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Definition
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
Participants
The classes and/or 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
Sample 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.
// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">// Respect the work.// </copyright>// <summary>// Structural Iterator Design Pattern.// </summary>// --------------------------------------------------------------------------------------------------------------------namespace CSharpLearning{ using System; using System.Collections; /// <summary> /// Startup class for Structural Iterator Design Pattern. /// </summary> internal static class Program { #region Methods /// <summary> /// Entry point into console application. /// </summary> private static void Main() { var a = new ConcreteAggregate(); a[0] = "Item A"; a[1] = "Item B"; a[2] = "Item C"; a[3] = "Item D"; // Create Iterator and provide aggregate var i = new ConcreteIterator(a); Console.WriteLine("Iterating over collection:"); object item = i.First(); while (item != null) { Console.WriteLine(item); item = i.Next(); } } #endregion } /// <summary> /// The 'Aggregate' abstract class /// </summary> internal abstract class Aggregate { #region Public Methods and Operators /// <summary> /// The create iterator. /// </summary> /// <returns> /// The <see cref="Iterator"/>. /// </returns> public abstract Iterator CreateIterator(); #endregion } /// <summary> /// The 'ConcreteAggregate' class /// </summary> internal class ConcreteAggregate : Aggregate { #region Fields /// <summary> /// The _items. /// </summary> private ArrayList items = new ArrayList(); #endregion // Gets item count #region Public Properties /// <summary> /// Gets the count. /// </summary> public int Count { get { return this.items.Count; } } #endregion // Indexer #region Public Indexers /// <summary> /// The this. /// </summary> /// <param name="index"> /// The index. /// </param> /// <returns> /// The <see cref="object"/>. /// </returns> public object this[int index] { get { return this.items[index]; } set { this.items.Insert(index, value); } } #endregion #region Public Methods and Operators /// <summary> /// The create iterator. /// </summary> /// <returns> /// The <see cref="Iterator"/>. /// </returns> public override Iterator CreateIterator() { return new ConcreteIterator(this); } #endregion } /// <summary> /// The 'Iterator' abstract class /// </summary> internal abstract class Iterator { #region Public Methods and Operators /// <summary> /// The current item. /// </summary> /// <returns> /// The <see cref="object"/>. /// </returns> public abstract object CurrentItem(); /// <summary> /// The first. /// </summary> /// <returns> /// The <see cref="object"/>. /// </returns> public abstract object First(); /// <summary> /// The is done. /// </summary> /// <returns> /// The <see cref="bool"/>. /// </returns> public abstract bool IsDone(); /// <summary> /// The next. /// </summary> /// <returns> /// The <see cref="object"/>. /// </returns> public abstract object Next(); #endregion } /// <summary> /// The 'ConcreteIterator' class /// </summary> internal class ConcreteIterator : Iterator { #region Fields /// <summary> /// The _aggregate. /// </summary> private ConcreteAggregate aggregate; /// <summary> /// The _current. /// </summary> private int current; #endregion // Constructor #region Constructors and Destructors /// <summary> /// Initializes a new instance of the <see cref="ConcreteIterator"/> class. /// </summary> /// <param name="aggregate"> /// The aggregate. /// </param> public ConcreteIterator(ConcreteAggregate aggregate) { this.aggregate = aggregate; } #endregion #region Public Methods and Operators /// <summary> /// The current item. /// </summary> /// <returns> /// The <see cref="object"/>. /// </returns> public override object CurrentItem() { return this.aggregate[this.current]; } /// <summary> /// The first. /// </summary> /// <returns> /// The <see cref="object"/>. /// </returns> public override object First() { return this.aggregate[0]; } /// <summary> /// Gets whether iterations are complete. /// </summary> /// <returns> /// The <see cref="bool"/>. /// </returns> public override bool IsDone() { return this.current >= this.aggregate.Count; } /// <summary> /// The next. /// </summary> /// <returns> /// The <see cref="object"/>. /// </returns> public override object Next() { object ret = null; if (this.current < this.aggregate.Count - 1) { ret = this.aggregate[++this.current]; } return ret; } #endregion }}// Output:/*Iterating over collection:Item AItem BItem CItem D*/This real-world code demonstrates the Iterator pattern which is used to iterate over a collection of items and skip a specific number of items each iteration.
// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">// Respect the work.// </copyright>// <summary>// Real-World Iterator Design Pattern.// </summary>// --------------------------------------------------------------------------------------------------------------------namespace CSharpLearning{ using System; using System.Collections; /// <summary> /// The 'Aggregate' interface /// </summary> internal interface IAbstractCollection { #region Public Methods and Operators /// <summary> /// The create iterator. /// </summary> /// <returns> /// The <see cref="Iterator"/>. /// </returns> Iterator CreateIterator(); #endregion } /// <summary> /// The 'Iterator' interface /// </summary> internal interface IAbstractIterator { #region Public Properties /// <summary> /// Gets the current item. /// </summary> Item CurrentItem { get; } /// <summary> /// Gets a value indicating whether is done. /// </summary> bool IsDone { get; } #endregion #region Public Methods and Operators /// <summary> /// The first. /// </summary> /// <returns> /// The <see cref="Item"/>. /// </returns> Item First(); /// <summary> /// The next. /// </summary> /// <returns> /// The <see cref="Item"/>. /// </returns> Item Next(); #endregion } /// <summary> /// Startup class for Real-World Iterator Design Pattern. /// </summary> internal static class Program { #region Methods /// <summary> /// Entry point into console application. /// </summary> private static void Main() { // Build a collection var collection = new Collection(); collection[0] = new Item("Item 0"); collection[1] = new Item("Item 1"); collection[2] = new Item("Item 2"); collection[3] = new Item("Item 3"); collection[4] = new Item("Item 4"); collection[5] = new Item("Item 5"); collection[6] = new Item("Item 6"); collection[7] = new Item("Item 7"); collection[8] = new Item("Item 8"); // Create iterator var iterator = new Iterator(collection) { Step = 2 }; // Skip every other item Console.WriteLine("Iterating over collection:"); for (Item item = iterator.First(); !iterator.IsDone; item = iterator.Next()) { Console.WriteLine(item.Name); } } #endregion } /// <summary> /// A collection item /// </summary> internal class Item { #region Fields #endregion // Constructor #region Constructors and Destructors /// <summary> /// Initializes a new instance of the <see cref="Item"/> class. /// </summary> /// <param name="name"> /// The name. /// </param> public Item(string name) { this.Name = name; } #endregion // Gets name #region Public Properties /// <summary> /// Gets the name. /// </summary> public string Name { get; private set; } #endregion } /// <summary> /// The 'ConcreteAggregate' class /// </summary> internal class Collection : IAbstractCollection { #region Fields /// <summary> /// The items. /// </summary> private ArrayList items = new ArrayList(); #endregion // Gets item count #region Public Properties /// <summary> /// Gets the count. /// </summary> public int Count { get { return this.items.Count; } } #endregion // Indexer #region Public Indexers /// <summary> /// The this. /// </summary> /// <param name="index"> /// The index. /// </param> /// <returns> /// The <see cref="object"/>. /// </returns> public object this[int index] { get { return this.items[index]; } set { this.items.Add(value); } } #endregion #region Public Methods and Operators /// <summary> /// The create iterator. /// </summary> /// <returns> /// The <see cref="Iterator"/>. /// </returns> public Iterator CreateIterator() { return new Iterator(this); } #endregion } /// <summary> /// The 'ConcreteIterator' class /// </summary> internal class Iterator : IAbstractIterator { #region Fields /// <summary> /// The collection. /// </summary> private Collection collection; /// <summary> /// The current. /// </summary> private int current; /// <summary> /// The step. /// </summary> private int step = 1; #endregion // Constructor #region Constructors and Destructors /// <summary> /// Initializes a new instance of the <see cref="Iterator"/> class. /// </summary> /// <param name="collection"> /// The collection. /// </param> public Iterator(Collection collection) { this.collection = collection; } #endregion #region Public Properties /// <summary> /// Gets the current item. /// </summary> public Item CurrentItem { get { return this.collection[this.current] as Item; } } // Gets whether iteration is complete /// <summary> /// Gets a value indicating whether is done. /// </summary> public bool IsDone { get { return this.current >= this.collection.Count; } } /// <summary> /// Gets or sets the step. /// </summary> public int Step { get { return this.step; } set { this.step = value; } } #endregion #region Public Methods and Operators /// <summary> /// The first. /// </summary> /// <returns> /// The <see cref="Item"/>. /// </returns> public Item First() { this.current = 0; return this.collection[this.current] as Item; } // Gets next item /// <summary> /// The next. /// </summary> /// <returns> /// The <see cref="Item"/>. /// </returns> public Item Next() { this.current += this.step; if (!this.IsDone) { return this.collection[this.current] as Item; } return null; } #endregion }}// Output:/*Iterating over collection:Item 0Item 2Item 4Item 6Item 8*/给我老师的人工智能教程打call！http://blog.csdn.net/jiangjunshow
