Design Patterns

Getting started with Mac/iOS can be hard. This presentation helps you learn the common design patterns used by Cocoa and Cocoa Touch.

1. Design Patterns Robert Brown @robby_brown
2. What is a Design Pattern? After working on related problems, patterns often appear in the solutions. Formalized description of best practice.
3. Singleton
4. What is a Singleton? A singleton is an object that only has one instance for the lifetime of the application. Some things are not safe to have more than one instance running. For example, an updater. Objective-C classes are singletons by design. You can take advantage of this when creating singletons.
5. Creating a Singleton https://gist.github.com/1116294
6. Related Patterns Abstract Factory Plug-in Registry Notification Center
7. Delegate
8. What is a Delegate? A delegate defines the behavior of a generic class. This keeps implementation specific behavior out of the generic class. Data sources are similar to delegates, except they define content rather than behavior. UITableViews use both a delegate and a data source.
9. Delegate Interaction UIApplication Delegate UIApplication
10. Delegate Interaction UITableView DataSource UITableView Delegate UITableView
11. Gotchas Use “assign” instead of “retain” to avoid retain cycles. When using assign, be sure to nullify the delegate reference when deallocating the object. Use “weak” instead of “assign” when using ARC.
12. Related Patterns Data Source Block Delegation Callback Chain of Responsibility State Flyweight
13. Block Delegation
14. What is Block Delegation? Similar to a delegate object. Rather than give an object that has one or more methods defined, give a block that has the implementation of one method. Rule of thumb: If you have three or more delegate methods, use a delegate. Otherwise, use block delegation. Sometimes it’s appropriate to break this rule.
15. Block 2 Block 1 Block Delegation Interaction Object
16. Gotchas Watch out for retain cycles, particularly with “self” __block __unsafe_unretained id selfRef = self; __block __weak id selfRef = self; Don’t forget to copy your blocks. They are created on the stack.
17. Related Patterns Delegate Callback
18. Model View Controller
19. What is MVC? A strict organization of classes into their different roles. Model: Any underlying data. View: The graphical presentation of the data. Controller: Coordinates the interactions between the views and model. Keeps programs modular and makes it easy to swap out views when needed.
20. Model View Controller Model Controller View Interface Builder Core Data Xcode
21. Gotchas Some implementations of MVC have the View and Model directly connected. This can cause infinite update cycles.
22. Related Patterns Observer
23. Layers
24. What is the Layer Pattern? Modules of a program can be organized like a stack of layers. Strict Layering: A layer can only use the layer directly below it. Non-strict Layering: A layer can use any layer below it.
25. Core Data Layers XML SQLite Binary In Memory Custom NSPersistentStore NSPersistentStoreCoordinator NSManagedObjectModel NSManagedObject Context NSManagedObject Application NSManagedObject Subclass
26. Gotchas Occasionally you may find a need to call up a layer. To do this, you will need to use dependency inversion. Dependency inversion: A lower layer defines an interface to interact with other layers. This is often used with notifications. An upper layer can register itself to receive these notifications through the defined interface.
27. Façade
28. What is a Façade? A façade is a single entry point to a complex collection of classes. Makes interaction with complex collections easier and less dependent on internal implementation. This is the basis of the layer pattern.
29. Object Façade Object Object Object Object External Object External Object External Object
30. Related Patterns Class cluster Layer Proxy
31. Observer
32. What is an Observer? Sometimes you want to know when an object changes. Often used with MVC. When a model object changes, you want to update the view(s) accordingly. The observer pattern is built into every NSObject via Key-Value Observing (KVO).
33. Gotchas Don’t forget to remove an object as an observer when that object is deallocated.
34. Related Patterns Model View Controller Notification Center
35. Proxy
36. What is a Proxy? A proxy receives actions in behalf of another object. Many types of proxies: Lazy loading proxy Distributed (network) proxy (NSDistantObject) Immutable proxy NSProxy can be used to make any kind of proxy object.
37. Lazy Loading Proxy Photo Proxy Object
38. Distributed Proxy Network Proxy Object Remote Object
39. Immutable Proxy Proxy Object Mutable Object
40. Gotchas If you use NSProxy, you need to be very familiar with message forwarding.
41. Other Patterns Flyweight Visitor Builder Template Method Factory Notification Center Class Cluster
42. Want to Learn More? Design Patterns: Elements of Reusable Object- Oriented Software Every developer should own this book! Cocoa Design Patterns in the Apple Docs
43. Questions?