How to automatically update a statusbar message? - c++

Question

I am using Qt 5.15.1 and C++ to create a simple app which processes signals from hardware and displays images and driver status. I want to update the statusbar message when an int value defined in another class changes. I would like this to happen automatically, each time this value changes. I understand that I need signals and slots to achieve this. So far I have done the following:
signalprocessing.h
class SignalProcessing: public QObject
{
Q_OBJECT
public:
SignalProcessing(QObject *parent = nullptr);
private:
int status;
public slots:
int GetStatus();
signals:
void StatusChanged();
}
signalprocessing.cpp
SignalProcessing::SignalProcessing(QObject *parent)
: QObject(parent)
{
}
int SignalProcessing::GetStatus()
{
emit(StatusChanged());
return status;
}
mainwindow.h
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
virtual ~MainWindow();
SignalProcessing *signalProcessing;
}
mainwindow.cpp
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
signalProcessing= new SignalProcessing(this);
ui->statusbar->showMessage(QString::number(signalProcessing->GetStatus()));
}
The problem is that the statusbar message is currently not updated automatically, but set to a given value.
How do I make sure it will always display the current status?

Answer 1

The idea is that you want to emit your StatusChanged signal when the value has actually changed, not when you call GetStatus(). So you need a SetStatus() function, and your SignalProcessing class will need to know when to call that. Then you want to connect that signal to a slot that then updates your status bar. It might look something like this:
class SignalProcessing: public QObject
{
Q_OBJECT
public:
SignalProcessing(QObject *parent = nullptr);
int getStatus();
void setStatus(int value);
private:
int status;
signals:
void statusChanged(); // It's better to start signals with a lower case letter
}
signalprocessing.cpp
SignalProcessing::SignalProcessing(QObject *parent)
: QObject(parent)
{
}
int SignalProcessing::getStatus()
{
return status;
}
void SignalProcessing::setStatus(int value)
{
if (status != value)
{
status = value;
emit statusChanged();
}
}
mainwindow.h
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
virtual ~MainWindow();
SignalProcessing *signalProcessing;
public slots:
void updateStatus();
}
mainwindow.cpp
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
signalProcessing= new SignalProcessing(this);
connect(signalProcessing, &SignalProcessing::statusChanged, this, &MainWindow::updateStatus);
updateStatus();
}
void MainWindow::updateStatus()
{
ui->statusbar->showMessage(QString::number(signalProcessing->getStatus()));
}

Answer 2

First change singal declartion so it will provide new value:
class SignalProcessing: public QObject
{
Q_OBJECT
public:
SignalProcessing(QObject *parent = nullptr);
private:
int status;
public slots:
int setStatus(int value);
signals:
void statusChanged(const QString& message);
}
void SignalProcessing::setStatus(int value)
{
if (status != value)
{
status = value;
emit statusChanged(tr("Status is %1").arg(status));
}
}
Then connect this signal to QStatusBar::showMessage slot and you done.