Visitor (THIẾT kế đối TƯỢNG SLIDE)

public interface Note { void acceptNoteVisitor v; } public interface NoteVisitor { void visitAmazoneNoteAmazoneNote note; void visitGoogleNoteGoogleNote note; void visitNewsNoteNewsN

The Visitor Pattern

The Problem

Show me everything in a consistent manner …

… and send it to my friend via email

oh, and send it to her phone as well

and print it …

Some analysis music please

NewsNote

headingbyLineexcerpt

RssNote

headingtext

date

Commonality is in the eye of the beholder

NewsNote

headingbyLineexcerpt

RssNote

headingtext

date

Note

Need do operations in Note structure

• With a little casual application of polymorphism

6

Note

printHeading() printDetails() asPrintable() asEmailContent() asSMS()

…

Does this look bad to you?

The solution

Visitor Pattern

• "Represent an operation to be performed on the elements of an object structure Visitor lets you define a new operation without changing the

classes of the elements on which it operates"

GOF

ConcreteVisistor1

visitConcreteElement1(e : ConcreteElement1) visitConcreteElement2(e : ConcreteElement2)

ConcreteVisistor2

visitConcreteElement1(e : ConcreteElement1) visitConcreteElement2(e : ConcreteElement2)

Client

accept(v : Visitor)

visitor.visitConcreteElement1(this);

A solution to a problem in a context

AmazoneNote GoogleNote NewsNote RssNote

Note

accept(v : NodeVisitor)

NoteVisitor

+ visitAmazoneNote() + visitGoogleNote() + visitNewsNote() + visitRssNote()

<<Interface>>

NoteHeadingPrinter NoteDetailsPrinter

Let’s show it in code!

public interface Note {

void accept(NoteVisitor v);

}

public interface NoteVisitor {

void visitAmazoneNote(AmazoneNote note);

void visitGoogleNote(GoogleNote note);

void visitNewsNote(NewsNote note);

void visitRssNote(RssNote note);

}

public AmazoneNote(String title, String author,

String isbn, double price) {

this.title = title;

this.author = author;

this.isbn = isbn;

this.price = price;

}

public void accept(NoteVisitor visitor) {

visitor.visitAmazoneNote(this);

}

this.heading = heading;

this.excerpt = excerpt;

this.url = url;

this.heading = heading;

this.byLine = byLine;

this.excerpt = excerpt;

public RssNote(String heading, String text) {

this.heading = heading;

this.text = text;

this.date = new Date(System.currentTimeMillis());

NoteHeadingPrinter visitor

public class NoteHeadingPrinter implements NoteVisitor {

public void visitAmazoneNote(AmazoneNote note) {

System.out.println(note.title);

}

public void visitGoogleNote(GoogleNote note) {

System.out.println(note.heading);

}

public void visitNewsNote(NewsNote note) {

System.out.println(note.heading);

}

public void visitRssNote(RssNote note) {

System.out.println(note.heading);

}

}

public class NoteDetailsPrinter implements NoteVisitor { public void visitAmazoneNote(AmazoneNote note) {

System.out.println(note.title);

System.out.println(note.price);

}

public void visitGoogleNote(GoogleNote note) {

System.out.println(note.heading);

System.out.println(note.excerpt);

}

public void visitNewsNote(NewsNote note) {

System.out.println(note.heading);

System.out.println(note.byLine);

System.out.println(note.excerpt);

}

public void visitRssNote(RssNote note) {

System.out.println(note.heading);

System.out.println(note.text);

}

}

NoteDetailsPrinter visitor

public class NoteDetailsPrinter implements NoteVisitor { public void visitAmazoneNote(AmazoneNote note) {

System.out.println(note.title);

System.out.println(note.price);

}

public void visitGoogleNote(GoogleNote note) {

System.out.println(note.heading);

System.out.println(note.excerpt);

}

public void visitNewsNote(NewsNote note) {

System.out.println(note.heading);

System.out.println(note.byLine);

System.out.println(note.excerpt);

}

public void visitRssNote(RssNote note) {

System.out.println(note.heading);

System.out.println(note.text);

NoteIterator

private List<Note> sampleNotes() {

List<Note> notes = new ArrayList<Note>();

notes add( new NewsNote( "Melbourne Pushes Boundaries" , "Tim Colebatch" ,

"Melbourne is experiencing its biggest growth surge since 1960's" ));

notes add( new GoogleNote(

"Redhill Consulting Pty Ltd - Ruby on Rails plugins" , "Here are some Ruby on Rails plugins we're

notes add( new RssNote( "let's talk about tests, baby " ,

"Some long babbling about test naming heuristics" )); return notes ;

}

for (Note note : sampleNotes()) {

note accept( visitor );

System.out println();

}

}

}

Test Drive

public class NoteTestDrive {

public static void main(String[] args) {

NoteIterator iterator = new NoteIterator();

Output

Melbourne Pushes Boundaries

The Wolves in the Wall

Redhill Consulting Pty Ltd - Ruby on Rails Plugins

Let's talk about tests, baby

Melbourne Pushes Boundaries

Tim Colebatch

Melbourne is experiencing its biggest growth surge since the 1960's

The Wolves in the Wall

12.06

Redhill Consulting Pty Ltd - Ruby on Rails Plugins

Here are some Ruby on Rails plugins we've developed

Let's talk about tests, baby

Some long babbling about test naming heuristics

Output of

NoteHeadingPrinter

Output of NoteDetailsPrinter

• “An object structure contains many classes of objects

with differing interfaces, and you want to perform

operations on these objects that depend on their

concrete classes”

• “Many distinct and unrelated operations need to be

performed on objects in an object structure, and you

want to avoid ‘polluting’ their classes with these

operations Visitor lets you keep unrelated operations

together by defining them in one class When the object structure is shared by many applications, use Visitor to put operations in just those applications that need them”

• “The classes defining the object structure rarely change, but you often want to define new

operations over the structure Changing the

object structure classes requires redefining the interface to all visitors, which is potentially costly

If the object structure classes change often, then it’s probably better to define the operations in

those classes”

22

• Visitor makes adding new operations easy

• A visitor gathers related operations and

separates unrelated ones

• Adding new ConcreteElement classes is hard

• Visitors can cross object hierarchies

• Visitors can accumulate state

• Visitors may compromise encapsulation

File Folder Examples

Folder children : ArrayList

addElement(FileSystemElement) removeElement(FileSystemElement) getElement(name)

createIterator() accept(v : Visitor)

Visitor

visitFile(file : File) visitFolder(folder : Folder)

PrintVisitor

visitFile(file : File) visitFolder(folder : Folder)

CountFileVisitor visitFile(file : File) visitFolder(folder : Folder)

Shape Example

Dot Circle

radius : int

Square size : int

Rectangle heigth : int width : int

Area IsContain

point : CartesianPoint BoundingBox

Shape location : CartesianPoint accept()

Client

ShapeVisitor

forDot() forCircle() forSquare() forRectanle()

<<Interface>>