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
