Understanding the Decorator Pattern and Open-Closed Principle in Software Design

1. CSC 313 – Advanced Programming Topics Lecture 10:Decorator Pattern

2. Open-Closed Principle Classes should be open for extension, but closed to modification • So, what does this mean?

3. Reality Check Classes should be open for extension… • Want to update other’s code • Other is important emphasis • Everyone is other to someone • Easily create subclasses to: • Add functionality • Includeadditional fields • Specialize behavior via overriding

4. More Reality Check …But closed to modification

5. More Reality Check …But closed to modification • Nobody better mess with your code • YOUR code being beauty incarnate usually • Only add errors & will soil your perfection • Never let anyone: • Make changes which violate basic assumptions • Alter tasks it performs perfectly already • Anything that might introduce bugs

6. Strategy Pattern & OCP • Pattern stays true to open-closed principle • Using pattern, context open for extension • New functionality created by changing strategy • If more data is needed, subclasses can add fields • Context closed to modification despite all this • Actions limited by data provided to strategies • Strategies focused on task & cannot do other acts

7. Observer Pattern & OCP • Observer pattern devotee of principle, too • Using this pattern, subject open for extension • Responding to events open to each Observer • In pull model, extend Subject to make more info • Cannot modify Subject using this pattern • Limits of pattern means Observers only re-act • Subject chooses events to expose to Observers

8. Warning Remainder of lecture unsafe for: Beginning programmers Unwilling to consider new viewpoints Stubbornly dogmatic Stupid Liberal arts majors

9. Inspiration For Pattern

10. Making a Cultured Pearl • Pearl made when piece of shell enters oyster • Irritant required to start making of pearl • To avoid irritation, shell grown around it • Layer after layer of material added by the oyster • As it grows, each layer adds new affect • Color, shine, shape modified by every layer added • So each layer adds something, but always a pearl

11. Problem At Hand • Have single key main concept • Coffee • Paycheck • Pizza • Characters • But many ways to adjust and extend concept • Cream, sugar, soy, mocha, caffeine, whip, chains • Federal & State income tax, FICA, health insurance • Bacon, pineapple, ham, anchovies, mushrooms • Bold, italic, underline, color, small caps, error

12. Add Fields for properties • Add field for each possible property • Is double whip, decaf, soy water possible? • Multiple states need taxes; how to handle this? • Using fields is both hard & inefficient • Must modify class for each new property we need • Creates huge number of rarely used fields

13. Add Fields for properties • Add field for each possible property • Is double whip, decaf, soy water possible? • Multiple states need taxes; how to handle this? • Using fields is both hard & inefficient • Must modify class for each new property we need • Creates huge number of rarely used fields • Class open to everyone to play with & change • This very clear violation of open close principle

14. Create Subclasses • Define subclass for each possible situation • Need both MochaWhipCoffee & WhipMochaCoffee? • Every pizza topping combination must be written • Good news: this follows open-close principle • But code duplicated in many places • Who’ll modify classes to update New York tax rate? • Maintaining this code will be pain for someone

15. Decorator Pattern Intent • Add, but not destroy, existing class instances • Double whip, soy, decaf coffee is just coffee • Theseare still letters ‘t’, ‘h’, ‘e’, ‘s’, & ‘e’ • After taxes, my paycheck still (barely) paycheck • Add functionality using these additions • Wrapping around character changes its display • Add $5.00 to price for calling coffee “tall” • This pattern can also add information • Many attachments included with my e-mail

16. Decorator Pattern Usage • Flexibly add adjectives to main class’s noun • These decorations provide additional description • Not required to use each decoration in each case • Wrappings can be stacked in any order • Enhance class’s actions by adding adverbs • Use additional information to improve result • Include additional tasks that must be performed • Lets main instance be invisiblyaugmented

17. For Next Lecture • Lab #2 due before next lab • Asks you to implement the Strategy Pattern • Read rubric • Read pages 95 – 105 in the book • How do we implement the design pattern? • Can anyone actually use this? • What is the main point of this pattern?