字符串转字典：
NSString * jsonString = @“123”;
NSData *jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
NSDictionary *dic= [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers error:&err];
字典转字符串：
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dic
options:NSJSONWritingPrettyPrinted
error:&parseError];
NSString * str = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
// 字符串转Data
NSString *str =@"jesfds";
NSData *data =[str dataUsingEncoding:NSUTF8StringEncoding];
//NSData 转NSString
NSString *result =[[ NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
//data 转char
NSData *data;
char *test=[data bytes];
// char 转data
byte* tempData = malloc(sizeof(byte)*16);
NSData *content=[NSData dataWithBytes:tempData length:16];
