example RQT plugin in C++ that uses signals/slots

edit

I finally figured it out. Thanks for all the helpful comments below.

I was trying to define both the signal and slot in my own class, per the various QT examples I had found, but I really wanted to signal the existing slot in the existing QLabel class. For this scenario the solution is way simpler than the QT examples I was using which do not signal an existing slot in an existing class. This is confusing to new ROS users who need to signal existing QT widgets in their RQT plugins. This is exactly why I was (and still am) pushing for this signal/slot example code to be added to the ROS C++ RQT Plugin tutorial.

Here is a simple diagram for newbies:

Here is the GUI definition file my_plugin.ui:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MyPluginWidget</class>

 <widget class="QWidget" name="MyPluginWidget">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>

  <!-- kurt added this widget -->
  <widget class="QLabel" name="label_myLabel">
   <property name="text">
    <string>Label:</string>
   </property>
   <property name="geometry">
    <rect>
     <x>10</x>
     <y>10</y>
     <width>50</width>
     <height>30</height>
    </rect>
   </property>
  </widget>

  <!-- kurt added this widget -->
  <widget class="QLabel" name="data_myData">
   <property name="text">
    <string>TBD</string>
   </property>
   <property name="geometry">
    <rect>
     <x>70</x>
     <y>10</y>
     <width>50</width>
     <height>30</height>
    </rect>
   </property>
   <property name="styleSheet">
    <string notr="true">color: green; background-color: black;</string>
   </property>
  </widget>

 </widget>

 <resources/>
 <connections/>
</ui>

Here is the header file my_plugin.h:

#ifndef rqt_example_cpp_my_plugin_H
#define rqt_example_cpp_my_plugin_H

#include <rqt_gui_cpp/plugin.h>
#include <rqt_example_cpp/ui_my_plugin.h>
#include <QWidget>

// kurt added these 3 includes
#include <ros/ros.h>
#include <std_msgs/Int32.h>
#include <QString>

namespace rqt_example_cpp {

class MyPlugin
  : public rqt_gui_cpp::Plugin
{
  Q_OBJECT
public:
  MyPlugin();
  virtual void initPlugin(qt_gui_cpp::PluginContext& context);

private:
  Ui::MyPluginWidget ui_;
  QWidget* widget_;

  // kurt added these lines to get data from a ROS Topic
  ros::NodeHandle ros_node_handle;
  ros::Subscriber my_subscriber;
  void ros_data_callback(const std_msgs::Int32 message);

// kurt added these signal definitions to pass data to QLabel on GUI
Q_SIGNALS:
  void setText(const QString str);
  void setStyleSheet(const QString str);

};

}  // namespace rqt_example_cpp
#endif  // rqt_example_cpp_my_plugin_H

And here is the source file my_plugin.cpp:

#include "rqt_example_cpp/my_plugin.h"
#include <pluginlib/class_list_macros.h>
#include <QStringList>

namespace rqt_example_cpp {

MyPlugin::MyPlugin()
  : rqt_gui_cpp::Plugin()
  , widget_(0)
{
  setObjectName("MyPlugin");
}

void MyPlugin::initPlugin(qt_gui_cpp::PluginContext& context)
{
  QStringList argv = context.argv();
  widget_ = new QWidget();
  ui_.setupUi(widget_);
  context.addWidget(widget_);

  // kurt added this ROS Topic subscriber
  my_subscriber = ros_node_handle.subscribe("my_data", 1, &MyPlugin::ros_data_callback, this);
   // kurt added these lines to connect the emitted signals below to the GUI's QLabel's public slots
  QObject::connect( this, SIGNAL(setText(const QString)),
            ui_.data_myData, SLOT(setText(const QString))   );
  QObject::connect( this, SIGNAL(setStyleSheet(const QString)),
            ui_.data_myData, SLOT(setStyleSheet(const QString)) );
}

// kurt added this ROS topic callback to get data from ROS and send signals to QLabel on GUI
void MyPlugin::ros_data_callback(const std_msgs::Int32 message ...