Categories

     当你想要为某个 class 新增 methods，你通常会扩充（extend，即继承）它。然而这不一定是个完美解法，特别是你想要重写一个 class 的某个功能，但你却没有原始码时。Categories 允许你在现有的 class 加入新功能，但不需要扩充它。Ruby 语言也有类似的功能。

FractionMath.h#import "Fraction.h"@interface Fraction (Math)-(Fraction*) add: (Fraction*) f;-(Fraction*) mul: (Fraction*) f;-(Fraction*) div: (Fraction*) f;-(Fraction*) sub: (Fraction*) f;@endFractionMath.m#import "FractionMath.h"@implementation Fraction (Math)-(Fraction*) add: (Fraction*) f {return [[Fraction alloc] initWithNumerator: numerator * [f denominator] + denominator * [f numerator]denominator: denominator * [f denominator]];}-(Fraction*) mul: (Fraction*) f {return [[Fraction alloc] initWithNumerator: numerator * [f numerator]denominator: denominator * [f denominator]];}-(Fraction*) div: (Fraction*) f { return [[Fraction alloc] initWithNumerator: numerator * [f denominator]denominator: denominator * [f numerator]];} -(Fraction*) sub: (Fraction*) f { return [[Fraction alloc] initWithNumerator: numerator * [f denominator] -denominator * [f numerator]denominator: denominator * [f denominator]];}@endmain.m#import <stdio.h>#import "Fraction.h"#import "FractionMath.h"int main( int argc, const char *argv[] ) {// create a new instanceFraction *frac1 = [[Fraction alloc] initWithNumerator: 1 denominator: 3];Fraction *frac2 = [[Fraction alloc] initWithNumerator: 2 denominator: 5];Fraction *frac3 = [frac1 mul: frac2];// print it[frac1 print];printf( " * " );[frac2 print];printf( " = " );[frac3 print]; printf( "/n" ); // free memory[frac1 release];[frac2 release]; [frac3 release];return 0;}output1/3 * 2/5 = 2/15 

 

 重点是 @implementation 跟 @interface 这两行：@interface Fraction (Math) 以及 @implementation Fraction (Math).（同一个 class）只能有一个同名的 category，其他的 categories 得加上不同的、独一无二的名字。

 Categories 在建立 private methods 时十分有用。因为 Objective-C 并没有像 Java 这种 private/protected/public methods 的概念，所以必须要使用 categories 来达成这种功能。作法是把 private method 从你的 class header (.h) 档案移到 implementation (.m) 档案。以下是此种作法一个简短的范例。

MyClass.h#import <Foundation/NSObject.h>@interface MyClass: NSObject-(void) publicMethod;@endMyClass.m#import "MyClass.h"#import <stdio.h>@implementation MyClass-(void) publicMethod {printf( "public method/n" );}@end// private methods@interface MyClass (Private)-(void) privateMethod;@end@implementation MyClass (Private)-(void) privateMethod {printf( "private method/n" );}@end main.m#import "MyClass.h"int main( int argc, const char *argv[] ) {MyClass *obj = [[MyClass alloc] init];// this compiles[obj publicMethod];// this throws errors when compiling//[obj privateMethod];// free memory[obj release];return 0;}outputpublic method