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Session # 5Muhamad Hesham
Know me well to get your goals!
Agenda
Why design patterns? Dynamic behaviors everywhere Our pattern exposed Applicability Design Changes Why all of this?
Where are we?
1. Make sure your software does what
customer wants it to do
Great Software
Where are we?
1. Make sure your software does what
customer wants it to do
Great Software
2. Apply basic OO princibles to add
more flexibility
Where are we?
1. Make sure your software does what
customer wants it to do
Great Software
2. Apply basic OO princibles to add
more flexibility
3. Strive for maintainability and
reusable design
Guess what?
“The one constant in software development”
Guess what?
“The one constant in software development”
CHANGE
Why Design Patterns? Job Requirements
Pocket Money
You: Dad, I need pocket money.
Pocket Money
Angry Dad
Pocket Money
Troubled Dad
Pocket Money
Happy Dad
Pepsi Machine
Out of
Pepsi
Pepsi Sold
No Mone
y
Has Mone
y
insert money
eject money
pepsi > 0
pepsi = 0
order pepsi
Pepsi State Machine
Platformer Game
Enemy State Machine
Running Idle
reach cliff
after a while
Coding by Sense®
/// <summary>/// Paces back and forth along a platform, waiting at either end./// </summary>public void Update(GameTime gameTime){ if (State == EnemyState.Idle)
{// If time-out go-to state Running// Else wait for some amount of time
} else if (State == EnemyState.Running) {
// Update enemy position // If reach cliff go-to state Idle
}}
Messenger
Messenger State Machine
Away
Available
Busy
busy
sign inOfflin
e
away
away
available
busy
available
All states are connected to offline
via sign out
Coding by Sense®
/// <summary>/// Occurs when a remote client sends a message/// </summary>public void OnMessageSent(Message msg){ if (State == MessengerState.Aways)
{ } else if (State == MessengerState.Busy) {
}...…
}
Coding by Sense®
/// <summary>/// Occurs when a remote client sends a message/// </summary>public void OnContactSignIn(Message msg){ if (State == MessengerState.Aways)
{ } else if (State == MessengerState.Busy) {
}...…
}
Coding by Sense®
/// <summary>/// Occurs when a remote client sends a message/// </summary>public void OnNudge(Message msg){ if (State == MessengerState.Aways)
{ } else if (State == MessengerState.Busy) {
}...…
}
Our Code Smells Bad
Violates Open Closed Principle.
Further additions may cause bugs.
State transitions are buried between if-else.
Exploring the pattern Let’s Google it!
Explaining the pattern Pattern class diagram
Explaining the pattern Participants
Context defines the interface of interest to clients. maintains an instance of a ConcreteState subclass
that defines the current state. State
defines an interface for encapsulating the behavior associated with a particular state of the Context.
ConcreteState subclasses each subclass implements a behavior associated with
a state of the Context.
Explaining the pattern Example
How they interact?
ContextCurrent-
State
Context
ContextSet Initial State
Foo Foo( )
Applicability
What are the symptoms should I take care of ?
Applicability
But take care! OVER DESIGN nightmare
Design Challenges
Who defines the state transitions? (Classes Dependency)
Design Challenges
Creating and destroying State objects (Memory Management)
What’s Next
State Pattern: Chapter 10 in Head First Strategy Pattern: Chapter 1 in Head First GoF: Behavioral Pattern Section Section: Enhance way of drawing shapes
Further Reading
Finally
Where are our scientists? HF & DS authors. Taha & Elmasri. Previous:▪ Algorithm, Algebra, 1,2,3, Camera, NASA.
Hospital. Nueclar scientist. Stat.
What about efforts? Thomas Edison. Reading books. Ebn Makhld. Quotes.
“No pain, no gain”Anonymous.
براحة “ العلم =ستطاع ُي ال”الجسم
كثير بن ُيحى
“My inventions comes from 2% inspiration and 98% of efforts”Thomas Edison.
“Science is what we understand well enough to explain to a computer. Art is everything else we do”Donald Knuth.
Why all of this?
Thank you ...
Questions?mhesham.fcis@gmail.com
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