How to get data from a non-member function into a member function in Qt?-CodePudding

I have a Qt project (in Win10) that requires a non-member function in my MainWindow cpp file. It's a callback function that is triggered automatically from an external library when data from an attached sensor arrives. I like to display this data on a MainWindow widget (a plainTextEdit), but of course I cannot access any MainWindow elements from my callback function, and also no signal-slot concept works so far. Below is a minimal example of the problem:

MainWindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QMutex>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

    void displayData();

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

MainWindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

// define global variables
QMutex mutex;
bool new_data_arrived = false;
QString newDataStr;

void newData() {
// This is a callback function triggered when data from a sensor attached via USB arrives
// I want to pass the result as a QString to MainWindow::displayData() to display it on screen 
    mutex.lock();
    new_data_arrived = true;
    newDataStr = "new data";
    mutex.unlock();
};

void MainWindow::displayData()
{
    mutex.lock();
    if (new_data_arrived == true) {
        new_data_arrived = false;
        ui->plainTextEdit->appendPlainText(newDatastr);
    }  
    mutex.unlock();
}

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

The function MainWindow::displayData() is then running in a loop (but that’s unimportant for my problem, so I removed it from the minimal example above…).

As mentioned, newData() cannot access any elements of MainWindow, which is clear. The only workaround I found so far is to use global variables as shown above, to get data from the non-member function newData() into the member function displayData(). I also included a mutex to avoid possible share violation (no idea if that's necessary, but it doesn't harm...).

Is there a more elegant way? In principle, what I need is either a signal-slot concept, but newData() cannot emit signals of MainWindow, or to get this into the function newData(), so that ui elements can be accessed.

CodePudding user response：

You should use QMetaObject::invokeMethod to call a slot of a QObject from a non QObject slot (your callback funtion).

Here is an example:

class MainWindow : public QMainWindow
{
    Q_OBJECT
    //...
    
public slots:
    void displayData( QString szNewData );
};


//Cpp MainWindow.cpp file
static MainWindow * mainWindowObj = nullptr;

void setMainWindowObj( MainWindow * ptr ){
    mainWindowObj = ptr;
}

void newData() {
    // This is a callback function triggered when data from a sensor attached via USB arrives
    // I want to pass the result as a QString to MainWindow::displayData() to display it on screen 

    if( mainWindowObj == nullptr )
        return;
        
    QMetaObject::invokeMethod( mainWindowObj, //pointer to MainWindow instance (see main.cpp)
                               "displayData", //slot name
                               Qt::QueuedConnection, //connection type
                               Q_ARG( QString, newDataStr ) ); //Param passed to the slot
};

void MainWindow::displayData( QString szNewData )
{
    ui->plainTextEdit->appendPlainText( szNewData );
}


//main.cpp file or where MainWindow instance is initialized

void setMainWindowObj( MainWindow * ptr );

int main( int argc, char *argv[] ){

    //....
    QApplication app(argc, argv);
    
    //....
    MainWindow mainWindow;
    setMainWindowObj ( &mainWindow );
    
    //....
}