Simple ViewModel graphs

When dealing with data editing and the MVVM pattern we need to be aware that the shortest path from the model to the UI is not always the best solution.

Imagine a scenario where we want to edit a Person instance that is loaded from a persistente storage, such as a database, the Person instance can be directly bound to the UI but it requires us to implement the INotifyPropertyChanged interface and if we want to enable it for the ChangeTrackingService we need to inherit from a base class. Both are not an option when dealing with the Single Responsibility Principle and with POCO objects.

In the above scenario we need to introduce at least two more actors, other than the Person data model:

1. A PersonViewModel that will be responsible to enrich the Person with property change notification support and with change tracking capabilities;

2. An EditorViewModel that will allow a clean separation of responsibilities owning all the relationship with the memento.

The second bullet is especially true when dealing with complex graph and/or with more than one tracked entity at the same time. Given a Person class like the following:

class Person
{
    public String FirstName { get; set; }
    public String LastName { get; set; }
}

We can create a PersonViewModel such as:

class PersonViewModel : MementoEntity
{
    public void Initialize( Person person, Boolean registerAsTransient )
    {
        if( registerAsTransient )
        {
            this.RegisterTransient();
        }

        this.SetInitialPropertyValue( () => this.FirstName, person.FirstName );
        this.SetInitialPropertyValue( () => this.LastName, person.LastName );
    }

    public String FirstName
    {
        get { return this.GetPropertyValue( () => this.FirstName ); }
        set { this.SetPropertyValue( () => this.FirstName, value ); }
    }

    public String LastName
    {
        get { return this.GetPropertyValue( () => this.LastName ); }
        set { this.SetPropertyValue( () => this.LastName, value ); }
    }
}

The first thing is to build a memento-enabled facade, that can grow adding feature, to enable change tracking and property change notifications in a Person-like class. In the above sample the PersonViewModel and the Person class are basically the same, we can say that this is corner case, most of the time in real scenarios there will be a huge difference between the model and the editing view model.

We are introducing a Initialize method, for the sake of the sample we can do the same thing using a constructor, using a Initialize method allows us to easily resolve PersonViewModel instances using an inversion of control container without the need to deal with the currently edited Person runtime instance. At initialization time we are doing 2 important things:

1. calling the RegisterTransient() method of the base class to register the current instance as transient, if required; To dive into the meaning of a transient entity look at the Change Tracking Service API;
2. using the SetInitialPropertyValue() method to initialize the default value of the PersonViewModel properties without affecting its tracking state;

Once we have setup our ViewModel we can build the editor:

public class EditorViewModel : AbstractViewModel
{
    readonly IChangeTrackingService service = new ChangeTrackingService();

    public EditorViewModel()
    {
        var observer = MementoObserver.Monitor( this.service );

        this.UndoCommand = DelegateCommand.Create()
            .OnCanExecute( o => this.service.CanUndo )
            .OnExecute( o => this.service.Undo() )
            .AddMonitor( observer );

        this.RedoCommand = DelegateCommand.Create()
            .OnCanExecute( o => this.service.CanRedo )
            .OnExecute( o => this.service.Redo() )
            .AddMonitor( observer );

        var person = new Person()
        {
            FirstName = "Mauro",
            LastName = "Servienti"
        };

        var entity = new PersonViewModel();
        this.service.Attach( entity );
        entity.Initialize( person, false );

        this.Entity = entity;
    }

    public ICommand UndoCommand { get; private set; }
    public ICommand RedoCommand { get; private set; }

    public PersonViewModel Entity
    {
        get { return this.GetValue( () => this.Entity ); }
        private set { this.SetValue( () => this.Entity, value ); }
    }
}

There is a lot going on here we are creating an editor and at first we setup our ChangeTrackingService instance, that in this specific sample is bound to the editor itself. In the constructor we are setting up a MementoObserver to watch the memento instance and we are binding that observer to 2 commands whose role is to expose Undo/Redo functionalities to the UI. Last we create a Person instance, in real scenarios the Person instance is expected to arrive from a persistent storage or a remote resource, we create the PersonViewModel, attach it to the memento service and finally initialize it with the person data source.

We finally expose both commands and the PersonViewModel instance to the View.