how to force Qt mainWindow to wait for some changes - c++

Question

I am designing UI for my opencv application and I'm new in Qt.
I have a QString in my Qt code that is a licence plate. and it can be change when a car arrive to our camera, but we don't know when it change.
and when car arrive we need to show it's plate on a QTableView on MainWindow.
how can I handle it in Qt?
I make my code simple here:
mainWindow.h
class MainWindow : public QMainWindow
{
Q_OBJECT
.
.
.
public slots:
void set_plate(QString p);
signals:
void plate_changed(QString newPlate);
private:
Ui::MainWindow *ui;
QString plate;
};
mainWindow.cpp
void MainWindow::set_plate(QString newPlate)
{
if(plate != newPlate){
plate = newPlate;
emit plate_changed(newPlate);
}
}
MainWindow::MainWindow(QString p, QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow),
plate(p)
{
.
.
.
QTableWidgetItem* item2;
item2 = new QTableWidgetItem(plate);
ui->table->setItem(table->rowCount() - 1, 0, item2);
}
main.cpp
int main(int argc, char *argv[])
{
QString plate;
.
.
.
//some changes on plate
QApplication a(argc, argv);
MainWindow w(plate);
w.show();
return a.exec();
}
thanks

Answer 1

When you detect a car, send a signal to your instance. Connect your emitted signal to your slot and that's all.
https://wiki.qt.io/Qt_for_Beginners#Creating_custom_signals_and_slots

Answer 2

Let's begin with a paraphrase of the code for main that you've posted
void someChangesOnPlate(QString &);
int main(int argc, char *argv[])
{
QString plate;
someChangesOnPlate(plate);
QApplication a(argc, argv);
MainWindow w(plate);
w.show();
return a.exec();
}
It seems that someChangesOnPlate would be a plate detection code that runs continuously in a loop, taking images from the camera, detecting plates, and extracting the plate numbers.
The main mistake in your approach is: while the plate detection code is running, the application window isn't shown yet - the code after someChangesOnPlate hasn't executed yet! The execution is within someChangesOnPlate, after all.
What you need to do, instead, is to run someChangesOnPlate in a separate thread, and to have that thread invoke the main window's newPlate method in a thread-safe fashion. Given C++11, this is a very straightforward change:
void someChangesOnPlate(MainWindow *);
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w(plate);
w.show();
std::thread thread(someChangesOnPlate, &w);
int rc = a.exec();
thread.join(); // wait for the thread to finish
return rc;
}
Within the someChangesOnPlate code, the thread-safe call to newPlate is done as follows:
void someChangesOnPlate(MainWindow * w) {
while (...) {
// process camera images
...
QString plate = ....;
...
QMetaObject::invoke(w, "newPlate", Qt::QueuedConnection, Q_ARG(QString, plate));
...
}
}
Finally, I understand that you want a new item to be added to the table whenever a new plate is indicated. Here's how to do it:
class MainWindow : public QMainWindow {
Q_OBJECT
Ui::MainWindow ui; // no need for it to be a pointer
QString m_lastPlate;
public:
MainWindow(QWidget * parent = 0) : QMainWindow(parent) {
ui.setupUi(this);
}
Q_SLOT void newPlate(const QString & plate) {
if (plate == m_lastPlate) return;
m_lastPlate = plate;
auto item = new QTableWidgetItem(plate);
ui->table->setItem(table->rowCount() - 1, 0, item);
}
}