Collections

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

We can go further and introduce collection change tracking:

var list = new MementoEntityCollection<String>();
memento.Attach(list);

list.Add("a");
list.Add("b");
list.Add("c");

var count = list.Count; //3

memento.Undo();
//list.Count 2

memento.Redo();
//list.Count 3

The MementoEntityCollection<T> being a memento entity will keep track of changes applied to collection structure. Each change will be tracked: Add, Remove, Clear, Insert, InsertAt, etc...

var list = new MementoEntityCollection<Person>();
memento.Attach(list);

list.Add(new Person());

Adding a MementoEntity, such as Person, will automatically trigger the memento that will start tracking the Person instance:

var list = new MementoEntityCollection<Person>();
memento.Attach( list );

var person = new Person();
list.Add( person );

person.FirstName = "name";

memento.Undo(); //the person first name is reverted
memento.Undo(); //the person instance is removed from the collection
memento.Redo(); //the person instance is added once again to the collection

As expected the changes stack is handled in the correct order. The next step is to understand how to handle change tracking in complex objects graph.