一般常用方法

+ (NSDictionary *)dictionaryWithJsonString:(NSString *)jsonString
{
    if (jsonString == nil) {
        return nil;
    }

    NSData *jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
    NSError *err;
    NSDictionary *dic = [NSJSONSerialization JSONObjectWithData:jsonData
                                                        options:NSJSONReadingMutableContainers
                                                          error:&err];
    if(err)
    {
        NSLog(@"json解析失败：%@",err);
        return nil;
    }
    return dic;
}

有时候我们需要的把字典转成Json字符串（去掉空格、换行符等）、我们一般的做法

+ (NSString *)jsonStringWithDict:(NSDictionary *)dict {
    NSError *error;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dict options:NSJSONWritingPrettyPrinted error:&error];
    
    NSString *jsonString;
    if (!jsonData) {
        NSLog(@"%@",error);
    }else{
        jsonString = [[NSString alloc]initWithData:jsonData encoding:NSUTF8StringEncoding];
    }
    
    NSMutableString *mutStr = [NSMutableString stringWithString:jsonString];
    NSRange range = {0,jsonString.length};
    //去空格
    [mutStr replaceOccurrencesOfString:@" " withString:@"" options:NSLiteralSearch range:range];
    NSRange range2 = {0,mutStr.length};
    //去换行
    [mutStr replaceOccurrencesOfString:@"\n" withString:@"" options:NSLiteralSearch range:range2];
    return mutStr;
}

！！！这个方法有很大弊端：键值对里面的key,value的空格也去掉了。

自定义递归拼接方法：

//dic 转json 自定义的无空格。
+ (NSString *)returnJSONStringWithContainerObj:(NSObject *)obj{
    NSString *jsonStr = @"";
    if ([obj isKindOfClass:[NSArray class]]) {
        jsonStr = @"[";
        NSArray *arr = (NSArray *)obj;
        for (NSObject *subObj in arr) {
            NSString *subStr = [NSString returnJSONStringWithContainerObj:subObj];
            jsonStr = [NSString stringWithFormat:@"%@%@,",jsonStr,subStr];
        }
        jsonStr = [NSString stringWithFormat:@"%@%@",[jsonStr substringWithRange:NSMakeRange(0, jsonStr.length-1)],@"]"];
        
    }else if ([obj isKindOfClass:[NSDictionary class]]){
        NSDictionary *dic = (NSDictionary *)obj;
        jsonStr = @"{";
        NSArray * keys = [dic allKeys];
        for (NSString * key in keys) {
            NSObject *subObj = [dic objectForKey:key];
            NSString *subStr = [NSString returnJSONStringWithContainerObj:subObj];
            jsonStr = [NSString stringWithFormat:@"%@\"%@\":%@,",jsonStr,key,subStr];
        }
        jsonStr = [NSString stringWithFormat:@"%@%@",[jsonStr substringWithRange:NSMakeRange(0, jsonStr.length-1)],@"}"];
    }else{
        jsonStr =  [NSString stringWithFormat:@"\"%@\"", obj];
    }
    return jsonStr;
}

iOS11 后有简单方法

NSJSONWritingOptions

typedef NS_OPTIONS(NSUInteger, NSJSONWritingOptions) {
   NSJSONWritingPrettyPrinted = (1UL << 0),

   /* Sorts dictionary keys for output using [NSLocale systemLocale]. Keys are compared using NSNumericSearch. >The specific sorting method used is subject to change.
    */
   NSJSONWritingSortedKeys API_AVAILABLE(macos(10.13), ios(11.0), watchos(4.0), tvos(11.0)) = (1UL << 1)
} API_AVAILABLE(macos(10.7), ios(5.0), watchos(2.0), tvos(9.0));

对上面字段的解释：
NSJSONWritingPrettyPrinted = (1UL << 0) //是将生成的json数据格式化输出，这样可读性高
NSJSONWritingSortedKeys //输出的json字符串就是一整行

作者：Simple_Code
链接：https://www.jianshu.com/p/1e242df1335b