MQTT Subscriptions Example

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 #include "subscriptionwindow.h"
 #include "ui_subscriptionwindow.h"

 SubscriptionWindow::SubscriptionWindow(QMqttSubscription *sub, QWidget *parent) :
     QWidget(parent),
     ui(new Ui::SubscriptionWindow),
     m_sub(sub)
 {
     ui->setupUi(this);

     ui->labelSub->setText(m_sub->topic().filter());
     ui->labelQoS->setText(QString::number(m_sub->qos()));
     updateStatus(m_sub->state());
     connect(m_sub, &QMqttSubscription::messageReceived, this, &SubscriptionWindow::updateMessage);
     connect(m_sub, &QMqttSubscription::stateChanged, this, &SubscriptionWindow::updateStatus);
     connect(m_sub, &QMqttSubscription::qosChanged, [this](quint8 qos) {
         ui->labelQoS->setText(QString::number(qos));
     });
     connect(ui->pushButton, &QAbstractButton::clicked, m_sub, &QMqttSubscription::unsubscribe);
 }

 SubscriptionWindow::~SubscriptionWindow()
 {
     m_sub->unsubscribe();
     delete ui;
 }

 void SubscriptionWindow::updateMessage(const QMqttMessage &msg)
 {
     ui->listWidget->addItem(msg.payload());
 }

 void SubscriptionWindow::updateStatus(QMqttSubscription::SubscriptionState state)
 {
     switch (state) {
     case QMqttSubscription::Unsubscribed:
         ui->labelStatus->setText(QLatin1String("Unsubscribed"));
         break;
     case QMqttSubscription::SubscriptionPending:
         ui->labelStatus->setText(QLatin1String("Pending"));
         break;
     case QMqttSubscription::Subscribed:
         ui->labelStatus->setText(QLatin1String("Subscribed"));
         break;
     case QMqttSubscription::Error:
         ui->labelStatus->setText(QLatin1String("Error"));
         break;
     default:
         ui->labelStatus->setText(QLatin1String("--Unknown--"));
         break;
     }
 }