design patterns. what is wrong with this picture? ball double gravity 32.1 private: enemy double...
TRANSCRIPT
Design Patterns
What is wrong with this picture?
Ball
double gravity 32.1Private:
Enemy
double gravity 32.1Private:
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 withdifferent 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 aboveThis 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: http://hillside.net
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 name Observer
Description Separates 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 7.12.
Consequences The 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 Classclass 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
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.
façadeA 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 <0,0,1>
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) differentcollision detection algorithms.
cPhysicsSlow
In the future I may want to use a super slow algorithm.
Encapsulate change
UML of Strategy DP
cPhysicsEngine cDetectCollision
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
Shapedraw() Graphics
package
DirectXadapter
OpenGL adapter
OpenGL DirectX
Circledraw()
Rectangledraw()
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
PaintProgramprocessKey
processMouse
ModeprocessKey
processMouse
EraserSprayPaint
Fill
State Design Pattern
1
supports open-closed, encapsulatechange, single responsibility principles
PaintProgramprocessKey
processMouse
ModeprocessKey
processMouse
EraserSprayPaint
Fill
State Design Pattern
1
ModeManagerprocessKey
processMouse
1 1 1
1
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
MouseKey Menu
Command Design Pattern
MVC Design PatternModel-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 cohesionand low coupling
