Simple model

We briefly introduced MementoEntity and MementoEntityCollection and seen a code snipped that allows us to attach the memento tracking system to an entity:

var memento = new ChangeTrackingService();

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

person.FirstName = "first name value";
person.LastName = "last name value";

var isChanged = memento.IsChanged; //true
var canUndo = memento.CanUndo; //true

What is happening is that each change performed on each tracked entity will be recorded by che ChangeTrackingService, on each tracked entity means that the following will work as expected:

var memento = new ChangeTrackingService();

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

person.FirstName = "first name value";
person.LastName = "last name value";
customer.CompanyName = "sample company";

var isChanged = memento.IsChanged; //true
var canUndo = memento.CanUndo; //true

Tracking at the same time both the Person instance and the Customer instance. The following sample highlights how changes are tracked:

var memento = new ChangeTrackingService();

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

person.FirstName = "a name";
person.LastName = "last name";

var isChanged = memento.IsChanged;
var state = memento.GetEntityState( person );

memento.Undo();
var _lastNameAfterUndo = person.LastName; //null

memento.Redo();
var _lastNameAfterRedo = person.LastName; //"last name"

The memento keeps tracks of a stack of changes in the exact same order they happened to the tracked models, each time Undo() is called the last change in the stack will be reverted and moved into the forward changes stack allowing the caller to call Redo() in order to apply it once again.

Calling AcceptChanges() on a memento instance will flush all the recorded changes considering the models as unchanged. Calling RejectChanges() will revert all the tracked models to their original state, or to the state we called AcceptChanges() last time.

The same applies also to collections: Handling Change Tracking in collections