Complex objects graph

We briefly introduced MementoEntity and MementoEntityCollection and seen how to track changes in a simple graph and in collections.

The last type of graph we want to be able to track is a complex graph, where at least one of the properties of the root tracked object is a memento entity or a memento collection itself:

class Person : MementoEntity
{
    public Person()
    {
        this.Addresses = new MementoEntityCollection<Address>();
    }

    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 ); }
    }

    public IList<Address> Addresses { get; private set; }
}

class Address : MementoEntity
{   
    public String Street
    {
        get { return this.GetPropertyValue( () => this.Street ); }
        set { this.SetPropertyValue( () => this.Street, value ); }
    }
}

In the above sample the Person class has a property, Addresses, whose type is itself a memento entity, a MementoEntityCollection<Address> in this specific case. Using the following snippet:

var memento = new ChangeTrackingService();

var person = new Person();
memento.Attach( person );

the Addresses collection is not automatically tracked, the memento does not know anything of the structure of the graph. We can update the Person class so to instruct the memento that also the Addresses collection needs to be tracked:

class Person : MementoEntity
{
    protected override void OnMementoChanged( IChangeTrackingService newMemento, IChangeTrackingService oldMemento )
    {
        base.OnMementoChanged( newMemento, oldMemento );
        if( oldMemento != null ) 
        {
            oldMemento.Detach( this.Addresses );
        }
        if( newMemento != null ) 
        {
            newMemento.Attach( this.Addresses );
        }

    //rest of the Person class code
}

We are intercepting the moment in which the Person instance is tracked by the memento service overriding the OnMementoChanged and we are manually propagating the memento to inner instances. It is important to remove, detach, the memento entity from the previous memento instance if any, a memento entity can be tracked by one memento only at a time.

Note: changes to a MementoEntityCollection<T> automatically propagates the memento service to list items if they are a memento entity.