stringByAppendingString, stringByAppendingPathComponent, stringByAppendingPathExtension

    1.//stringByAppendingString是在path加后缀的意思

     NSString *imagePath = [path stringByAppendingString:[NSString stringWithFormat:@"/%@",imageName]];

    2.//stringByAppendingPathComponent是在path后面加上“／”号连接 "字符串" 让它成为完整的路径
     stringByAppendingPathComponent是添加/号，使之变成一个完整的路径
     NString * str=@“路径”;
     str=[stringByAppendingPathComponent:@"img.png"];
     打印:str就变成 路径/img.png了
     
     3.//stringByAppendingPathExtension 是加后缀的意思"   自动加上 "."    "
     如果是stringByAppendingPathExtension是加后缀的意思
     NString * str=@“路径”;
     str=[str stringByAppendingPathExtension:@"jpg"];
     打印:str就变成 路径.jpg