• It provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
• A common programming task is to traverse and manipulate a collection of objects.
  • These collections may be stored as an array, a list, or perhaps something more complex, such as a tree or graph structure.
  • You may need access the items in the collection in a certain order, such as, front to back, back to front, depth first (as in tree searches), skip evenly numbered objects.
  • The Iterator design pattern solves this problem by separating the collection of objects from the traversal of these objects by implementing a specialized iterator class.

Participants

• Iterator (AbstractIterator)
  • Defines interface for accessing & traversing elements.
• ConcreteIterator (Iterator)
  • Implements the Iterator interface.
  • Keeps track of the current position in the traversal of the aggregate.
• Aggregate (AbstractCollection)
  • Defines an interface for creating Iterator object.
• ConcreteAggregate (Collection)
  • Implements the Iterator creation interface to return an instance of the proper ConcreteIterator.

var collection = new Collection();
collection[0] = new Item("Item 1");
collection[1] = new Item("Item 2");
collection[2] = new Item("Item 3");
collection[3] = new Item("Item 4");
collection[4] = new Item("Item 5");

var iterator = collection.CreateIterator();
iterator.Step = 2;

for (var item = iterator.First(); !iterator.IsDone; item = iterator.Next())
	Console.WriteLine(item.Name);

public class Item
{
	public string Name { get; set; }
	public Item(string name) => Name = name;
}

public interface IAbstractCollection
{
	Iterator CreateIterator();
}

public class Collection : IAbstractCollection
{
	private readonly ArrayList _Items = new ArrayList();

	public Iterator CreateIterator() => new Iterator(this);
	public int Count => _Items.Count;

	public object this[int index]
	{
		get => _Items[index];
		set => _Items.Add(value);
	}
}

public interface IAbstractIterator
{
	Item First();
	Item Next();
	bool IsDone { get; }
	Item CurrentItem { get; }
}

public class Iterator : IAbstractIterator
{
	private Collection _Collection;
	private int _Current = 0;

	public Iterator(Collection collection) => _Collection = collection;

	public Item First()
	{
		_Current = 0;
		return _Collection[_Current] as Item;
	}

	public Item Next()
	{
		_Current += Step;
		return IsDone ? null : _Collection[_Current] as Item;
	}

	public int Step { get; set; } = 1;
	public Item CurrentItem => _Collection[_Current] as Item;
	public bool IsDone => _Current >= _Collection.Count;
}

Rules of Thumb

• The abstract syntax tree of Interpreter is a Composite (therefore Iterator and Visitor are also applicable).
• Iterator can traverse a Composite.
  • Visitor can apply an operation over a Composite.
• Polymorphic Iterators rely on Factory Methods to instantiate the appropriate Iterator subclass.
• Memento is often used in conjunction with Iterator.
  • An Iterator can use a Memento to capture the state of an iteration.
  • The Iterator stores the Memento internally.