类目举例

 

 

#import "ViewController.h"

#import "NSString+test.h"

  NSString *sss=@"123";
    NSString *ss=[sss subStringWithString:@"456"];
    NSLog(@"%@",ss);

//NSString+test.h

#import <Foundation/Foundation.h>

@interface NSString (test)
-(NSString *) subStringWithString:(NSString *)str;
@end

//NSString+test.m
#import "NSString+test.h"

@implementation NSString (test)
-(NSString *)subStringWithString:(NSString *)str
{
    NSString *str1=[[NSString alloc]initWithFormat:@"%@--%@",self,str];
    return str1;
}
@end