iOS中的分类(category)

Student *stu = [Student studentWithName:@"xiaomin" gender:@"man" age:118];
        [stu study];
        [stu playGame];

@interface NSString (SayHi)
//分类(类目,Category),为米有源代码的类,扩充方法,并且只能扩充方法
//一但添加过方法之后,这些方法就是该类自己的方法,由该类的对象或者该类调用
+ (void)sayHi;
- (void)sayHello;
//添加获取首字母的方法,不是大写,改成大写
- (NSString *)changeStringFirstBig;
//添加获取尾字母的方法,不是小写,改成小写
- (NSString *)changeStringOverlitter;
@end

@implementation NSString (SayHi)
+ (void)sayHi{
    NSLog(@"牛比");
}

- (void)sayHello
{
    NSLog(@"还是你比");
}

- (NSString *)changeStringFirstBig
{
 return [[self substringToIndex:1] uppercaseString] ;
}
//添加获取尾字母的方法,不是小写,改成小写
- (NSString *)changeStringOverlitter
{
    return [[self substringFromIndex:[self length] -1] lowercaseString];
}
@end

@interface Student : NSObject
{
    NSString *_name;
    NSString *_gender;
    NSInteger _age;
    
}
//自定义初始化
- (id)initWIthName:(NSString *)name gender:(NSString *)gender age:(NSInteger)age;
+ (id)studentWithName:(NSString *)name gender:(NSString *)gender age:(NSInteger)age;

- (void)study;
- (void)playGame;
@end

#import "Student.h"


/**
 *  延展  外部访问不到,只为源代码的类,扩充功能(实例变量,方法),延展中的实例变量以及方法是私有的,只能在本类中访问
 
 
 如果本类中的某个方法只想在本类中使用,不想让外界调用,那么此时就可以把该方法在本类的.m文件的延展中申明,实现此时,这些方法都是该类的私有方法.
 
 
OC中,没有绝对的私有
 */
@interface Student ()
{
    CGFloat _score;
}

- (void)sayMorning;

- (void)sayAfternoon;

- (void)sayGoodNight;
@end

@implementation Student

- (id)initWIthName:(NSString *)name gender:(NSString *)gender age:(NSInteger)age{
    if (self = [super init]) {
        _name = name;
        _age = age;
        _gender = gender;
    }
    return self;
}
+ (id)studentWithName:(NSString *)name gender:(NSString *)gender age:(NSInteger)age{
   return [[Student alloc] initWIthName:name gender:gender age:age];
}
- (void)study
{
    NSLog(@"study 学生苏恶习");
}
- (void)playGame{
    NSLog(@"学哥屁  去玩");
    
    [self sayAfternoon];
    [self sayGoodNight];
    [self sayMorning];
}

- (void)sayMorning{
    NSLog(@"早上好!");
}

- (void)sayAfternoon
{
    NSLog(@"下午好");
}

- (void)sayGoodNight
{
    NSLog(@"晚安");
}
@end