Iterator.md

Iterator

Definition

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

Frequency of use:

High

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UML class diagram

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Participants

    The classes and objects participating in this pattern are:

• Iterator  (AbstractIterator)
  • defines an interface for accessing and 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 an Iterator object
• ConcreteAggregate  (Collection)
  • implements the Iterator creation interface to return an instance of the proper ConcreteIterator

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Structural code in C#

This structural code demonstrates the Iterator pattern which provides for a way to traverse (iterate) over a collection of items without detailing the underlying structure of the collection.

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Iterator.Structural
{
    /// <summary>
    /// MainApp startup class for Structural 
    /// Iterator Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        static void Main()
        {
            ConcreteAggregate a = new ConcreteAggregate();
            a[0] = "Item A";
            a[1] = "Item B";
            a[2] = "Item C";
            a[3] = "Item D";

            // Create Iterator and provide aggregate
            ConcreteIterator i = new ConcreteIterator(a);

            Console.WriteLine("Iterating over collection:");

            object item = i.First();
            while (item != null)
            {
                Console.WriteLine(item);
                item = i.Next();
            }

            // Wait for user
            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Aggregate' abstract class
    /// </summary>
    abstract class Aggregate
    {
        public abstract Iterator CreateIterator();
    }

    /// <summary>
    /// The 'ConcreteAggregate' class
    /// </summary>
    class ConcreteAggregate : Aggregate
    {
        private ArrayList _items = new ArrayList();

        public override Iterator CreateIterator()
        {
            return new ConcreteIterator(this);
        }

        // Gets item count
        public int Count
        {
            get { return _items.Count; }
        }

        // Indexer
        public object this[int index]
        {
            get { return _items[index]; }
            set { _items.Insert(index, value); }
        }
    }

    /// <summary>
    /// The 'Iterator' abstract class
    /// </summary>
    abstract class Iterator
    {
        public abstract object First();
        public abstract object Next();
        public abstract bool IsDone();
        public abstract object CurrentItem();
    }

    /// <summary>
    /// The 'ConcreteIterator' class
    /// </summary>
    class ConcreteIterator : Iterator
    {
        private ConcreteAggregate _aggregate;
        private int _current = 0;

        // Constructor
        public ConcreteIterator(ConcreteAggregate aggregate)
        {
            this._aggregate = aggregate;
        }

        // Gets first iteration item
        public override object First()
        {
            return _aggregate[0];
        }

        // Gets next iteration item
        public override object Next()
        {
            object ret = null;
            if (_current < _aggregate.Count - 1)
            {
                ret = _aggregate[++_current];
            }

            return ret;
        }

        // Gets current iteration item
        public override object CurrentItem()
        {
            return _aggregate[_current];
        }

        // Gets whether iterations are complete
        public override bool IsDone()
        {
            return _current >= _aggregate.Count;
        }
    }
}

Output

Iterating over collection:\
Item A\
Item B\
Item C\
Item D