Detect when a widget looses focus?

[thomaspu]

This might be a stupid question, but I'll ask it anyway. I need to inherit the QTextEdit and reimpliment the focusOutEvent to be able to be able to send a signal that it lost focus right? Or is there a better way to do it?

Something like..

class PaulsQTextEdit: public QTextEdit
{
  Q_OBJECT
      .....
   void focusOutEvent(QFocusEvent *e);
 
public signals:
  void lostFocus();
 
};
 
void PaulsQTextEdit::focusOutEvent(QFocusEvent *e)
{
    if (e->lostFocus() )
      emit(lostFocus() );
 
   QTextEdit::focusOutEvent(e);
 
}

class PaulsQTextEdit: public QTextEdit
{
  Q_OBJECT
      .....
   void focusOutEvent(QFocusEvent *e);

public signals:
  void lostFocus();

};

void PaulsQTextEdit::focusOutEvent(QFocusEvent *e)
{
    if (e->lostFocus() )
      emit(lostFocus() );

   QTextEdit::focusOutEvent(e);

}

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[jpn]

Yes, something like that. You can drop the "public" keyword in front of signals declaration, though.

By the way, there is another way to catch events too, namely event filter. Installing an event filter doesn't require subclassing but in my humble opinion leads to a bit messier code.

[thomaspu]

I really have no idea how to implement that as an event filter.

Wait, if the event filter that I implement sends out, say, a lostFocus signal and that event filter is installed on the QTextEdit, does the QTextEdit essentially now have a lostFocus signal that I can connect too?

Paul

[jpn]

Perhaps you wouldn't need a signal at all if you used an event filter. The object who must know when a QTextEdit loses focus could be the filtering object itself.

[thomaspu]

I'm not sure I follow you. Like in my scenario I have two widgets, textEdit and LineEdit and a button that inserts text. I want the button click to insert text in the widget that had focus lost. So are you saying that the button could some way filter the textEdit's loosing focus event? Or am I looking at this wrong?

Paul

[jpn]

Let the example speak for itself:

// main.cpp
#include <QtGui>
 
class Window : public QWidget
{
	Q_OBJECT
 
public:
	Window(QWidget* parent = 0) : QWidget(parent), focused(0)
	{
		QVBoxLayout* layout = new QVBoxLayout(this);
		QPushButton* button = new QPushButton("Paste", this);
		connect(button, SIGNAL(clicked()), this, SLOT(paste()));
		layout->addWidget(button);
 
		for (int i = 0; i < 5; ++i)
		{
			QLineEdit* lineEdit = new QLineEdit(this);
			lineEdit->installEventFilter(this); // <--
			layout->addWidget(lineEdit);
		}
	}
 
	bool eventFilter(QObject* object, QEvent* event)
	{
		if (event->type() == QEvent::FocusIn)
		{
			focused = qobject_cast<QLineEdit*>(object); // <--
		}
		return false;
	}
 
private slots:
	void paste()
	{
		if (focused)
		{
			focused->setText(qApp->clipboard()->text());
		}
	}
 
private:
	QLineEdit* focused;
};
 
int main(int argc, char* argv[])
{
	QApplication a(argc, argv);
	Window w;
	w.show();
	return a.exec();
}
 
#include "main.moc"

// main.cpp
#include <QtGui>

class Window : public QWidget
{
	Q_OBJECT

public:
	Window(QWidget* parent = 0) : QWidget(parent), focused(0)
	{
		QVBoxLayout* layout = new QVBoxLayout(this);
		QPushButton* button = new QPushButton("Paste", this);
		connect(button, SIGNAL(clicked()), this, SLOT(paste()));
		layout->addWidget(button);

		for (int i = 0; i < 5; ++i)
		{
			QLineEdit* lineEdit = new QLineEdit(this);
			lineEdit->installEventFilter(this); // <--
			layout->addWidget(lineEdit);
		}
	}

	bool eventFilter(QObject* object, QEvent* event)
	{
		if (event->type() == QEvent::FocusIn)
		{
			focused = qobject_cast<QLineEdit*>(object); // <--
		}
		return false;
	}

private slots:
	void paste()
	{
		if (focused)
		{
			focused->setText(qApp->clipboard()->text());
		}
	}

private:
	QLineEdit* focused;
};

int main(int argc, char* argv[])
{
	QApplication a(argc, argv);
	Window w;
	w.show();
	return a.exec();
}

#include "main.moc"

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[GrayOwl]

This might be a stupid question, but I'll ask it anyway. I need to inherit the QTextEdit and reimpliment the focusOutEvent to be able to be able to send a signal that it lost focus right? Or is there a better way to do it?

Something like..

class PaulsQTextEdit: public QTextEdit
{
  Q_OBJECT
      .....
   void focusOutEvent(QFocusEvent *e);
 
public signals:
  void lostFocus();
 
};
 
void PaulsQTextEdit::focusOutEvent(QFocusEvent *e)
{
    if (e->lostFocus() )
      emit(lostFocus() );
 
   QTextEdit::focusOutEvent(e);
 
}

class PaulsQTextEdit: public QTextEdit
{
  Q_OBJECT
      .....
   void focusOutEvent(QFocusEvent *e);

public signals:
  void lostFocus();

};

void PaulsQTextEdit::focusOutEvent(QFocusEvent *e)
{
    if (e->lostFocus() )
      emit(lostFocus() );

   QTextEdit::focusOutEvent(e);

}

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An alternative way is to use:

connect(qApp, &QApplication::focusChanged, [this](QWidget *from, QWidget *to) {
        auto w = from;
 
        if ( (w != nullptr) && (w == [B]__yourWidget__[/B]) )
           // send your signal
    } );

connect(qApp, &QApplication::focusChanged, [this](QWidget *from, QWidget *to) {
        auto w = from;

        if ( (w != nullptr) && (w == [B]__yourWidget__[/B]) )
           // send your signal
    } );

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Doc: https://doc.qt.io/qt-5/qapplication.html#focusChanged

[d_stranz]

@GrayOwl - you do realize that this thread is almost 14 years old, right? Please don't pull old threads out of their graves if all you are adding is a link to the documentation.