json格式

• json元素类型

	/* cJSON Types: */
#define cJSON_Invalid (0)
#define cJSON_False  (1 << 0)
#define cJSON_True   (1 << 1)
#define cJSON_NULL   (1 << 2)
#define cJSON_Number (1 << 3)
#define cJSON_String (1 << 4)
#define cJSON_Array  (1 << 5)
#define cJSON_Object (1 << 6)
#define cJSON_Raw    (1 << 7) /* raw json */

• json数组
  • 中括号[整形，字符串，布尔类型，json数组，josn对象]
  • [123,123.4,true, false, [12,34,56,“hello”,“world”]]
  • 细节：最后一个元素是没有逗号的
• json 对象
  • { }中是一些键值对
    • {“name”:“ZhangSan”,
      “name”:“li4”}
    • key值：必须是字符串，且不重复
    • value值：可以使json对象，json数组，布尔，整形，字符串
• json数组对象
• json数组+json对象
  • {“name”:“ZhangSan”,
    “name”:“li4”，
    “张三”:{
    “别名”:“老王”，
    “性别”:“男”，
    “孩子”:"[“小红”,“小黑”,“小白”]"
    }
    }

Cjson

• 下载cjson-master.zip包,解压命令 unzipt CJSON-master.zip
• cJson对象

typedef struct cJSON
	{
		/* next/prev allow you to walk array/object chains. Alternatively, use GetArraySize/GetArrayItem/GetObjectItem */
		struct cJSON *next;
		struct cJSON *prev;
		/* An array or object item will have a child pointer pointing to a chain of the items in the array/object. */
		struct cJSON *child;

		/* The type of the item, as above. */
		int type;

		/* The item's string, if type==cJSON_String  and type == cJSON_Raw */
		char *valuestring;
		/* writing to valueint is DEPRECATED, use cJSON_SetNumberValue instead */
		int valueint;
		/* The item's number, if type==cJSON_Number */
		double valuedouble;

		/* The item's name string, if this item is the child of, or is in the list of subitems of an object. */
		char *string;
	} cJSON;

• 创建一个json对象

CJSON_PUBLIC(cJSON *) cJSON_CreateObject(void)
{
    cJSON *item = cJSON_New_Item(&global_hooks);
    if (item) {
        item->type = cJSON_Object;
    }

    return item;
}

• 往json对象中添加数据成员

CJSON_PUBLIC(void) cJSON_AddItemToObject(
cJSON *object,	//json对象，即上面创建的json对象 
const char *string, //key 
cJSON *item			//value(int,string, array, obj)
);

• 创建一个整形值

cJSON *cJSON_CreateNumber(double num);

• 创建一个字符串

cJSON *cJSON_CreateString(const char *string);

写入json数据

{
	//json数据
	"奔驰"：{
		"factory":"北京奔驰",
		"last":31,
		"price":83,
		"sell":49,
		"other":[123,
				true,
				"hello",
				{"梅赛德斯奔驰":"The best or nothing"}
				]
	}
}

int main()
{
	//创建对象
	cJSON *obj = cJSON_CreateObject();
	//创建以子对象
	cJSON *subobj=cJson_CreateObject();
	cJSON_AddItemToObject(subobj,"factory",cJSON_CreateString("北京奔驰"));
	cJSON_AddItemToObject(subobj,"last",cJSON_CreateNumber(31));
	cJSON_AddItemToObject(subobj,"price",cJSON_CreateNUmer(83));
	cJSON_AddItemToObject(subobj,"sell",cJSON_CreateNUmer(49));
	//创建一个空数组
	cJSON *array = cJSON_CreateArray();
	//数组中添加元素
	cJSON_AddItemToArray(array,cJSON_CreateNumber(123));
	cJSON_AddItemToArray(array,cJSON_CreateBool(1));
	cJSON_AddItemToArray(array,cJSON_CreateString("hello"));
	//数组中的对象
	cJSON *subsub = cJSON_CreateObject();
	cJSON_AddItemToObject(subsub,"梅赛德斯奔驰",cJSON_CreateString("The best or Nothing"));
	cJSON_AddItemToArray(array, subsub);
	cJSON_AddItemToObject(subobj,"other",array);
	//obj中添加键值对
	cJson_addItemtoObj("obj","奔驰"，subobj);
	//数据格式化
	char *data = cJSON_Print(obj);
	FILE* fp = fopen("car.json","w");
	fwrite(data, sizeof(char), strlen(data)+1, fp);
	fclose(fp);
	
	return 0;
}	

• 错误:在函数‘print_number’中：cJSON.c：对‘floor’未定义的引用
  将函数对应的库加入 编译时增加 -lm ,math库

读json数据

• 将字符串解析为json结构

//返回值需要使用cJSON_Delete释放
(cJSON *) cJSON_Parse(
const char *value;
);

• 根据json key查找

(cJSON *) cJSON_GetObjectItem(
const cJSON * const object, //当前json对象 
const char * const string   //key值
);

typedef struct cJSON
	{
		struct cJSON *next;
		struct cJSON *prev;
		struct cJSON *child;
		//重点在下面4行===========================
		int type; //The type of the item, as above.指向对顶端的类型
		
		char *valuestring;  //从这里取 字符串
		int valueint;  	    //从这里取 整形值
		double valuedouble; //从这里取 浮点值
		//=======================================
		char *string;
	} cJSON;

• 获取json数组中的元素的个数

int cJSON_GetArraySize(cJSON *array);

• 获取数组下标找到对应的数组元素

cJSON *cJSON_GetArrayItem(cJSON *array,int index);

• 判断是否有可以值对应的键值对

//如果有这样一个key， 如果有这样一个key值 ，就会有一个value值
cJSON *cJSON_HasObjectItem(cJSON *object,const char* string);

• 判断数据

json = cJSON_Parse(topic_info->payload);
json_id = cJSON_GetObjectItem(json,id); 
if(json_id.type == cJSON_String) //判断id键值的type是否是cJSON_String类型
{
	HAL_Printf("id:%s\r\n", json_id->valuestring);//获取valuestring并打印
}
json_params = cJSON_GetObjectItem(json, "params");
if(json_params)
{
	json_led = cJSON_GetObjectItem(json_params, "led");
	if(json_led->type == cJSON_Number)  //判断led键值的type是否是cJSON_Number类型
	{
		HAL_Printf("LED:%d\r\n", json_led->valueint);  //获取valueint并打印
	}
}