refactor(simulator): extract HostSerialBackend interface

Phase 1 of pluggable host serial backends. Introduces an abstract
HostSerialBackend QObject interface and extracts the existing
QSerialPort handling into a QSerialPortBackend implementation.

HostSerialConnector becomes a thin per-port owner of a backend
pointer. Per-port encoding and baudrate are cached on the connector
so they survive backend swaps and reconnects.

Error reporting moves out of the connector: backends emit
errorOccurred, the connector forwards via a backendError signal,
and SimulatorMainWindow shows the message box.

No behaviour change. Sets up the next phase, which adds a
QLocalSocketBackend so user scripts can stand in for hardware on
simulator aux ports.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: raphaelcoeffic

companion/src/simulation/CMakeLists.txt

@@ -20,6 +20,7 @@ set(${PROJECT_NAME}_NAMES
   widgets/radiowidget
   widgets/virtualjoystickwidget
   serialportsdialog
+  hostserialbackend_serialport
   hostserialconnector
   simulateduiwidgetGeneric
   wasmsimulatorinterface
@@ -36,6 +37,7 @@ AddHeadersSources()
 
 set(${PROJECT_NAME}_HDRS
   ${${PROJECT_NAME}_HDRS}
+  hostserialbackend.h
   radiouiaction.h
   widgets/buttonswidget.h
   widgets/radiofaderwidget.h

companion/src/simulation/hostserialbackend.h

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) EdgeTX
+ *
+ * Based on code named
+ *   opentx - https://github.com/opentx/opentx
+ *   th9x - http://code.google.com/p/th9x
+ *   er9x - http://code.google.com/p/er9x
+ *   gruvin9x - http://code.google.com/p/gruvin9x
+ *
+ * License GPLv2: http://www.gnu.org/licenses/gpl-2.0.html
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#pragma once
+
+#include <QByteArray>
+#include <QObject>
+#include <QString>
+
+// Abstract host-side transport for a simulator aux serial port.
+//
+// The simulator firmware sees a generic byte stream; on the host side
+// the bytes can come from a real QSerialPort device, a local socket,
+// or any other source. Each backend implementation hides the
+// transport details behind this interface so HostSerialConnector can
+// stay transport-agnostic.
+//
+// Backends are owned by HostSerialConnector and live on the same
+// thread as the connector. Implementations should emit dataReceived
+// for incoming bytes and errorOccurred (with a user-facing message)
+// when something goes wrong opening or operating the transport.
+class HostSerialBackend : public QObject
+{
+    Q_OBJECT
+
+  public:
+    explicit HostSerialBackend(QObject * parent = nullptr) : QObject(parent) {}
+    ~HostSerialBackend() override = default;
+
+    virtual bool open() = 0;
+    virtual void close() = 0;
+    virtual bool isOpen() const = 0;
+
+    virtual void write(const QByteArray & data) = 0;
+
+    // No-ops for transports without a notion of bit-level encoding /
+    // baudrate (e.g. local sockets). Serial backends override these.
+    virtual void setBaudrate(quint32 /*baudrate*/) {}
+    virtual void setEncoding(quint8 /*encoding*/) {}
+
+    // Human-readable identifier (port name, socket path, ...) shown
+    // in dialogs and log output.
+    virtual QString displayName() const = 0;
+
+  signals:
+    void dataReceived(const QByteArray & data);
+    void errorOccurred(const QString & message);
+};

companion/src/simulation/hostserialbackend_serialport.cpp

@@ -0,0 +1,96 @@
+/*
+ * Copyright (C) EdgeTX
+ *
+ * Based on code named
+ *   opentx - https://github.com/opentx/opentx
+ *   th9x - http://code.google.com/p/th9x
+ *   er9x - http://code.google.com/p/er9x
+ *   gruvin9x - http://code.google.com/p/gruvin9x
+ *
+ * License GPLv2: http://www.gnu.org/licenses/gpl-2.0.html
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include "hostserialbackend_serialport.h"
+
+#include "simulatorinterface.h"
+
+#include <QtDebug>
+
+QSerialPortBackend::QSerialPortBackend(const QString & portName, QObject * parent)
+  : HostSerialBackend(parent),
+    port(new QSerialPort(portName, this))
+{
+  connect(port, &QSerialPort::readyRead, this, [this]() {
+    emit dataReceived(port->readAll());
+  });
+}
+
+QSerialPortBackend::~QSerialPortBackend()
+{
+  if (port->isOpen())
+    port->close();
+}
+
+bool QSerialPortBackend::open()
+{
+  if (port->open(QIODevice::ReadWrite)) {
+    qDebug() << "Opened host serial" << port->portName();
+    return true;
+  }
+  emit errorOccurred(port->errorString());
+  return false;
+}
+
+void QSerialPortBackend::close()
+{
+  port->close();
+}
+
+bool QSerialPortBackend::isOpen() const
+{
+  return port->isOpen();
+}
+
+void QSerialPortBackend::write(const QByteArray & data)
+{
+  port->write(data);
+}
+
+void QSerialPortBackend::setBaudrate(quint32 baudrate)
+{
+  if (!port->setBaudRate(baudrate))
+    qDebug() << "Failed to set baudrate";
+}
+
+void QSerialPortBackend::setEncoding(quint8 encoding)
+{
+  switch (encoding) {
+    case SERIAL_ENCODING_8N1:
+      port->setDataBits(QSerialPort::Data8);
+      port->setParity(QSerialPort::NoParity);
+      port->setStopBits(QSerialPort::OneStop);
+      break;
+    case SERIAL_ENCODING_8E2:
+      port->setDataBits(QSerialPort::Data8);
+      port->setParity(QSerialPort::EvenParity);
+      port->setStopBits(QSerialPort::TwoStop);
+      break;
+    default:
+      // QSerialPort can't do SERIAL_ENCODING_PXX1_PWM
+      break;
+  }
+}
+
+QString QSerialPortBackend::displayName() const
+{
+  return port->portName();
+}

companion/src/simulation/hostserialbackend_serialport.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) EdgeTX
+ *
+ * Based on code named
+ *   opentx - https://github.com/opentx/opentx
+ *   th9x - http://code.google.com/p/th9x
+ *   er9x - http://code.google.com/p/er9x
+ *   gruvin9x - http://code.google.com/p/gruvin9x
+ *
+ * License GPLv2: http://www.gnu.org/licenses/gpl-2.0.html
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#pragma once
+
+#include "hostserialbackend.h"
+
+#include <QSerialPort>
+
+// HostSerialBackend implementation backed by a real QSerialPort
+// device (USB-serial adapter, native UART, ...). Mirrors the original
+// HostSerialConnector behaviour 1:1.
+class QSerialPortBackend : public HostSerialBackend
+{
+    Q_OBJECT
+
+  public:
+    QSerialPortBackend(const QString & portName, QObject * parent = nullptr);
+    ~QSerialPortBackend() override;
+
+    bool open() override;
+    void close() override;
+    bool isOpen() const override;
+
+    void write(const QByteArray & data) override;
+
+    void setBaudrate(quint32 baudrate) override;
+    void setEncoding(quint8 encoding) override;
+
+    QString displayName() const override;
+
+  private:
+    QSerialPort * port;
+};