Design Patterns

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Design Patterns
ECE 417/617Elements of Software Engineering
Stan Birchfield Clemson University
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What is a Design Pattern?

• A design pattern
• abstracts a recurring design structure
• comprises class and/or object
• dependencies,
• structures,
• interactions, or
• conventions
• distills design experience

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Re-use

• Code re-use
• Dont reinvent the wheel
• Requires clean, elegant, understandable, general,
  stable code
• leverage previous work
• Design re-use
• Dont reinvent the wheel
• Requires a precise understanding of common,
  recurring designs
• leverage previous work

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Some design patterns

• Abstract factory
• Adapter
• Bridge
• Command
• Composite
• Façade
• Subject / Observer
• Proxy
• Strategy

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Subject-observer
from Vlissides
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Subject-observer (cont.)
1

Subject Register(Observer) Unregister(Observer) N
otifyAll()
Observer OnUpdate()
for all o in observers o.OnUpdate()
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Subject-observer (cont.)
1

Subject Register(Observer) Unregister(Observer) N
otifyAll()
Observer virtual OnUpdate()
for all o in observers o.OnUpdate()
ConcreteSubject
ConcreteObserver virtual OnUpdate()
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Model / view / controller (MVC)
(displays data)
View
Model m Controller c(m) View v(c)
(mediates)
Controller
(holds data)
Model
calls Register()
Main
Create()
View
Create()
Register()
Create()
Controller
Model
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MVC (cont.)
1

Subject Register(Observer) Unregister(Observer) N
otifyAll()
Observer virtual OnUpdate()
for all o in observers o.OnUpdate()
Controller
View virtual OnUpdate()
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MVC (cont.)
class Observer protected virtual void
OnUpdate(MsgId message_id) 0 class
Subject public enum MsgId void
RegisterObserver(Observer obs) virtual void
NotifyAllObservers(MsgId message_id)
for (int i0 iltm_observers.size() i)
m_observersi-gtOnUpdate(message_id)
private stdvectorltObservergt
m_observers
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MVC (cont.)
class Controller public Subject
Controller(Data d) m_data(d) const
Data GetData() const void AddSphere(const
Sphere s) m_data-gtAddSphere(s)
NotifyAllObservers(ADD_SPHERE) private
Data m_data
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MVC (cont.)
class MainWnd public Observer public
MainWnd(Controller c) m_controller(c)
c.Register(this) virtual void
OnUpdate(int message_id) switch
(message_id) case SubjectADD_SPHERE
... private
Controller m_controller
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Adapter

• You have
• legacy code
• current client
• Adapter changes interface of legacy code so
  client can use it
• Adapter fills the gap b/w two interfaces
• No changes needed for either
• legacy code, or
• client

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Adapter (cont.)
class NewTime public int GetTime()
return m_oldtime.get_time() 1000 8
private OldTime m_oldtime
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Command

• You have commands that need to be
• executed,
• undone, or
• queued
• Command design pattern separates
• Receiver from Invoker from Commands
• All commands derive from Command and implement
  do(), undo(), and redo()

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Facade

• You
• have a set of related classes
• want to shield the rest of the system from these
  details
• Facade provides a simplified interface
• Encapsulates a subsystem

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Composite

• You want uniformly to treat
• items (atomic elements), and
• groups (containing items or other groups)
• Composite interface specifies operations that are
  shared between items and groups
• Examples hierarchy of files and directories,
  groups of drawable elements

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Composite (cont.)
Composite
Group
Item
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Proxy

• You want to
• delay expensive computations,
• use memory only when needed, or
• check access before loading an object into memory
• Proxy
• has same interface as Real object
• stores subset of attributes
• does lazy evaluation

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Strategy

• You want to
• use different algorithms depending upon the
  context
• avoid having to change the context or client
• Strategy
• decouples interface from implementation
• shields client from implementations
• Context is not aware which strategy is being
  used Client configures the Context
• strategies can be substituted at runtime
• example interface to wired and wireless networks

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Strategy (cont.)
Client
Policy
Strategy
Context
Concrete StrategyA
Concrete StrategyB
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Bridge

• You
• have several different implementations
• need to choose one, possibly at run time
• Bridge
• decouples interface from implementation
• shields client from implementations
• Abstraction creates and initializes the
  ConcreteImplementations
• Example stub code, slow code, optimized code

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Bridge (cont.)
Client
Implementor
Abstraction
Concrete ImplementorA
Concrete ImplementorB
Refined Abstraction
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Design pattern space
from Vlissides