Decorator Pattern. CS356 Object-Oriented Design and Programming November 7, 2014

Transcription

1 Decorator Pattern CS356 Object-Oriented Design and Programming November 7, 2014 Yu Sun, Ph.D.

2 Decorator Intent Dynamically attach additional responsibilities to an object Provide a flexible alternative to subclassing (static) Decorating object is transparent to the core component Also Known As Wrapper

3 Motivation We want to add different kinds of borders and/or scrollbars to a TextView GUI component Borders Plain, 3D, or Fancy Scrollbars Horizontal and/or Vertical

4 Motivation An inheritance solution requires 15 subclasses to represent each type of view 1. TextView-Plain 2. TextView-3D 3. TextView-Fancy 4. TextView-Horizontal 5. TextView-Vertical 6. TextView-Horizontal-Vertical 7. TextView-Plain-Horizontal 8. TextView-Plain-Vertical 9. TextView-Plain-Horizontal-Vertical 10. TextView-3D-Horizontal 11. TextView-3D-Vertical 12. TextView-3D-Horizontal-Vertical 13. TextView-Fancy-Horizontal 14. TextView-Fancy-Vertical 15. TextView-Fancy-Horizontal-Vertical

5 Solution 1: Use Object Composition TextView border scrollbar Border Scrollbar PlainBorder 3DBorder FancyBorder Horizontal Scrollbar Vertical Scrollbar Horizontal Vertical Scrollbar Is it Open-Closed? Can you add new features without affecting TextView? e.g., what about adding sound to a TextView?

6 Decorator Pattern Solution VisualComponent TextView VisualDecorator component Border Scrollbar PlainBorder 3DBorder FancyBorder Change the Skin, not the Guts! Horizontal Scrollbar TextView has no borders or scrollbars! Vertical Scrollbar Horizontal Vertical Scrollbar Add borders and scrollbars on top of a TextView

7 Structure What is significance of this? Component ConcreteComponent Decorator component Component->; ConcreteDecoratorA addedstate ConcreteDecoratorB AddedBehavior() Decorator::; AddBehavior(); The decorator forwards requests to the component and may perform additional actions (such as drawing a border) before or after any forwarding

8 Component u Defines the interface for objects that can have responsibilities added dynamically Component ConcreteComponent Decorator component Component->; ConcreteDecoratorA addedstate ConcreteDecoratorB AddedBehavior() Decorator::; AddBehavior();

9 ConcreteComponent u The "base" object to which additional responsibilities can be added Component ConcreteComponent Decorator component Component->; ConcreteDecoratorA addedstate ConcreteDecoratorB AddedBehavior() Decorator::; AddBehavior();

10 Decorator u Maintains a reference to a Component object u Defines an interface conformant to Component's interface Component ConcreteComponent Decorator component Component->; ConcreteDecoratorA addedstate ConcreteDecoratorB AddedBehavior() Decorator::; AddBehavior();

11 ConcreteDecorator u Adds responsibilities to the component Component ConcreteComponent Decorator component Component->; ConcreteDecoratorA addedstate ConcreteDecoratorB AddedBehavior() Decorator::; AddBehavior();

12 Decorator Applicability Dynamically and transparently attach responsibilities to objects Responsibilities that can be withdrawn Extension by subclassing is impractical May lead to too many subclasses

13 Example Sales Ticket Printing

14 Example: Decorate Sales Ticket Printing Assume the SalesTicket currently creates an html sales receipt for an Airline Ticket New Requirements Add header with company name Add footer that is an advertisement During the holidays add holiday relevant header(s) and footer(s) We re not sure how many such things One solution Place control in SalesTicket Then you need flags to control what header(s) get printed

15 Decorator Approach A layered approach Start chain with decorators End with original object Decorator 1 Decorator 2 Concrete Component

16 Example Sales Ticket Printing Component SalesOrder main(string[]) SalesTicket component TicketDecorator TicketDecorator(Component) Configuration getsalesticket() HeaderDecorator1 HeaderDecorator1(Component) printheader() FooterDecorator1 FooterDecorator1(Component) printfooter() HeaderDecorator2 HeaderDecorator2(Component) printheader() FooterDecorator2 FooterDecorator2(Component) printfooter()

17 A SalesTicket Implementation Component SalesOrder main(string[]) SalesTicket HeaderDecorator1 HeaderDecorator1(Component) printheader() // Instances TicketDecorator of this class are the sales tickets // that may be decorated TicketDecorator(Component) Configuration public class SalesTicket extends Component { getsalesticket() public component void { // Hard coded here, but simpler than // adding a new Customer class now System.out.println("Customer: Bob"); FooterDecorator1 System.out.println("The sales ticket itself"); System.out.println("Total: FooterDecorator1(Component) $123.45"); printfooter() HeaderDecorator2 HeaderDecorator2(Component) printheader() FooterDecorator2 FooterDecorator2(Component) printfooter()

18 TicketDecorator Component SalesOrder main(string[]) SalesTicket HeaderDecorator1 HeaderDecorator1(Component) printheader() TicketDecorator TicketDecorator(Component) Configuration getsalesticket() component public abstract class TicketDecorator extends Component { private Component component; FooterDecorator1 public TicketDecorator(Component FooterDecorator1(Component) c) { component = c; printfooter() HeaderDecorator2 public void FooterDecorator2 { if(component!= null) HeaderDecorator2(Component) FooterDecorator2(Component) component.; printheader() printfooter()

