Visitor (THIẾT kế đối TƯỢNG SLIDE)

The Visitor Pattern The Problem 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 AmazoneNote title author isbn price GoogleNote heading excerpt url NewsNote heading byLine excerpt RssNote heading text date Commonality is in the eye of the beholder Note AmazoneNote GoogleNote title author isbn price heading excerpt url NewsNote heading byLine excerpt RssNote heading text date Need operations in Note structure • With a little casual application of polymorphism Note printHeading() printDetails() asPrintable() asEmailContent() asSMS() Does this look bad to you? … AmazoneNote GoogleNote NewsNote RssNote 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 Structure Visitor Client visitConcreteElement1(e : ConcreteElement1) visitConcreteElement2(e : ConcreteElement2) ConcreteVisistor1 ConcreteVisistor2 visitConcreteElement1(e : ConcreteElement1) visitConcreteElement2(e : ConcreteElement2) visitConcreteElement1(e : ConcreteElement1) visitConcreteElement2(e : ConcreteElement2) ObjectStructure Element accept(v : Visitor) visitor.visitConcreteElement1(this); ConcreteElement1 ConcreteElement2 accept(visitor : Visitor) accept(visitor : Visitor) A solution to a problem in a context Note accept(v : NodeVisitor) AmazoneNote GoogleNote NewsNote RssNote NoteVisitor + visitAmazoneNote() + visitGoogleNote() + visitNewsNote() + visitRssNote() 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); } 10 AmazoneNote public class AmazoneNote implements Note { String title; String author; String isbn; double price; 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); } } 11 GoogleNote public class GoogleNote implements Note { String heading; String excerpt; String url; public GoogleNote(String heading, String excerpt, String url) { this.heading = heading; this.excerpt = excerpt; this.url = url; } public void accept(NoteVisitor visitor) { visitor.visitGoogleNote(this); } } 12 NewsNote public class NewsNote implements Note { String heading; String byLine; String excerpt; public NewsNote(String heading, String byLine, String excerpt) { this.heading = heading; this.byLine = byLine; this.excerpt = excerpt; } public void accept(NoteVisitor visitor) { visitor.visitNewsNote(this); } } 13 RssNote public class RssNote implements Note { String heading; String text; Date date; public RssNote(String heading, String text) { this.heading = heading; this.text = text; this.date = new Date(System.currentTimeMillis()); } public void accept(NoteVisitor visitor) { visitor.visitRssNote(this); } } 14 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); } } 15 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); } 16 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); } 17 NoteIterator public class NoteIterator { private List sampleNotes() { List notes = new ArrayList(); 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 developed ", "http://www.redhillconsulting.com.au/rails_plugins.html")); notes.add(new AmazoneNote("The Wolves in the Wall", "Nail Gaiman", "ISBN XXX-XXXX-XXXXX", 12.06)); notes.add(new RssNote("let's talk about tests, baby ", "Some long babbling about test naming heuristics")); return notes; } public void visitAllUsing(NoteVisitor visitor) { for (Note note : sampleNotes()) { note.accept(visitor); 18 Test Drive public class NoteTestDrive { public static void main(String[] args) { NoteIterator iterator = new NoteIterator(); iterator.visitAllUsing(new NoteHeadingPrinter()); } } iterator.visitAllUsing(new NoteDetailsPrinter()); 19 Output Melbourne Pushes Boundaries The Wolves in the Wall Redhill Consulting Pty Ltd - Ruby on Rails Plugins Output of NoteHeadingPrinter 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 Output of NoteDetailsPrinter 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 20 When? • “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” 21 When? • “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 Consequences • 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 23 File Folder Examples FileSystemElement name : String getName() getElement(name) createIterator() accept(v : Vistor) File size : int Folder children : ArrayList getElement() createIterator() accept() addElement(FileSystemElement) removeElement(FileSystemElement) getElement(name) createIterator() accept(v : Visitor) Visitor visitFile(file : File) visitFolder(folder : Folder) PrintVisitor CountFileVisitor visitFile(file : File) visitFolder(folder : Folder) visitFile(file : File) visitFolder(folder : Folder) 24 Shape Example Shape Client location : CartesianPoint accept() Dot Circle Square radius : int size : int Rectangle heigth : int width : int ShapeVisitor forDot() forCircle() forSquare() forRectanle() Area IsContain point : CartesianPoint BoundingBox 25 ... : Visitor) visitor. visitConcreteElement1(this); ConcreteElement1 ConcreteElement2 accept (visitor : Visitor) accept (visitor : Visitor) A solution to a problem in a context Note accept(v : NodeVisitor)... Date(System.currentTimeMillis()); } public void accept(NoteVisitor visitor) { visitor. visitRssNote(this); } } 14 NoteHeadingPrinter visitor public class NoteHeadingPrinter implements NoteVisitor { public void visitAmazoneNote(AmazoneNote... • 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