GoF Design Patterns Tutorial

Twenty-three design patterns. Twenty-three interactive demos you actually drive. Most pattern references show you a diagram and a code listing, then leave you to imagine the rest. This app runs the pattern instead. Flip a switch and watch a Singleton split into three drifting instances. Drag a forest to 100,000 trees and watch memory stay flat — then turn the Flyweight off and watch it climb. Pull the Adapter out of a pipeline and see the two ends stop fitting. Every pattern gets three tabs: LEARN The intent, a real-world analogy, the problem it solves, the solution, when to reach for it, and — just as important — what to watch out for. PLAY A live demo you control. Not an animation of the pattern; the pattern actually running, with its internal state on screen as you change things. CODE A syntax-highlighted Swift distillation of the exact structure the demo runs on. Same shapes, same names, no pseudocode. ALL 23 GANG OF FOUR PATTERNS Creational — Singleton, Factory Method, Abstract Factory, Builder, Prototype Structural — Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy Behavioral — Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template Method, Visitor A FEW OF THE DEMOS • Command — draw shapes, undo, redo, with both stacks visible the whole time • State — a media player whose buttons change behavior per state, next to a live state diagram • Composite — expand a file tree, add files, then watch one size call recurse to every leaf • Decorator — stack and reorder wrappers, drawn literally as nested boxes • Observer — one price, four subscribers; unsubscribe one mid-run and it freezes • Visitor — run four operations across the same shapes without editing a shape • Interpreter — type an expression and watch it parse into a tree that evaluates itself BUILT FOR STUDYING Track which patterns you have visited and which you have marked as understood. Search the whole catalog by name or intent. Progress is saved between launches. Built for iPad, where there is room to show the demo and its internal state side by side. Fully offline. No account, no ads, no analytics, no data collection of any kind — your progress never leaves your device. Whether you are preparing for a systems design interview, working through the GoF book for the first time, or just want the Bridge pattern to finally click, this is the reference that lets you poke at the answer instead of reading about it.