Design Patterns Part 2: Factory, Builder, & Memento

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Last time Observer: Singleton: Adapter: Keeping up with the times Making sure there is only one Adapter: Filling the gaps

Today’s Objectives: Three more patterns Factory Builder Memento

Lots of patterns out there! Abstract Factory Builder Factory Method Prototype Singleton Adapter Bridge Composite Decorator Strategy Template Method Visitor Façade Flyweight Proxy Chain of responsibility Command Interpreter Iterator Mediator Memento Observer State

Lets talk video games… How to create a dynamic number of varying objects? Handling all of the initialization can be messy Clash of Clans – spawn units Candy Crush – generate endless blocks Fortnite – create projectiles

Solution: Factory Decouple the “owner” from creating the “object” Encapsulation Would be difficult to change later Works well with subtyping E.g., the level class would use specific block classes Instead, Factory knows how to create it Owner passes parameters to Factory

Factory

Lets talk video games… Clash Royale: Candy Crush: Fortnite: Each unit type has a Factory, player has 8 factories Pass info to Factory, such as card, level, and location Candy Crush: Each level has a Factory Level uses info like difficulty, probability, and what blocks should be allowed Fortnite: Each player can have a Factory Generates the correct projectile based on the weapon

Running Example: Tiger Dining

Recall: Adapter Pattern Solution to Support Multiple Payment Methods

Goal: To add additional payment methods PaymentsController new create … DiningDollars charge(uid) …

Solution: Adapter Interface which bridge a gap in payment methods Key objects are Target, Adapter and Adaptee Target request() Client Adapter request() Adaptee specificrequest()

DiningDollarsAdapter PaymentProcessor process_payment() PaymentsController … DiningDollarsAdapter process_payment() VisaPayAdapter process_payment() DiningDollars charge(uid) VisaPay pay(debitcard, cvv, exp)

Visa processes the payment Control Flow PaymentsController create new vpa:VisaPayAdapter new vp:VisaPay process_payment pay Visa processes the payment

Yuck!

Clean up payment initialization Design problem: Clean up payment initialization

Solution: Separate construction code Needs to separate construction of a complex object Same construction interface creates different representation

Solution: Builder Director construct() Builder build() ConcreteBuilder Product

… PaymentsController create() PaymentProcessorBuilder build(params) DiningDollarsAdapterBuilder build(params) VisaPayAdapterBuilder build(params) DiningDollarsAdapter VisaPayAdapter

Goal: Implement Builder pattern Step 1: Create Builder Classes Step 2: Move constructor code out of controller inside build Step 3: instantiate build class inside construct

Goal: Implement Builder pattern Step 1: Create Builder Classes Step 2: Move constructor code out of controller inside build Step 3: instantiate build class inside construct

Goal: Implement Builder pattern Step 1: Create Builder Classes Step 2: Move constructor code out of controller inside build Step 3: instantiate build class inside construct

Goal: Implement Builder pattern Step 1: Create Builder Classes Step 2: Move constructor code out of controller inside build Step 3: instantiate build class inside construct

@builder = … @builder = …

vpab:VisaPayAdapterBuilder Control Flow : PaymentsController create new vpab:VisaPayAdapterBuilder build

vpab:VisaPayAdapterBuilder Control Flow : PaymentsController create new vpab:VisaPayAdapterBuilder build new vpa:VisaPayAdapter process_payment

Delete from cart and Undo Design problem: Delete from cart and Undo

Cannot retrieve back

OrdersController new create … destroy

Solution: Memento Needs to store the state of the order Caretaker Originator state … create_memento(): Memento set_memento(memento) Memento state … getters and setters … In Java, the Memento is often a nested class inside the Originator class Caretaker

OrderMementoCaretaker Originator state … create_memento(): Memento set_memento(memento) Memento state … getters and setters … Caretaker Order … create_memento set_memento(version) OrderMemento order … initialize OrderMementoCaretaker

Summary: Keep one or more previous states of an object Provide a way to revert to those states

Java Example

Java Example

Goal: Implement memento pattern Order “create_memento” and “set_memento” OrderMemento initialize OrderMementoCaretaker “push_memento” and “pop_memento”

Memento : Examples Editor (VS Code, Sublime, Notepad++) Provides features of undo and redo What actions create Mementos? Does each editor have its own stack of Mementos?

For more design patterns, see the “Gang of Four” book (GoF)

Challenge for Iteration 3: Use a design pattern https://en.wikipedia.org/wiki/Software_design_pattern Probably already using several Identify them and clean up the code to match the “pattern”