Category and Class Extension in Objective C

1. Category

 Category in Objective-C allows you to add new methods (instance and class methods) to an existing class, even if you don’t have the source code for that class.

Categories provide a powerful way to extend the behavior of system or third-party classes without subclassing or modifying their implementation.

You can use categories to keep related methods together, organize large classes, or add utility methods to built-in classes such as NSString, NSArray, etc

@interface NSString (Reverse)
- (NSString *)reverseString;
@end

@implementation NSString (Reverse)
- (NSString *)reverseString {
    NSMutableString *result = [NSMutableString string];
    for (NSInteger i = self.length - 1; i >= 0; i--) {
        [result appendFormat:@"%C", [self       characterAtIndex:i]];
    }
    return result;
}
@end

Usage:

NSString *str = @"Hello";
NSLog(@"%@", [str reverseString]); 
// Output: olleH

---

2. Class Extension

• Class Extension is a special category with empty parentheses:

@interface NewClass ()
@end

Unlike categories, extensions:

1. Can add private properties (with backing ivars).
2. An extension (or class extension) in Objective-C is a special, unnamed category usually declared within a class’s implementation (.m) file.
3. Extensions enable you to add private methods and properties (including backing instance variables) to a class. These members are only visible within the class implementation itself.
4. Only classes for which you have the original source code can be extended in this way

// NewClass.h
@interface NewClass : NSObject
@property (nonatomic, strong) NSString *publicName;
- (void)publicMethod;
@end

// NewClass.m
@interface NewClass ()
@property (nonatomic, strong) NSString *privateName; //  Private property
- (void)privateMethod;                            // Private method
@end

@implementation NewClass
- (void)publicMethod {
    self.privateName = @"Hello"; // Accessible here
    [self privateMethod];
}
- (void)privateMethod {
    NSLog(@"Private method called");
}
@end

Important:

• Use class extensions to:
  • Hide internal state of a class (private properties).
  • Implement private helper methods not exposed to public API.
• Common in encapsulation and clean API design.

Diffrences: