QT not responding to touch event with external touch screen

Question

I am trying to use an HMDI/USB touchscreen with a QT application on Ubuntu. I am able to use the touchscreen to navigate in the OS, but my QT application is completely unresponsive to the touch events. The application type is a Desktop QT 6.8.1 with a simple button: enter image description here

This is the content of my mainwindow.cpp

#include "mainwindow.h"
#include "./ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent)
 : QMainWindow(parent)
 , ui(new Ui::MainWindow)
{
 ui->setupUi(this);
 setAttribute(Qt::WA_AcceptTouchEvents, true);
}
MainWindow::~MainWindow()
{
 delete ui;
}
void MainWindow::on_pushButton_pressed()
{
 qDebug() << "button pressed";
}
void MainWindow::on_pushButton_released()
{
 qDebug() << "button released";
}
bool MainWindow::event(QEvent* event)
{
 switch (event->type())
 {
 case QEvent::TouchBegin:
 qDebug() << "touch!";
 return true;
 case QEvent::TouchEnd:
 qDebug() << "touch end!";
 return true;
 case QEvent::MouseButtonDblClick:
 qDebug() << "double click";
 return true;
 case QEvent::MouseButtonPress:
 qDebug() << "pressed";
 return true;
 case QEvent::MouseButtonRelease:
 qDebug() << "released";
 return true;
 default:
 // call base implementation
 return QMainWindow::event(event);
 }
}

I am enabling touch events in the constructor. When I run the application, I can see the events debug messages when using the mouse. However, when using the touchscreen, none of the events are triggered. Not even the mouse ones!

Is there a reason why the touch event are not recognized?

Note: touch events are working completely fine on another UI framework (flutter).

• musicamante: Does tapping on the button trigger those functions (assuming they're properly set as slots and they also work when using the mouse)?
• ESD: @musicamante yes, but only with the mouse. not with the touchscreen.
• ESD: @AhmedAEK It should still print a debug message if the event is triggered at all no?
• musicamante: Note that you're using a QMainWindow with an UI set, which means that it has a central widget. Since you only set the attribute on the main window, the central widget will receive the event as a mouse event, and will then propagate it to the main window as such. As far as I know, in this case the event is not "restored" as a touch event, even if it wasn't accepted by the children. Try setting the attribute to the central widget and the button as well.
• Swift - Friday Pie: Is touch screen functional otherwise? It needs a correct configuration. Desktop and X11 should react on that. Also, HID-compatible driver for touch screen generates normal pointer events, including mouse ones, that goes to central widget, not to main window.

Answer 1

I had a similar problem (Qt5, however), and found out that accepting touch events wasn't recursively transmitted to children widgets...

So I wrote this function:

void AddTouchEventSupport ( QWidget* qw ) {
 qw->setAttribute(Qt::WA_AcceptTouchEvents, true) ;
 for ( auto&& w : qw->findChildren<QWidget*>(QString(),Qt::FindChildrenRecursively))
 w->setAttribute(Qt::WA_AcceptTouchEvents, true) ;
}

Replace your setAttribute(Qt::WA_AcceptTouchEvents, true); by AddTouchEventSupport(this); and test if it works.

• musicamante: That requirement is already explained in the documentation; besides, the OP has issues receiving touch events on the main window, not its children.