how to get touchevent for mainwindow in qt - c++

Question

I am writing an application for arm board. I have a mainwindow and I need to get the touch co-ordinates for single touch. Due to lack of Qt knowledge I am facing a problem of getting the touch event.
As per the QTouchEvent Class document, I have declared a slot as below.
#include <QTouchEvent>
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(int x, int y, QWidget *parent = 0);
public slots:
void touchEvent(QTouchEvent *ev);
};
and in mainwindow.cpp I redefine it as below. The FFLabel[1] is already declared by me and I am writing it to confirm if the event is received.
void MainWindow::touchEvent(QTouchEvent *ev)
{
ui->FFLabel[1]->setText(QString("Event"));
switch (ev->type())
{
case QEvent::TouchBegin:
case QEvent::TouchEnd:
case QEvent::TouchUpdate:
{
}
}
}
I am not able to get the touchEvent. Can someone please help me.
I have verified the /dev/input/event2 and am receiving the event there.

Answer 1

QWidget has a number of protected virtual members, e.g., QWidget::mousePressEvent. They are called when the event occurs.
Usually one overrides such a function to run some code when a certain event happens.
The problem is, that, unlike for mouse press, there is no such virtual function QWidget::touchEvent (as of Qt 5.12). That is, you can't override it and it cannot be called when a touch event happens as QWidget is not aware of such a method.
C++11 introduced the override-keyword. If you had used it, you would have noticed that issue.
In order to receive touch events, you must override the more general QWidget::event method. Then filter for the events you're actually interested in:
bool MainWindow::event(QEvent* event)
{
switch (event->type()) {
case QEvent::TouchBegin:
qDebug() << "touch!";
return true;
default:
// call base implementation
return QMainWindow::event(event);
}
}
Note that you must state setAttribute(Qt::WA_AcceptTouchEvents); in order to receive any touch events (put it in the MainWindow-constructor).
You can read more details in the docs.

Answer 2

Events aren't slots. Declare it as a protected function.
protected:
void touchEvent(QTouchEvent *ev);
Edit: And you didn't put anything in your switch statement to show if you get the event.
void MainWindow::touchEvent(QTouchEvent *ev)
{
switch (ev->type())
{
case QEvent::TouchBegin:
ui->FFLabel[1]->setText("Event began.");
break;
case QEvent::TouchEnd:
ui->FFLabel[1]->setText("Event ended.");
break;
case QEvent::TouchUpdate:
{
ui->FFLabel[1]->setText("Event updated.");
break;
}
}
}
Edit: If it's still not working, it means your mainwindow isn't accepting touch events. You need to do this in your mainwindow's constructor:
setAttribute(Qt::WA_AcceptTouchEvents, true);

Answer 3

Disclaimer:
I had a similar issue and this is not a solution but may be a workaround depending on what you want to do.
In my case, the touch events were interpreted as mouse events so, MainWindow::touchEvent was never called but, the mouse events handler were.
I used MainWindow::mousePressEvent to get the touch point and MainWindow::mouseMoveEvent to get the drag.
This does not handle the multi-touch (it works for the separate contact points but you will have only one move event for all your points)
From Qt5.4, one may try (I didn't test as I'm stuck with Qt4), in the mouse events handler, to check the MouseEventSource If it's MouseEventSynthesizedBySystem it means from the documentation that:
the mouse event was synthesized from a touch event by the platform.
One can find more details checking this solution provided by Chirag Sachdeva in this post
And of course, with regards to the OP code, as user4516901 said, events aren't slots.