Categories allow extending classes without subclassing by adding methods to existing classes. They are useful for spreading implementation across multiple files when working with others, fixing bugs in classes when you don't have access to the source code, and working with Cocoa class clusters that are difficult to subclass. However, categories can cause conflicts if two categories implement the same method, so they should be used carefully.

1. Categories

2. Categories
@interface NSString (MyExtension)
-(NSString *)randomizeCharacters;
@end
//in code
NSString *stringToScramble = @”Tom Marvolo Riddle”
NSString *scrambledString = [stringToScramble randomizeCharacters];

3. When should you use a
category?

4. Categories
• Extend classes without subclassing.
• Create informal protocols.
• Spread implementation over multiple files.
• Simply development when multiple
programmers are working on the same file.
• Fix bugs in classes when you don’t have
access to the source (in a last resort).

5. Some Cocoa classes
are “class clusters” so
subclassing is very
difficult (NSString,
NSArray, etc).

6. Sometimes, Apple uses
“informal protocols.”
(categories with no
implementation)
-(void)awakeFromNib;

7. (Maybe this is
obsolete.)
@protocol MyProtocol
@optional:
-(void)thisMethod;
-(void)thatMethod;
@end

8. Anonymous Categories
are like private
methods.
@implementation MyClass()
-(void)methodThatIWantToBePrivate;
@end

9. But not really.
(The Objective-C runtime isn’t that secure.)
@implementation MyClass()
-(void)methodThatIWantToBePrivate;
@end
// in another class
MyClass *myClass = [[MyClass alloc] init];
[myClass methodThatIWantToBePrivate]; // calls it

10. Be Careful About
• Overriding existing methods
• Causing conflicts with other categories (if
two categories implement the method with
the same name, the result is on predictable)

11. Subclassing vs Categories
• Many times, the difference is not clear-cut
• Subclass when you need to add a property
(but in iOS 5 categories can add
properties)
• Don’t subclass Class Clusters

12. One example when I use categories is
using Core Data. Core Data will auto
generate class files.
If I want to add methods to those
classes, I add them in a category so that, if
I need to regenerate the classes, they
won’t overwrite my changes.