October 30, 2001
Aditya P. Mathur
Purdue University

Patterns:
Behavioral:
Observer
Structural:
Façade
Creational
Abstract Factory
Factory Method
Singleton
Excellent reference:
Design Patterns book by Erich Gamma, et al., Addison-Wesley, 1994.
CS 406: Design Patterns 2

• A solution to a problem that occurs repeatedly in a variety of contexts.
• Each pattern has a name .
• Use of each pattern has consequences .
CS 406: Design Patterns 3

• Generally at a “higher level” of abstraction.
• Not about designs such as linked lists or hash tables.
• Generally descriptions of communicating objects and classes .
CS 406: Design Patterns 4

• Need to separate presentational aspects with the data, i.e. separate views and data.
• Classes defining application data and presentation can be reused .
• Change in one view automatically reflected in other views. Also, change in the application data is reflected in all views.
• Defines one-to-many dependency amongst objects so that when one object changes its state, all its dependents are notified.
CS 406: Design Patterns 5

Relative Percentages
X
Y
Z
A B C D
15 35 35 15
10 40 30 20
10 40 30 20
A B C D
Change notification
Requests, modifications
A=10%
B=40%
C=30%
D=20%
CS 406: Design Patterns
D
A
B
C
Application data
6

observers
Subject attach (Observer) detach (Observer)
Notify ()
For all x in observers{ x

Update(); }
Observer
Update()
Concrete Subject
GetState()
SetState() subjectState subject
Concrete Observer
Update()
CS 406: Design Patterns observerState observerState= subject
 getState();
7

: ConcreteSubject :ConcreteObserver-1 :ConcreteObserver-2
Notify()
Update()
SetState()
GetState()
Update()
GetState()
CS 406: Design Patterns 8

class Subject; class observer { public: virtual ~observer;
Abstract class defining the Observer interface.
virtual void Update (Subject* theChangedSubject)=0; protected: observer ();
};
Note the support for multiple subjects.
CS 406: Design Patterns 9

class Subject {
Abstract class defining the Subject interface.
public: virtual ~Subject; virtual void Attach (observer*); virtual void Detach (observer*) ; virtual void Notify(); protected:
Subject (); private:
List <Observer*> *_observers;
};
CS 406: Design Patterns 10

}
} void Subject :: Attach (Observer* o){
_observers -> Append(o); void Subject :: Detach (Observer* o){
_observers -> Remove(o);
} void Subject :: Notify (){
ListIterator<Observer*> iter(_observers); for ( iter.First(); !iter.IsDone(); iter.Next()) { iter.CurrentItem() -> Update(this);
}
CS 406: Design Patterns 11

} class ClockTimer : public Subject { public:
ClockTimer(); virtual int GetHour(); virtual int GetMinutes(); virtual int GetSecond(); void Tick ();
CS 406: Design Patterns 12

ClockTimer :: Tick {
// Update internal time keeping state.
// gets called on regular intervals by an internal timer.
Notify();
}
CS 406: Design Patterns 13

class DigitalClock: public Widget, public Observer { public:
DigitalClock(ClockTimer*); virtual ~DigitalClock();
Override Observer operation.
virtual void Update(Subject*); virtual void Draw();
Override Widget operation.
private:
ClockTimer* _subject;
}
CS 406: Design Patterns 14

}
}
DigitalClock ::DigitalClock (ClockTimer* s) {
_subject = s;
_subject

Attach(this);
DigitalClock ::~DigitalClock() {
_subject->Detach(this);
CS 406: Design Patterns 15

} void DigitalClock ::Update (subject* theChangedSubject ) {
If (theChangedSubject == _subject) {
Draw();
}
Check if this is the clock’s subject.
void DigitalClock ::Draw () { int hour = _subject->GetHour(); int minute = _subject->GeMinute(); // etc.
}
// Code for drawing the digital clock.
CS 406: Design Patterns 16

ClockTimer* timer = new ClockTimer;
DigitalClock* digitalClock = new DigitalClock (timer);
CS 406: Design Patterns 17

• When an abstraction has two aspects : one dependent on the other. Encapsulating these aspects in separate objects allows one to vary and reuse them independently.
• When a change to one object requires changing others and the number of objects to be changed is not known .
• When an object should be able to notify others without knowing who they are. Avoid tight coupling between objects.
CS 406: Design Patterns 18

• Abstract coupling between subject and observer.
Subject has no knowledge of concrete observer classes.
(What design principle is used?)
• Support for broadcast communication . A subject need not specify the receivers; all interested objects receive the notification.
• Unexpected updates : Observers need not be concerned about when then updates are to occur. They are not concerned about each other’s presence. In some cases this may lead to unwanted updates.
CS 406: Design Patterns 19

Client Classes
Need to communicate with
Subsystem classes
CS 406: Design Patterns 20

Client Classes
Fac ade
CS 406: Design Patterns
Subsystem classes
21

• Subsystems often get complex as they evolve.
• Need to provide a simple interface to many, often small, classes. But not necessarily to ALL classes of the subsystem.
• Façade provides a simple default view good enough for most clients.
• Facade decouples a subsystem from its clients.
• A façade can be a single entry point to each subsystem level. This allows layering.
CS 406: Design Patterns 22

