Iterator PatternIterator Pattern

Dr. NealDr. Neal

CIS 480CIS 480

IteratorIterator

• 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# InterfacesMicrosoft 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 DiagramInterface Class Diagram

IEnumerable

GetEnumerator() : IEnumerator

<<Interface>>IEnumerator

Current : object

MoveNext() : boolReset() : void

<<Interface>>

Iterator Pattern ExampleIterator 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 InterfaceExample User Interface

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

Classes Classes for our for our

ExampleExampleCart

cart : string[]length : int

Cart()GetEnumerator() : IEnumerator

EnumCart

count : intlist : object[]Current : object

EnumCart(c : object[])MoveNext() : boolReset() : void

Form1

btnDumpCart : ButtonlblCartEnumeration : LabellblResults : Label

Form1()dumpCartClick(o : object, e : Eventargs) : voidDispose(b : bool) : voidInitializeComponent() : void

IEnumerable

GetEnumerator() : IEnumerator

(from .NET)

<<Interface>>IEnumerator

Current : object

MoveNext() : boolReset() : void

(from .NET)

<<Interface>>

instantiates

instantiatesuses

Cart Cart ClassClass

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 enumerationpublic IEnumerator GetEnumerator(){

return new EnumCart(cart);}

#endregion}

Implemention of

IEnumerable Interface

EnumCart EnumCart ClassClass

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 FormEvent 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 The Magic of Objects and CompilersCompilers

• 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 A foreachforeach Enumeration in C# 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";

}

ProgrammerCreated

Code

C# CompilerGenerated

Code

ResultingResultingSequence Sequence DiagramDiagram

: User : User : Form1 : Form1 : Cart : Cart : EnumCart : EnumCart

2: Cart( )

1: dumpCartClick(object, Eventargs)

3: GetEnumerator( )

4: MoveNext( )

5: Current

Complier Generated Code