApp.Data.IEmployeeDataProvider EmployeeDataProvider { get; private set; }
}

Now, again, it’s important to understand that as soon as you have set the [Inject(typeof(IEmployeeDataProvider))] attribute on your ViewModel class, the EmployeeDataProvider property is immediately available for you to use, as the code generation happens on-the-fly while you type your code. That means, in your ViewModel, you can immediately use the EmployeeDataProvider property for example in the Save method like you see it in the following code snippet:

[Inject(typeof(IEmployeeDataProvider))]
[ViewModel(typeof(Employee))]
public partial class EmployeeViewModel
{
  [Property] private string _updateComment;

  partial void OnInitialize()
  {
    Model = new Employee
    {
      FirstName = "Thomas Claudius"
    };
  }

  [Command(CanExecuteMethod = nameof(CanSave))]
  private void Save()
  {
    EmployeeDataProvider.Save(Model);
  }

  [CommandInvalidate(nameof(FirstName))]
  private bool CanSave()
  {
    return !string.IsNullOrEmpty(FirstName);
  }
}

Define a Custom Property Name

By default, MvvmGen generates a property for your service like you would expect it. For an injected interface like IEmployeeDataProvider, a property with the name EmployeeDataProvider is generated, as you saw in the preview code snippets.

If you want to use a different generated property name, you can use the PropertyName property of the Inject attribute:

[Inject(typeof(IEmployeeDataProvider),PropertyName ="EmpDataProv")]
[ViewModel]
public partial class EmployeeViewModel  { }

Instead of using the PropertyName property, you can also pass the property name directly to the constructor of the Inject attribute:

[Inject(typeof(IEmployeeDataProvider), "EmpDataProv")]
[ViewModel]
public partial class EmployeeViewModel  { }

When you look now at the generated code, you can see that the generated property has the name EmpDataProv, exactly like specified with the Inject attribute.

partial class EmployeeViewModel : ViewModelBase
{
  public EmployeeViewModel(MyWpfApp.ViewModel.IEmployeeDataProvider empDataProv)
  {
    this.EmpDataProv = empDataProv;
    this.OnInitialize();
  }

  partial void OnInitialize();

  protected MyWpfApp.ViewModel.IEmployeeDataProvider EmpDataProv { get; private set; }
}
}

Set the Access Modifier

The generated property for an injected service is by default protected. If needed, you can specify a different access modifier by using the PropertyAccessModifier property of the Inject attribute:

[Inject(typeof(IEmployeeDataProvider), 
  PropertyAccessModifier = AccessModifier.Public)]
[ViewModel]
public partial class EmployeeViewModel  { }

This generates the following ViewModel. Note that the EmployeeDataProvider property is now public.

partial class EmployeeViewModel : ViewModelBase
{
  public EmployeeViewModel(MyWpfApp.ViewModel.IEmployeeDataProvider employeeDataProvider)
  {
    this.EmployeeDataProvider = employeeDataProvider;
    this.OnInitialize();
  }

  partial void OnInitialize();

  public MyWpfApp.ViewModel.IEmployeeDataProvider EmployeeDataProvider { get; private set; }
}

Next, let's look at how to communicate between ViewModels.