• Participants: Façade and subsystem classes
• Clients communicate with subsystem classes by sending requests to façade.
• Façade forwards requests to the appropriate subsystem classes.
• Clients do not have direct access to subsystem classes.
CS 406: Design Patterns 23

• Shields clients from subsystem classes; reduces the number of objects that clients deal with.
• Promotes weak coupling between subsystem and its clients.
• Helps in layering the system. Helps eliminate circular dependencies.
CS 406: Design Patterns 24

Stream
Compiler
Compile()
Scanner Token
BytecodeStream
Parser Symbol
CodeGenerator
PnodeBuilder Pnode
Invocations
RISCCodegenerator
StatementNode
StackMachineCodegenerator
ExpressionNode
CS 406: Design Patterns 25

class
Scanner { // Takes a stream of characters and produces a stream of tokens.
public:
Private:
Scanner (istream&); virtual Scanner(); virtual Token& Scan(); istream& _inputStream;
};
CS 406: Design Patterns 26

class parser { // Builds a parse tree from tokens using the PNodeBuilder.
public:
Parser (); virtual ~Parser() virtual void Parse (Scanner&, PNodeBuilder&);
};
CS 406: Design Patterns 27

class
Pnodebuilder {
// Builds a parse tree incrementally. Parse tree
// consists of Pnode objects.
public:
Pnodebuilder ();
// Node for a variable.
virtual Pnode* NewVariable (
) const;
Char* variableName
Private: virtual Pnode* NewAssignment (
// Node for an assignment.
) const;
Pnode* variable, Pnode* expression
// Similarly...more nodes.
Pnode* _node;
}; CS 406: Design Patterns 28

class
Pnode {
// An interface to manipulate the program node and its children.
public:
// Manipulate program node.
virtual void GetSourcePosition (int& line, int& index);
// Manipulate child node.
virtual void Add (Pnode*); virtual void Remove (Pnode*);
// ….
virtual void traverse (Codegenerator&); protected:
PNode();
};
// Traverse tree to generate code.
CS 406: Design Patterns 29

class
CodeGenerator { // Generate bytecode.
public:
// Manipulate program node.
virtual void Visit (StatementNode*); virtual void Visit (ExpressionNode*);
// ….
Protected:
CodeGenerator (BytecodeStream&);
BytecodeStream& _output;
};
CS 406: Design Patterns 30

void ExpressionNode::Traverse (CodeGenerator& cg) { cg.Visit (this);
};
ListIterator<Pnode*> i(_children);
For (i.First(); !i.IsDone(); i.Next();{ i.CurrentItem()

Traverse(cg);
};
CS 406: Design Patterns 31

class public:
Compiler {
Compiler ();
// Façade. Offers a simple interface to compile and
// Generate code.
Could also take a CodeGenerator
Parameter for increased generality.
virtual void Compile (istream&, BytecodeStream&);
} void Compiler:: Compile (istream& input, BytecodeStream& output) {
Scanner scanner (input);
PnodeBuilder builder;
Parser parser; parser.Parse (scanner, builder);
RISCCodeGenerator generator (output);
Pnode* parseTree = builder.GetRootNode(); parseTree  Traverse (generator);
}
CS 406: Design Patterns 32

• Assume that rules are desired to invalidate an action:
– Suppose that when a new Sale is created, it will be paid by a gift certificate
– Only one item can be purchased using a gift certificate.
– Hence, subsequent enterItem operations must be invalidated in some cases. (Which ones?)
How does a designer factor out the handling of such rules?
CS 406: Design Patterns 33

• Define a “rule engine” subsystem (e.g.
POSRuleEngineFacade ).
• It evaluates a set of rules against an operation and indicates if the rule has invalidated an operation.
• Calls to this façade are placed near the start of the methods that need to be validated.
– Example: Invoke the façade to check if a new salesLineItem created by makeLineItem is valid or not. (See page 370 of
Larman.)
CS 406: Design Patterns 34

Main body of an
Application
Calls a procedure or
A method
Reuse the main body of an
Application and write the code it calls
Defines the architecture
Of the application
Toolkit Framework
Toolkits: Collection of related and reusable classes e.g. C++ I/O stream library
Framework: A set of cooperating classes that make up a reusable design for a specific class of applications e.g. drawing, compilers,
CAD/CAM etc.
CS 406: Design Patterns 35

Advantages and disadvantages of using frameworks .
1. When using frameworks, what defines the architecture of the application?
2. What is more difficult to design: Application, toolkit, or frameworks?
3. How do changes in framework effect an application?
4. How do design patterns differ from frameworks?
CS 406: Design Patterns 36

1. Consider a user interface toolkit to support multiple lookand-feel standards.
2. For portability an application must not hard code its widgets for one look and feel.
How to design the application so that incorporating new look and feel requirements will be easy?
CS 406: Design Patterns 37

