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Design Patterns

Design Patterns. Composite: Shapes Organisation State Observer. Designeksempel: Composite-pattern. Composite: Grafisk editor, Tekstbehandling, Køkkenlager mmm. Hvordan ser en Show-metode ud på Shape, Circle og Picture. Har I en løsning?. abstract public class Shape{

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Design Patterns

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  1. Design Patterns Composite: Shapes Organisation State Observer

  2. Designeksempel:Composite-pattern Composite: Grafisk editor, Tekstbehandling, Køkkenlager mmm. Hvordan ser en Show-metode ud på Shape, Circle og Picture Har I en løsning?

  3. abstract public class Shape{ protected Position pos; //figurens position protected Colorcolor; //figurensfarve //øvrigeattributter public virtual void MoveTo(Position newPos){ // PRE none // POST pos'=newPos } public abstract void Show(); // Abstrakt operation // - kan ikke implementeres for en vilkårlig figur. // PRE none // POST figuren er tegnet public abstract void Hide(); // Abstrakt operation // - kan ikke implementeres for en vilkårlig figur. // PRE none // POST figuren er skjult // øvrige operationer }//end Shape

  4. public class Circle: Shape{ private int r; //radius //øvrige attributter - pos og color arves public override void Show(){ //PRE none //POST cirklen er tegnet //Denne operation kan nu implementeres for en cirkel //ved hjælp af en passende grafikrutine. } public override void Hide(){ //PRE none //POST cirklen er skjult //Denne operation kan nu implementeres for en cirkel //ved hjælp af en passende grafikrutine. } // øvrige operationer - MoveTo() arves} }//end Circle;

  5. public class Picture: Shape{ private ArrayList shapes; // operationer til at tilføje og slette figurer mv. public void override Show(){ //PRE none //POST den sammensatte figur er tegnet foreach(Shape s in shapes) s.Show(); } public void override Hide(){ //PRE none //POST den sammensatte figur er skjult foreach(Shape s in shapes) s.Hide(); } public void MoveTo(Position newPos){ //PRE none //POST pos'=newPos foreach(Shape s in shapes) s.MoveTo(newPos); } }//end Picture Statisk type Dynamisk type definerer Show() demos\Shapes

  6. (OO) Design Patterns • The concept of patterns originates from architecture (Christopher Alexander, 1977): “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice” (Christopher Alexander e. a.: “A Pattern Language”. Oxford University Press, New York, 1977.)

  7. (OO) Design Patterns • A well known and widely accepted concept in software engineering • Developed in the early 1990s and published by Gamma e.a. (“Gang of Four”, GoF) in 1995: “(…) design patterns (…) are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.” (Erich Gamma e.a.:”Design Patterns. Elements of Reusable Object-Oriented Software”. Addison-Wesley. 1995.)

  8. The benefits of patterns • A pattern captures a proven good design: • A pattern is based on experience • A pattern is discovered – not invented • It introduces a new (and higher) level of abstraction, which makes it easier: • to talk and reason about design on a higher level • to document and communicate design • One doesn’t have to reinvent solutions over and over again • Patterns facilitate reuse not only of code fragments, but of ideas.

  9. Patterns as a learning tool • It is often said that good skills in software construction require experience and talent • …and neither can be taught or learned at school • Patterns capture experience (and talent) in a way that is communicable and comprehensible • …and hence experience can be taught (and learned) • So one should put a lot of effort in studying patterns

  10. Eksempel: Organisation demos\CompositeOrgStruct

  11. The Object-Oriented Implementation: Composite Pattern 0..* AbstractTree Leaf Tree demos\RecursiveDirectoryTraverse

  12. State Pattern • Implements state machines (DFA) encapsulating state. Provides addition of new states without changing existing code. • Examples: • Dialog box for editing parameters to a program • XML • Parsing protocols • Parser/scanner in a compiler or a browser or… • Event handling in a windows system • …..

  13. State Pattern Implements the loop that gets next state and calls any operations connected to current state

  14. The Classes of the Pattern • Context:Defines the objects that we want maintain state information about (for instance DialogBox) . This class has a reference (static type: ContextState – the abstract super class) to some concrete state (that is an object of one of the sub classes – dynamic type). • ContextState:The abstract super class defining a common interface to all the concrete states. • ConcreteState1,...:The sub classes to ContextState. One sub class to each state in the DFA. The key to the design is in the processEvent-method, which takes an event as input and returns the next state.

  15. DFA defining Integers

  16. OO Implementation • State is an object • State Pattern can be applied: • abstract class State specifies one or more abstract methods: • transition(-) – returns state corresponding to input symbol • action(-) – if any processing is to be done when a transistion occurs (code generation, event handling etc.) • each concrete state inherits from State and implements the abstract methods • The parser loop uses references having the abstract class State as static type. • Polymorphism and dynamic binding handles the rest!

  17. Integer Scanner

  18. OO Parser Loop public bool Scan(string input) { //input.Length>0 bool ok = false; int i = 0; char nextChar = input[i]; State currState = s1.Transition(nextChar); while (currState != s5 && currState != s4) { i++; if (i == input.Length) nextChar = '\0'; else nextChar = input[i]; currState = currState.Transition(nextChar); } if (currState == s5) ok = true; return ok; } Let’s try input =+123 View the C# Code

  19. Observer Pattern

  20. Observer Pattern(Call-back) • A number of different objects (observers) wants to be notified when some given object (observable or subject) changes state. • Subject must allow observers to enroll dynamically, and subject shall have no knowledge about the observers. • This pattern is in sometimes known as call-back.

  21. Observer Pattern • Example: a number of objects are interested in being told when some other object is changing state. • These objects (the observers) must implement the interface IObserver. • This means implementing the method NotifyMe. • This method will be called when the observed object (subject) changes state. • Subject must maintain a collection of observers and be able to notify all objects in the collection (IObservable). • Subject calls NotifyAll, when a change in state happens View source

  22. Exercise • Add a Realist to the example. A realist will act as a pessimist, if the bottle amount is less than 50 cl. and like an optimist otherwise.

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