Iterator Pattern Dr. Neal CIS 480

Iterator An iterator pattern can be used when one class is a collection of things and would like to provide a standardized method of accessing it’s collection to another class. In Microsoft’s C# the.NET framework defines two interfaces for solving the iterator problem.

Microsoft C# Interfaces IEnumerable –Defines a method GetEnumerator() which return a class of type IEnumerator IEnumerator –Defines methods for MoveNext() and ReSet() which allow sequential movement through the collection or reset to the beginning –Defines an attribute/property Current that contains the current object in the collection

Interface Class Diagram

Iterator Pattern Example Say we have a Shopping Cart of some items, call this class “Cart”. This class must implement the “IEnumerable” interface. We will create a class called “EnumCart” which is the iterator for “Cart”. This class must implement the “IEnumerator” interface. Finally, we will create a Form to display our results using a C# foreach statement to test our iterator.

Example User Interface Our user interface will just display the items in Cart when the DumpCart button is pushed.

Classes for our Example

Cart Class public class Cart : IEnumerable { string[] cart; int length; public Cart() { length = 4; cart = new string[length]; cart[0] = "Item one"; cart[1] = "Item two"; cart[2] = "Item three"; cart[3] = "Item four"; } #region IEnumerable Members // this method returns a object which implements the // the IEnumerator interface so that it can be used // in an enumeration public IEnumerator GetEnumerator() { return new EnumCart(cart); } #endregion } Implemention of IEnumerable Interface

EnumCart Class public class EnumCart : IEnumerator { object[] list; int count; public EnumCart(object[] ol) { list = ol; count = -1; } #region IEnumerator Members public void Reset() { count = -1; } public object Current { get { return list[count]; } public bool MoveNext() { count++; if (count < list.Length) return true; else return false; } Implemention of IEnumerator Interface

Event Procedure in Form private void dumpCartClick(object sender, System.EventArgs e) { Cart c = new Cart(); foreach(string s in c) { lblResults.Text = lblResults.Text + s + "\n"; } Use foreach to iterate through the Cart

The Magic of Objects and Compilers Note that in our implementation no code never references the class “EnumCart” or calls the GetEnumerator() method in “Cart” Let’s look behind the scenes and determine what the compiler is doing to our code.

A foreach Enumeration in C# Cart c = new Cart(); foreach (string s in c) { lblResults.Text = lblResults.Text + s + "\n"; } Cart c = new Cart(); IEnumerator e = c.GetEnumerator(); while (e.MoveNext()) { string s = (string) e.Current; lblResults.Text = lblResults.Text + s + "\n"; } Programmer Created Code C# Compiler Generated Code

Resulting Sequence Diagram