1.
Define an abstract WidgetFactory class.
2.
This class declares an interface to create different kinds of widgets.
3.
There is one abstract class for each kind of widget and concrete subclasses implement widgets for different standards.
4.
WidgetFactory offers an operation to return a new widget object for each abstract widget class. Clients call these operations to obtain instances of widgets without being aware of the concrete classes they use.
CS 406: Design Patterns 38

WidgetFactory
CreateScrollbar()
CreateWindow()
Window
Client
MacWindow WWindow
ScrollBar
WWidgetFactory
MacWidgetFactory
One for each standard.
MacScrollBar
CS 406: Design Patterns
WScrollBar
39

AbstractFactory
CreateScrollbar()
CreateWindow()
AbstractProductA
Client
ProductA2 ProductA1
AbstractProductB
ConcreteFactory1
ConcreteFactory2
CreateProductA()
CreateProductB()
ProductB2
CS 406: Design Patterns
ProductB1
40

• AbstractFactory : Declares the interface for operations to create abstract product objects
• ConcreteFactory : Implements the operations to create concrete product objects.
• AbstractProduct: Declares an interface for a type of product object.
• ConcreteProduct: Defines a product object to be created by the corresponding factory.
• Client: Uses only the interface declared by the abstractFactory and
AbstractProduct classes.
CS 406: Design Patterns 41

class
MazeFactory {
// Creates components of mazes.
// Builds rooms, walls, and doors.
public:
MazeFactory(); virtual Maze* MakeMaze() const
{ return new Maze;} virtual Wall* MakeWall() const
{ return new Wall;}
// This factory is a collection of
// factory methods. Also, this class
// acts both as Abstract and Concrete
// Factory
} virtual Wall* MakeRoom(int n) const
{ return new Room;}
// more methods.
CS 406: Design Patterns 42

Maze* MazeGame:: CreateMaze (MazeFactory& factory)
// Builds a maze.
}
Maze* aMaze = factory.MakeMaze();
Room* myroom = factory.MakeRoom(1);
Room* herroom = factory.MakeRoom(2);
Door* aDoor = factory.MakeDoor(myRoom,herRoom) aMaze  AddRoom(myRoom) aMaze  AddRoom(herRoom)
// More code to add walls.
CS 406: Design Patterns
// One can also create a
// BombedMazeFactory with
// different types of Rooms
// and Walls .
43

1. Frameworks use abstract classes to define and maintain relationships between objects
2. Consider a framework for applications that present multiple documents to the user. A drawing application is an example.
3. This framework defines two abstract classes: application and document. These ought to be sub classed by clients for application specific implementation.
4. The application class will create and manage documents when required, e.g. when a New command is selected from the menu.
CS 406: Design Patterns 44

5. Document sub class is application specific . Hence the
Application class does not know what kind of document to create!
6.
Problem : The framework must instantiate classes but it only knows about the abstract classes, which it cannot initiate!
CS 406: Design Patterns 45

1.
The Factory Method pattern encapsulates the knowledge of which Document subclass to create and moves this knowledge out of the framework.
2.
Application subclasses redefine an abstract CreateDoc() method to return the appropriate Document subclass.
3.
When an Application is instantiated, it can instantiate application specific Documents without knowing their class.
CS 406: Design Patterns 46

Document
Open()
Close()
Save()
* docs
1
Application
Factory method
CreateDoc()
NewDoc()
OpenDoc()
MyDocument
MyApplication
CreateDoc()
Document* doc=CreateDoc(); docs.Add(doc); doc

Open();
CS 406: Design Patterns 47

Product
Creator
FactoryMethod()
SomeOperation() product=Factory method
ConcreteProduct
ConcreteCreator
FactoryMethod()
Return new ConcreteProduct
CS 406: Design Patterns 48

• Product (Document): Defines the interface of objects the factory method creates.
• ConcreteProduct (MyDocument): Implements the Product interface.
• Creator (Application): Declares factory method which returns an object of type Product. Also, may define the factory method to create a Product object.
• ConcreteCreator (MyApplication): Overrides the factory method to return an instance of a ConcreteProduct .
CS 406: Design Patterns 49

• Used to ensure that a class has only one instance. For example, one printer spooler object, one file system, one window manager, etc.
• One may use a global variable to access an object but it does not prevent one from creating more than one instance.
• Instead the class itself is made responsible for keeping track of its instance. It can thus ensure that no more than one instance is created. This is the singleton pattern.
CS 406: Design Patterns 50

Singleton static uniqueInstance singletonData static Instance()
SingletonOp ()
GetSingletonData () return uniqueinstance
CS 406: Design Patterns 51

class Singleton { // Only one instance can ever be created.
public: static Singleton* Instance(); protected:
Singleton();
// Creation hidden inside Instance().
} private:
Static Singleton* _instance
// Cannot access directly.
CS 406: Design Patterns 52

Singleton* Singleton::_instance=0;
Singleton* Singleton:: Instance(){ if (_instance ==0) {
_instance=new Singleton;
}
Return _instance;
}
// Clients access the singleton
// exclusively via the Instance member
// function.
CS 406: Design Patterns 53