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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

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