我们使用的是paho mqtt软件包，这里介绍一下怎么使用mqtt协议编程。关于鸿蒙系统的mqtt移植好的软件包，相关github链接如下：

https://gitee.com/qidiyun/harmony_mqtt?

这里提供一个简单的编程示例：

这里我们使用MQTTClient编程模型，他支持多任务多线程，非常适合用在鸿蒙系统上。

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1. 网络初始化

这里定义一个 Network 结构体，然后指定我们的MQTT服务器的IP和端口号。

Network n;
   //初始化结构体
 NetworkInit(&n);
 //连接到指定的MQTT服务器IP、端口号
 NetworkConnect(&n, “XXX.XXX.XXX.XXX”, XXXX);

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2. 设置MQTT缓存和启动MQTT线程

我们这里使用的是MQTT线程功能。

 MQTTClientInit(&c, &n, 1000, buf, 100, readbuf, 100);
 MQTTStartTask(&c);

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3. 设置MQTT相关参数

接下来我们设置MQTT的相关参数，包括版本号、客户端ID、账户密码等

MQTTPacket_connectData data = MQTTPacket_connectData_initializer; 
 data.willFlag = 0;
 //MQTT版本为 v3
 data.MQTTVersion = 3;
 //设置客户端ID
 data.clientID.cstring = opts.clientid;
 //设置客户端账户
 data.username.cstring = opts.username;
 //设置客户端密码
 data.password.cstring = opts.password;
 data.keepAliveInterval = 10;
 data.cleansession = 1;

 //连接到MQTT服务器
 rc = MQTTConnect(&c, &data);

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4. 订阅主题和接收消息

订阅主题可以使用如下函数

MQTTSubscribe(&c, topic, opts.qos, messageArrived);

它的函数原型如下：

DLLExport int MQTTSubscribe(MQTTClient* client, const char* topicFilter, enum QoS, messageHandler);

其中：

MQTTClient* c ：我们前面定义的MQTTClient结构体

const char* topicFilter：订阅的主题

messageHandler messageHandler ：接收到主题信息后的回调处理函数。

例如上面我们的回调函数是 ?messageArrived ，它的原型如下：

void messageArrived(MessageData* md)
{
 MQTTMessage* message = md->message;
 //打印接收到的消息的长度、和消息内容
 printf("%.*s", (int)message->payloadlen, (char*)message->payload);
}

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5. 发送消息

发送消息也比较简单，我们只需要设置好我们的主题和消息内容即可

memset(&pubmsg, ‘\0‘, sizeof(pubmsg));
 //消息内容为 hello harmonyOS !
   pubmsg.payload = (void*)"hello harmonyOS !";
 //消息长度
   pubmsg.payloadlen = strlen((char*)pubmsg.payload);
   pubmsg.qos = QOS0;
   pubmsg.retained = 0;
   pubmsg.dup = 0;

 //推送消息，主题为 pubtest
 MQTTPublish(&c, "pubtest", &pubmsg);

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完整源码如下：

#include <stdio.h>

#include <unistd.h>

#include "ohos_init.h"
#include "cmsis_os2.h"

#include <unistd.h>
#include "hi_wifi_api.h"
//#include "wifi_sta.h"
#include "lwip/ip_addr.h"
#include "lwip/netifapi.h"

#include "lwip/sockets.h"

#include "MQTTClient.h"

/**
 * MQTT URI farmat:
 * domain mode
 * tcp://iot.eclipse.org:1883
 *
 * ipv4 mode
 * tcp://192.168.10.1:1883
 * ssl://192.168.10.1:1884
 *
 * ipv6 mode
 * tcp://[fe80::20c:29ff:fe9a:a07e]:1883
 * ssl://[fe80::20c:29ff:fe9a:a07e]:1884
 */
#define MQTT_URI                "tcp://106.13.62.194:1883"

struct opts_struct
{
	char* clientid;
	int nodelimiter;
	char* delimiter;
	enum QoS qos;
	char* username;
	char* password;
	char* host;
	int port;
	int showtopics;
} opts =
{
	(char*)"stdout-subscriber", 0, (char*)"\n", QOS2, NULL, NULL, (char*)"106.13.62.194", 1883, 1
};


void messageArrived(MessageData* md)
{
	MQTTMessage* message = md->message;

	if (opts.showtopics)
		printf("%.*s\t", md->topicName->lenstring.len, md->topicName->lenstring.data);
	if (opts.nodelimiter)
		printf("%.*s", (int)message->payloadlen, (char*)message->payload);
	else
		printf("%.*s%s", (int)message->payloadlen, (char*)message->payload, opts.delimiter);
	//fflush(stdout);
}


unsigned char buf[100];
unsigned char readbuf[100];

int mqtt_test(void)
{
	int rc = 0;
	
	MQTTMessage pubmsg;

	
	char* topic = "test";

	if (strchr(topic, ‘#‘) || strchr(topic, ‘+‘))
		opts.showtopics = 1;
	if (opts.showtopics)
		printf("topic is %s\n", topic);

	Network n;
	MQTTClient c;

	NetworkInit(&n);
	NetworkConnect(&n, opts.host, opts.port);
	
	MQTTClientInit(&c, &n, 1000, buf, 100, readbuf, 100);
	MQTTStartTask(&c);

	MQTTPacket_connectData data = MQTTPacket_connectData_initializer;       
	data.willFlag = 0;
	data.MQTTVersion = 3;
	data.clientID.cstring = opts.clientid;
	data.username.cstring = opts.username;
	data.password.cstring = opts.password;

	data.keepAliveInterval = 10;
	data.cleansession = 1;
	printf("Connecting to %s %d\n", opts.host, opts.port);
	
	rc = MQTTConnect(&c, &data);
	printf("Connected %d\n", rc);
    
	

    printf("Subscribing to %s\n", topic);
	rc = MQTTSubscribe(&c, topic, opts.qos, messageArrived);
	printf("Subscribed %d\n", rc);

	memset(&pubmsg, ‘\0‘, sizeof(pubmsg));
  	pubmsg.payload = (void*)"hello harmonyOS !";
  	pubmsg.payloadlen = strlen((char*)pubmsg.payload);
  	pubmsg.qos = QOS0;
  	pubmsg.retained = 0;
  	pubmsg.dup = 0;

	

	while (1)
	{
		MQTTPublish(&c, "pubtest", &pubmsg);
		sleep(1);	
	}
	
	printf("Stopping\n");

	MQTTDisconnect(&c);
	NetworkDisconnect(&n);

	return 0;
}