19 A Header Decorator Component public class HeaderDecorator1 extends TicketDecorator { SalesOrder public HeaderDecorator1(Component c) { super(c); main(string[]) SalesTicket TicketDecorator public void { this.printheader(); TicketDecorator(Component) super.; component Configuration getsalesticket() HeaderDecorator1 HeaderDecorator1(Component) printheader() public void printheader() { System.out.println("@@ Header FooterDecorator1 FooterDecorator1(Component) printfooter() HeaderDecorator2 HeaderDecorator2(Component) printheader() FooterDecorator2 FooterDecorator2(Component) printfooter()

20 Example Sales Ticket Printing Component public class FooterDecorator2 extends TicketDecorator { public FooterDecorator2(Component c) { super(c); SalesTicket TicketDecorator public void { super.; this.printfooter(); TicketDecorator(Component) component SalesOrder main(string[]) Configuration getsalesticket() public void printfooter() { System.out.println("## FOOTER HeaderDecorator1 FooterDecorator1 Two ##"); HeaderDecorator1(Component) FooterDecorator1(Component) printheader() printfooter() HeaderDecorator2 HeaderDecorator2(Component) printheader() FooterDecorator2 FooterDecorator2(Component) printfooter()

21 SalesOrder (Client) Component public class SalesOrder { public static void main(string[] args) { SalesOrder SalesTicket s = new SalesOrder(); TicketDecorator s.; TicketDecorator(Component) public void component { // Get an object decorated dynamically Component myst = Configuration.getSalesTicket(); myst.; HeaderDecorator1 FooterDecorator1 HeaderDecorator1(Component) // calcsalestax... printheader() FooterDecorator1(Component) printfooter() SalesOrder main(string[]) Configuration getsalesticket() HeaderDecorator2 HeaderDecorator2(Component) printheader() FooterDecorator2 FooterDecorator2(Component) printfooter()

22 Example Configuration Component // This object will determine how to decorate the // SalesTicket. This could become a Factory public class Configuration { public static Component getsalesticket() { // SalesTicket Return a decorated SalesTicket TicketDecorator return TicketDecorator(Component) new HeaderDecorator1( new HeaderDecorator2( new FooterDecorator1( component new FooterDecorator2( new SalesTicket() )))); HeaderDecorator1 FooterDecorator1 HeaderDecorator1(Component) printheader() FooterDecorator1(Component) printfooter() SalesOrder main(string[]) Configuration getsalesticket() HeaderDecorator2 HeaderDecorator2(Component) printheader() FooterDecorator2 FooterDecorator2(Component) printfooter()

23 Output with Current Configuration Output: Header One >> Header Two << Customer: Bob The sales ticket itself Total: $ %% FOOTER One %% ## FOOTER Two ##

24 Implementation Issues Keep Decorators lightweight Don't put data members in Component Use it for shaping the interface Omitting the abstract Decorator class If only one decoration is needed Subclasses may pay for what they don't need

25 Return to TextView Example The TextView class knows nothing about Borders and Scrollbars VisualComponent public class TextView { TextView public void draw() { VisualDecorator // Code to draw this Text object component Border public void resize () { // Code to resize this Text object Scrollbar PlainBorder 3DBorder FancyBorder Horizontal Scrollbar Vertical Scrollbar Horizontal Vertical Scrollbar

26 A New Class The new ImageView class knows nothing about Borders and Scrollbars public class ImageView { public void draw() { // Code to draw this Image Object public void resize () { // Code to resize this Image Object

27 Decorators Contain Components The decorators don t need to know about components public class FancyBorder extends Border { VisualComponent public FancyBorder(VisualComponent c) { super(c); TextView public VisualDecorator void draw() { // forward draw message component component.draw(); Border // Code to draw this FancyBorder object Scrollbar PlainBorder 3DBorder FancyBorder Horizontal Scrollbar Vertical Scrollbar Horizontal Vertical Scrollbar

28 How to Use Decorators public class Client { public static void main(string[] args) { TextView data = new TextView(); Component borderdata = new FancyBorder(data); Component scrolleddata = new VertScrollbar(data); Component borderandscrolleddata = new HorzScrollbar(borderData);

29 Decorator Pattern in Java

30 Decorator Pattern in Java public class JavaIO { public static void main(string[] args) { // Open an InputStream. FileInputStream in = new FileInputStream("test.dat"); // Create a buffered InputStream. BufferedInputStream bin = new BufferedInputStream(in); // Create a buffered, data InputStream. DataInputStream dbin = new DataInputStream(bin); // Create a buffered, pushback, data InputStream. PushbackInputStream pbdbin = new PushbackInputStream(dbin); BufferedReader keyboard = new BufferedReader( new InputStreamReader(System.in));

31 Java Streams With > 60 streams in Java, you can create a wide variety of input and output streams This provides flexibility (good) It also adds complexity (bad) Flexibility made possible with inheritance and classes that accept many different classes that extend the parameter You can have an InputStream instance or any instance of a class that extends InputStream public InputStreamReader(InputStream in)

32 Consequences Ë Transparency very good Ë More flexibility than static inheritance Allows to mix and match responsibilities Allows to apply a property twice Ë Avoid feature-laden classes high-up in the hierarchy Pay-as-you-go approach Easy to define new types of decorations Ò A decorator and its component aren't identical Ò Lots of little objects Easy to customize, but hard to learn and debug