一，引入qmqtt 库

我是将整个头文件/源文件都添加到了工程中进行编译，这样 跨平台时  方便，直接编译就行了。

原始仓库路径：https://github.com/emqx/qmqtt/tree/master

二，使用

声明一个单例类，将订阅到主题，直接以属性的方式 进行暴露，这样QML文件中 就可以直接使用。 下边是订阅一个温度 和湿度的主题数据。

#ifndef MQTTMANAGERT_H
#define MQTTMANAGERT_H

#include <iostream>
#include <mqtt/qmqtt.h>

class MQTTManager:public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString inTemperature READ getInTemperature WRITE setInTemperature NOTIFY inTemperatureChanged FINAL)
    Q_PROPERTY(QString inHumidity READ getInHumidity WRITE setInHumidity NOTIFY inHumidityChanged FINAL)

public:
    static MQTTManager* instance();

    QString getInTemperature() const;
    void setInTemperature(QString newInTemperature);

    QString getInHumidity() const;
    void setInHumidity(QString newInHumidity);

private:
    ~MQTTManager();
    MQTTManager();

public slots:
    void doConnected();
    void onSubscribed(const QString& topic);
    void doDisconnected();
    void doDataReceived(QMQTT::Message message);

signals:
    void inTemperatureChanged();
    void inHumidityChanged();

private:
    QMQTT::Client *client;
    static MQTTManager *manager;

    QString inTemperature;
    QString inHumidity;
};
#endif // MQTTMANAGERT_H

#include "mqttManagert.h"

MQTTManager* MQTTManager::manager=nullptr;

const QString HOST= "127.0.0.1"; //服务器地址
//需要订阅的主题
const QString TOPIC_IN_TEMPERATURE_HUMIDITY ="qmqtt/in_temperature_humidity";

MQTTManager::MQTTManager(){
    client = new QMQTT::Client(QHostAddress(HOST),1883,this);
    connect(client,&QMQTT::Client::connected,this,&MQTTManager::doConnected);
    connect(client,&QMQTT::Client::disconnected,this,&MQTTManager::doDisconnected);
    connect(client,&QMQTT::Client::received,this,&MQTTManager::doDataReceived);
    connect(client, &QMQTT::Client::subscribed, this, &MQTTManager::onSubscribed);

    client->connectToHost();
}

MQTTManager::~MQTTManager(){
    client->disconnected();
}

MQTTManager* MQTTManager::instance(){
    if(!manager){
        manager = new MQTTManager();
    }
    return manager;
}

void MQTTManager::doConnected(){
    qDebug()<<"doConnected ok";
    //再此订阅主题
    client->subscribe(TOPIC_IN_TEMPERATURE_HUMIDITY);
}
void MQTTManager::onSubscribed(const QString& topic)
{
    qDebug() << "onSubscribed " << topic;
}

void MQTTManager::doDataReceived(QMQTT::Message message){
    //收到消息
    QString mes = QString(message.id())+" "+QString(message.qos())+" "+message.topic()+" "+message.payload()+"\n";
    qDebug()<<"dataReceived "<<mes;
   //根据主题过滤消息
    if(message.topic()==TOPIC_IN_TEMPERATURE_HUMIDITY){
        QStringList dataList = QString(message.payload()).split(",");
        if(dataList.size()==2){
            setInTemperature(dataList[0]);
            setInHumidity(dataList[1]);
        }
    }

}
void MQTTManager::doDisconnected(){
    qDebug()<<"doDisconnected ok";
}

QString MQTTManager::getInTemperature() const
{
    return inTemperature;
}

void MQTTManager::setInTemperature(QString newInTemperature)
{
    if (inTemperature == newInTemperature)
        return;
    inTemperature = newInTemperature;
    emit inTemperatureChanged();
}

QString MQTTManager::getInHumidity() const
{
    return inHumidity;
}

void MQTTManager::setInHumidity(QString newInHumidity)
{
    if (inHumidity == newInHumidity)
        return;
    inHumidity = newInHumidity;
    emit inHumidityChanged();
}