Iterator Pattern

Iterator Pattern 

Gamma Categorization: Behavioral Design Pattern
Summary: Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Sample Code

Problem: We want to apply several commands on the digital camera.

Code

public class Photo

{

    public string Name { get; set; }

    public Photo(string name)

    {

        this.Name = name;

    }

}

public interface IPhotoCollection

{

    IPhotoIterator CreateIterator();

}

public interface IPhotoIterator

{

    Photo First();

    Photo Next();

    bool IsDone { get; }

    Photo CurrentPhoto { get; }

}

public class PhotoCollection : IPhotoCollection

{

    private ArrayList _items = new ArrayList();

    public IPhotoIterator CreateIterator()

    {

        return new PhotoIterator(this);

    }

    public int Count

    {

        get { return _items.Count; }

    }

    public object this[int index]

    {

        get { return _items[index]; }

        set { _items.Add(value); }

    }

}

public class PhotoIterator : IPhotoIterator

{

    private PhotoCollection _photos;

    private int _current = 0;

    private int _step = 1;

    public Photo CurrentPhoto { get { return _photos[_current] as Photo; } }

    public bool IsDone { get { return _current >= _photos.Count; } }

    public PhotoIterator(PhotoCollection photos)

    {

        this._photos = photos;

    }

    public Photo First()

    {

        _current = 0;

        return _photos[_current] as Photo;

    }

    public Photo Next()

    {

        _current += _step;

        if (!IsDone)

            return _photos[_current] as Photo;

        else

            return null;

    }

}

Usage

PhotoCollection photoCollection = new PhotoCollection();

photoCollection[0] = new Photo("IMG_01");

photoCollection[1] = new Photo("IMG_02");

photoCollection[2] = new Photo("IMG_03");

photoCollection[3] = new Photo("IMG_04");

photoCollection[4] = new Photo("IMG_05");

photoCollection[5] = new Photo("IMG_06");

photoCollection[6] = new Photo("IMG_07");

photoCollection[7] = new Photo("IMG_08");

photoCollection[8] = new Photo("IMG_09");

PhotoIterator photoIterator = new PhotoIterator(photoCollection);

for (Photo p = photoIterator.First(); !photoIterator.IsDone; p = photoIterator.Next())

{

    System.Console.WriteLine($"{p.Name}");           

}

Output

IMG_01

IMG_02

IMG_03

IMG_04

IMG_05

IMG_06

IMG_07

IMG_08

IMG_09