TDDB84 Design Patterns Lecture 09. Interpreter, Facade

Transcription

1 Lecture 09 Interpreter, Facade Peter Bunus Dept of Computer and Information Science Linköping University, Sweden Interpreter Peter Bunus 2 1

2 The Interpreter Non Software Example Musical Notation (Abstract Expression) Notes (Terminal Expression) Signatures # The Interpreter pattern defines a grammatical representation for a language and an interpreter to interpret the grammar. Musicians are examples of Interpreters. The pitch of a sound and its duration can be represented in musical notation on a staff Musicians playing the music from the score are able to reproduce the original pitch and duration of each sound represented Peter Bunus 3 The Interpreter Use the Interpreter to build and interpreter for a language Define a language grammar Each grammar rule is represented by a class Represent sentences Sentences within language can be represented by abstract syntax trees of instances of these classes Peter Bunus 4 2

3 The Interpreter How it Works We need to implement an educational tool for children to learn programming. Using our tool, each child gets to control Ugly Duckly with the help of a simple language. Here is a example of the language: right; while (daylight) fly; quack; Peter Bunus 5 The Ugly Dukly Language right; while (daylight) fly; quack; expression ::= <command> <sequence> <repetition> sequence ::= <expression> ; <expression> command ::= right quack fly repetition := while ( <variable> ) <expression> variable :=[A-Z,a-z]; We got the grammar, now all we need is a way to represent and interpret sentences in the grammar Peter Bunus 6 3

4 The Ugly Dukly Language Classes We define a class based representation for the grammar along with an interpreter to interpret the sentences expression ::= <command> <sequence> <repetition> sequence ::= <expression> ; <expression> command ::= right quack fly vepetition := while ( <variable> ) <expression> variable :=[A-Z,a-z]; Peter Bunus 7 Interpreter UML Builds abstract syntax tree representing a particular sentence in the language and invokes the interpret operation. Context Contains information that s global to the interpreter Declares an abstract Interpret operation that is common to all nodes in the abstract syntax tree. Client AbstractExpression Interpret(Context) TerminalExpression Interpret(Context) NonterminalExpression Interpret(Context) Implements an Interpret operation associated with terminal symbols in the grammar. Implements an Interpret operation for nonterminal symbols in the grammar. Peter Bunus 8 4

5 Interpreter Looks familiar? Context Client AbstractExpression Interpret(Context) TerminalExpression Interpret(Context) NonterminalExpression Interpret(Context) Peter Bunus 9 Composite (Structural Pattern) Client Component Operation() Leaf Operation() Composite Operation() Peter Bunus 10 5

6 Interpretor Advantages and Disadvantages Representing each grammar rule in a class makes the language easy to implement Because the grammar is represented by classes, you can easily change or extend the language By adding additional methods to the class structure, you can add new behaviors beyond interpretation (pretty printing) Use for scripting and programming languages Use Interpreter when you need to implement a simple language Appropriate when you have a simple grammar and simplicity is more important than efficiency It can become cumbersome when the number of grammar rule is large. In this cases a parser/compiler generator may be more appropriate. Peter Bunus 11 Facade Peter Bunus 12 6

7 Facade Non Software Example Peter Bunus 13 Joe installs a Home Theater Peter Bunus 14 7

8 Joe s Home Theater Amplifier -tuner -dvdplayer -cdplayer +on() +off() +setcd() +setdvd() +setstereosound() +setsurroundsound() +settuner() +setvolume() Tuner -amplifier +on() +off() +setam() +setfm() +setfrequency() Screen +up() +down() PoppcornPopper +on() +off() +pop() CDPlayer -amplifier +on() +off() +eject() +pause() +play() +stop() TheaterLights +on() +off() +dim() DvdPlayer -amplifier +on() +off() +eject() +pause() +play() +setsurroundaudio() +settwochannelaudio() +stop() Projector -dvdplayer +on() +off() +tvmode() +widescreenmode() Peter Bunus 15 Watching a movie Call the Waitress Turn on the popcorn popper Start the popper popping Dim the lights Put the screen down Turn the projector on Set the projector input to DVD Put the projector on wide-screen mode Turn the sound amplifier on Set the amplifier to DVD input Set the amplifier to surround sound Set the amplifier volume to medium Turn the DVD player on Start the DVD player playing Peter Bunus 16 8

9 Watching a movie The Java Way popper.on(); popper.pop(); lights.dim(10); screen.down(); projector.on(); projector.setinput(); projector.widescreenmode(); amp.on(); amp.setdvd(dvd); amp.setsurroundsound(); amp.setvolume(); dvd.on(); dvd.play(movie); When the movie is over how do you turn everything off? If Joe decides to upgrade the system he will probably going to learn a slightly different procedure? What is the procedure when you only want to listen to the Radio or a CD. Peter Bunus 17 Time for Facade HomeTheaterFacade +watchmovie() +endmovie() +listentocd() +endcd() +listentoradio() +endradio() watchmovie() Peter Bunus 18 9

