WebSockets MQTT Subscription Example

 // Copyright (C) 2017 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

 #include "websocketiodevice.h"

 #include <QtCore/QDebug>

 WebSocketIODevice::WebSocketIODevice(QObject *parent)
     : QIODevice(parent)
 {
     connect(&m_socket, &QWebSocket::connected, this, &WebSocketIODevice::onSocketConnected);
     connect(&m_socket, &QWebSocket::binaryMessageReceived, this, &WebSocketIODevice::handleBinaryMessage);
 }

 bool WebSocketIODevice::open(QIODevice::OpenMode mode)
 {
     // Cannot use an URL because of missing sub protocol support
     // Have to use QNetworkRequest, see QTBUG-38742
     QNetworkRequest request;
     request.setUrl(m_url);
     request.setRawHeader("Sec-WebSocket-Protocol", m_protocol.constData());

     m_socket.open(request);

     return QIODevice::open(mode);
 }

 void WebSocketIODevice::close()
 {
     m_socket.close();
     QIODevice::close();
 }

 qint64 WebSocketIODevice::readData(char *data, qint64 maxlen)
 {
     qint64 bytesToRead = qMin(maxlen, (qint64)m_buffer.size());
     memcpy(data, m_buffer.constData(), bytesToRead);
     m_buffer = m_buffer.right(m_buffer.size() - bytesToRead);
     return bytesToRead;
 }

 qint64 WebSocketIODevice::writeData(const char *data, qint64 len)
 {
     QByteArray msg(data, len);
     const int length = m_socket.sendBinaryMessage(msg);
     return length;
 }

 void WebSocketIODevice::setUrl(const QUrl &url)
 {
     m_url = url;
 }

 void WebSocketIODevice::setProtocol(const QByteArray &data)
 {
     m_protocol = data;
 }

 void WebSocketIODevice::handleBinaryMessage(const QByteArray &msg)
 {
     m_buffer.append(msg);
     emit readyRead();
 }

 void WebSocketIODevice::onSocketConnected()
 {
     emit socketConnected();
 }