iOS NSDictionary <--> NSString(JSON) in Objc

+ (NSString*)stringINJSONFormatForObject:(id)obj
{
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:obj
options:0 // If that option is not set, the most compact possible JSON will be generated
error:nil];

NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
jsonString = [jsonString stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];  //去除掉首尾的空白字符和换行字符

return jsonString;
}

+ (NSDictionary *)dictForJSONString:(NSString *)str
{
NSData *jsonData = [str dataUsingEncoding:NSUTF8StringEncoding];
NSError *err;
NSDictionary *dic = [NSJSONSerialization JSONObjectWithData:jsonData
options:NSJSONReadingMutableContainers
error:nil];
return dic;
}