101，使用Category给API里面的类添加新方法

NSString+JS.h：

#import <Foundation/Foundation.h>

@interface NSString (JS)

+(int)countWithStr:(NSString *)str;

-(int)count;

@end

NSString+JS.m：

#import "NSString+JS.h"

@implementation NSString (JS)

+(int)countWithStr:(NSString *)str{

    int count = 0;

    for (int i =0; i < str.length; i++) {

        unichar c = [str characterAtIndex:i];

        if (c >= '0' && c <='9') {

            count++;

        }

    }

    return count;

}

-(int)count{

    //self：谁调用该函数就是谁

    int count = 0;

    for (int i =0; i < self.length; i++) {

        unichar c = [selfcharacterAtIndex:i];

        if (c >= '0' && c <='9') {

            count++;

        }

    }

    return count;

}

@end

main.m:

#import <Foundation/Foundation.h>

#import "NSString+JS.h"

int main(int argc,const char * argv[]) {

    /*

     问题：计算字符串中含有多少个数字？

     知识点：为NSString添加计算数字的方法

     */

    NSString *str =@"s123asf675ds3s5";

    int count = [NSStringcountWithStr:str];

    

    int count1 = [str count];

    

    NSLog(@"count = %i,count1 = %i",count,count1);

    

    return 0;

}

//2015-12-18 11:18:29.492 8，使用Category为系统类添加方法[982:50190] count = 8,count1 = 8

//Program ended with exit code: 0