Design Patterns

What is wrong with this picture? Ball double gravity 32.1 Private: Enemy double gravity 32.1 Private: Don’t repeat yourself!

How about this? Global double gravity 3.21 We’re not using a constant because we want to support different worlds with different gravitational forces.

Why not use globals? A.They make code hard to debug. B.They make code hard to change. C.Profs O’Neill and Kuenning will haunt your dreams if you do. Answer: All of the above This is a Design Pattern....

Design Patterns A.Description of the problem. B.Essence of a solution C.Description of accumulated knowledge. D.Book: Design Patterns, by gang of 4. E.Web page devoted to software Design Patterns:

Elements of a Design Pattern Name that is meaningful Description of the problem area that explains when the pattern may be applied A solution description of the parts of the design solution, their relationships, and their responsibilities. –not a concrete design description, but a template –many times expressed graphically - UML Statement of the consequences of applying the pattern –results and trade-offs –used to indicate applicability

The Observer pattern Pattern nameObserver DescriptionSeparates the display of the state of an object from the object itself and allows alternative displays to be provided. When the object state changes, all displays are automatically notified and updated to reflect the change. Problem description In many situations, you have to provide multiple displays of state information, such as a graphical display and a tabular display. Not all of these may be known when the information is specified. All alternative presentations should support interaction and, when the state is changed, all displays must be updated. This pattern may be used in all situations where more than one display format for state information is required and where it is not necessary for the object that maintains the state information to know about the specific display formats used. Solution description This involves two abstract objects, Subject and Observer, and two concrete objects, ConcreteSubject and ConcreteObject, which inherit the attributes of the related abstract objects. The abstract objects include general operations that are applicable in all situations. The state to be displayed is maintained in ConcreteSubject, which inherits operations from Subject allowing it to add and remove Observers (each observer corresponds to a display) and to issue a notification when the state has changed. The ConcreteObserver maintains a copy of the state of ConcreteSubject and implements the Update() interface of Observer that allows these copies to be kept in step. The ConcreteObserver automatically displays the state and reflects changes whenever the state is updated. The UML model of the pattern is shown in Figure ConsequencesThe subject only knows the abstract Observer and does not know details of the concrete class. Therefore there is minimal coupling between these objects. Because of this lack of knowledge, optimizations that enhance display performance are impractical. Changes to the subject may cause a set of linked updates to observers to be generated, some of which may not be necessary.

Observer Pattern: Multiple displays

UML model of the Observer pattern

Using Design Patterns Is a design process –develop a design –experiencing a problem –recognizing that an existing pattern can be used –most difficult step is the need for a taxonomy of Design Patterns

Singleton Pattern Problem: Ensure a class has only one instance and provide a global point of access to that instance.

Singleton Gravity Class class Gravity { public: static Gravity* getInstance(); double getGravity(); private: static Gravity* theGravityInstance; Gravity() {}; ~Gravity) {}; Gravity(const Gravity& toCopy) {}; Gravity& operator=(const Gravity& toCopy) {}; }; Gravity::Gravity* theGravityInstance = NULL; users can ask for a pointer to THE instance and then get the value for gravity the constructor is private

Implementation Gravity* getInstance() { if (theGravityInstance == NULL) theGravityInstance = new Gravity; return theGravityInstance; } int getGravity() { return double(3.21) } Singletons provide the functionality of global variables without the problems

façade Design Pattern Problem: You need to use a subset of a complex system or you need to interact with the system in a particular way.

façade A facade is an object that provides a simplified interface to a larger body of code, such as a class library. A facade can make a software lib easier to use, understand and test. A facade can make code more readable because it has convenient methods for common tasks A facade can reduce dependencies of outside code on library inner workings. A facade is a wrapper

Triangle2D Triangle3D tuple3D vertices[3] triangle() triangle(tuple3D v[3] ~triangle() set color(r, g, b) rotate(vector, angle) translate(dx, dy, dz) scale(sx, sy, sz) draw() set z=0 2d rotate about origin, use 3d rotate about etc. Triangle 2D is special case of Triangle 3D

facade: Triangle2D Triangle3D tuple3D vertices[3] triangle() triangle(tuple3D v[3]) ~triangle() set color(r, g, b) rotate(vector, angle) translate(dx, dy, dz) scale(sx, sy, sz) draw() Triangle2D Triangle3D* myTriangle triangle() triangle(tuple2D v[3]) ~triangle() set color(r, g, b) rotate(angle) translate(dx, dy) scale(sx, sy) draw() MORE INTUITIVE, Hides the 3D interface.

Strategy design pattern Problem: Want to be able to swap the algorithm used in an application at runtime Solution: Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable Strategy DP lets the algorithm vary independently from the clients that use it.

cPhysicsEngine cPhysicsFast I want to support two (or more) different collision detection algorithms. cPhysicsSlow In the future I may want to use a super slow algorithm. Encapsulate change

UML of Strategy DP

cPhysicsEnginecDetectCollision cDetectCollisionFast Strategy Design Pattern cDetectCollisionSlow supports several design principles: encapsulate change open-closed principle single responsibility principles favor composition over inheritance

Bridge Pattern Problem: Want to decouple implementation from abstraction so they can change independently A teacher (Communicator object) can talk to any kid (talkable object), and a kid should also allow any teacher (Communicator object) to start talking to him/her as well. Just like how printers work with computers. If we have a USB cable (bridge), then we can connect any printer to any computer to start printing. It really doesn't matter if it's a laser printer or a color printer, either Windows PC or Mac. Because we know all the printers will allow the computers to print, makes sense?

Bridge Shape draw() Graphics package DirectX adapter OpenGL adapter OpenGLDirectX Circle draw() Rectangle draw() Decouple abstraction from implementation abstracts interface wrappers

State Design Pattern Problem: want to allow an object to alter its behavior when its internal state changes

PaintProgram processKey processMouse Mode processKey processMouse Eraser Spray Paint Fill State Design Pattern 1 supports open-closed, encapsulate change, single responsibility principles

PaintProgram processKey processMouse Mode processKey processMouse Eraser Spray Paint Fill State Design Pattern 1 ModeManager processKey processMouse Mode mgr. returns pointer to correct mode, i.e., Key or Mouse handling function

Command Design Pattern Encapsulate a request as an object to permit logging, queuing, un-doing etc. an object is used to represent and encapsulate all the info needed to call a method at a later time. Includes method name, object that owns that method and values for any parms,.

Command MouseKeyMenu Command Design Pattern

MVC Design Pattern Model-view-Controller The pattern isolates “domain logic” (application logic for the user) from the user interface permitting independent development, testing and maintenance of each (separation of concerns) user interacts with user interface, e.g., button controller converts into understandable action for the model controller tells model of user action (model state may change) view queries model to generate user interface; view gets its date from model. view may render itself or is notified by model of changes in state that require screen update

MVC supports high cohesion and low coupling

Other design patterns wikipedia books