10 The Home Theater Facade public class HomeTheaterFacade { Amplifier amp; Tuner tuner; DvdPlayer dvd; CdPlayer cd; Projector projector; TheaterLights lights; Screen screen; PopcornPopper popper; public HomeTheaterFacade(Amplifier amp, Tuner tuner, DvdPlayer dvd, CdPlayer cd, Projector projector, Screen screen, TheaterLights lights, PopcornPopper popper){ this.amp = amp; this.tuner = tuner; this.dvd = dvd; this.cd = cd; this.projector = projector; this.screen = screen; this.lights = lights; this.popper = popper; Peter Bunus 19 The Home Theater Facade public void watchmovie(string movie) { System.out.println("Get ready to watch a movie..."); popper.on(); popper.pop(); lights.dim(10); screen.down(); projector.on(); projector.widescreenmode(); amp.on(); amp.setdvd(dvd); amp.setsurroundsound(); amp.setvolume(5); dvd.on(); dvd.play(movie); public void endmovie() { System.out.println("Shutting movie theater down..."); popper.off(); lights.on(); screen.up(); projector.off(); amp.off(); dvd.stop(); dvd.eject(); dvd.off(); Peter Bunus 20 10

11 Time to Watch a Movie public class HomeTheaterTestDrive { public static void main(string[] args) { Amplifier amp = new Amplifier("Top-O-Line Amplifier"); Tuner tuner = new Tuner("Top-O-Line AM/FM Tuner", amp); DvdPlayer dvd = new DvdPlayer("Top-O-Line DVD Player", amp); CdPlayer cd = new CdPlayer("Top-O-Line CD Player", amp); Projector projector = new Projector("Top-O-Line Projector", dvd); TheaterLights lights = new TheaterLights("Theater Ceiling Lights"); Screen screen = new Screen("Theater Screen"); PopcornPopper popper = new PopcornPopper("Popcorn Popper"); HomeTheaterFacade hometheater = new HomeTheaterFacade(amp, tuner, dvd, cd, projector, screen, lights, popper); hometheater.watchmovie("raiders of the Lost Ark"); hometheater.endmovie(); Peter Bunus 21 The Facade Pattern The Facade Pattern provides an unified interface to set of interfaces in a subystem. Facade defines a higher level interface that make the subsytem easier to use Peter Bunus 22 11

12 Design Principle Talk only to your immediate friends Peter Bunus 23 Principle of Least Knowledge What does it mean? When you designing a system, be careful on the number of classes it interacts with and how it comes to interact with thse classes When you build a lot of dependencies between many classes you are building a fragile system: changes in one part cascade in other part What I should do? It s easy. Invoke only methods that belong to: The object itself Objects passed in as parameters to the method Any object the method creates or instantiates Any components of the object Peter Bunus 24 12

13 Without The Principle Tone, What is the outside temperature now? Well Joe, I can give a thermometer and you can measure the temperature yourself? public float gettemperature(){ Thermometer thermometer = tone.getthermometer(); return thermometer.gettemperature(); Peter Bunus 25 Using The Principle of Least Knowledge Tone, What is the outside temperature now? It is 32 degrees Celsius? When we apply the principle, we add a method to Tone that makes the request to the thermometer for Joe. This reduces the number of classes Joe is dependent on public float gettemperature(){ return = tone.gettemperature(); Peter Bunus 26 13

14 Using The Principle of Least Knowledge public class Car{ Engine engine; // other instance variables public Car(){ // initialize engine, etc public void start(key key) { Doors doors = new Doors(); boolean authorized = key.turns(); if (authorized){ engine.start(); updatedashboearddisplay(); doors.lock(); public void updatedashboarddisplay(){ // update display Here s a component of this class. We can call its methods Here we are creating a new object, it s methods are legal You can call a method on an object passed as a parameter You can call a method on a component of the object You can call a local method within the object You can call a method on an object you create or instantiate Peter Bunus 27 The Constitution of Software Architects Encapsulate what varies. Program to an interface not to an implementation. Favor Composition over Inheritance. Classes should be open extension but closed for modification Don t call us, we ll call you A Class should have only one reason to change Depend upon abstractions. Do not depend upon concrete classes. Strive for loosely coupled designs between objects that interact Only talk to your friends Peter Bunus 28 14

15 Facade Compiler example Peter Bunus 29 Mediator versus Facade Façade Façade abstracts a subsystem of objects to provide a convenient interface. Unidirectional. Enables cooperative behaviour, that colleagues don t or can t provide. Multidirectional. Façade objects make requests of the subsystem, but not viceversa. Mediator Peter Bunus 30 15