[SOLVED] Updating the text on a QLabel

[notgary]

I trying to use the following code to change the text in a QLabel from "Ready." to "Boo!" when I press the button marked "Change text", but when the button is clicked, nothing happens. I've looked through the QLabel documentation and can't find anything that would indicate that I need to set some special flag to allow updating of text, so I was wondering if there was something else that I'm missing here.

@#include <QtGui>

int main( int argc, char** args )
{
QApplication app( argc, args );

QLabel *label = new QLabel;
label->setText( "Ready." );

QPushButton *button = new QPushButton;
button->setText( "Change text" );
QObject::connect( button, SIGNAL( clicked() ), label, SLOT( setText( "Boo!" ) ) );

QHBoxLayout *layout = new QHBoxLayout;
layout->addWidget( button );
layout->addWidget( label );

QMainWindow *mainWindow = new QMainWindow;
QWidget *centralWidget = new QWidget;
centralWidget->setLayout( layout );
mainWindow->setCentralWidget( centralWidget );
mainWindow->show();

return app.exec();
}@

[umerlaton]

Hi there,
hmmm i believe the problem is when you connect the signal and the slot. both must have the same parameter.Your button is emiting a clicked signal but nothing else and your qlabel slot settext(QString) is expecting a Qstring from the emitter . I believe you cant just put "Boo!". You can have a public slot in your class with no parameters and call seText("Boo!") from there.

[dbzhang800]

The following code is basically wrong.
@
QObject::connect( button, SIGNAL( clicked() ), label, SLOT( setText( "Boo!" ) ) );
@

When you running the application under debug mode, you should get a warning message says that, the slot you used doesn't exists.

In addition, this "connect" statement will return false instead of true.

[maxvanceffer]

Use for debug porpose Q_ASSERT(connect(SIGNAL()),SLOT(());

And u will see in console output, which connections not connected.

[mlong]

Using Q_ASSERT(connect(...)) is dangerous because in release mode the Q_ASSERT line will be ignored and your connect will not happen at all.

If you still want to use Q_ASSERT to test your connection, do it as follows:
@
bool connected = connect(...);
Q_ASSERT(connected);
@

[notgary]

Thanks a lot for your replies. It appears that the code I posted didn't effectively illustrate my situation. It was only an example I cobbled together for posting here. In my actual code, the signals are successfully connected, as the std::cout statements I placed in them for debugging were successfully triggered.

My problem seems to be that the label is not being updated on the UI, as I can successfully extract the new text using the labsl's text() method, but the text being displayed is still the old one. Does this behaviour sound familiar to anyone?

[mlong]

Are you certain that you don't somehow have a second QLabel somewhere, perhaps?

Can you post a snippet of your code, itself?

[notgary]

There are two classes involved in this process, the label itself and one of its members, a dialog, that is being used to accept the new text. The slot that's actually performing the rename is at the very bottom. The skeletons of both classes are as follows:

@class RenameTitleDialog : public QDialog
{
Q_OBJECT
public:
RenameTitleDialog( QWidget *parent = NULL );
QString editorText();

private:
QPushButton *m_confirmButton;
QLineEdit *m_titleEditor;
};

RenameTitleDialog::RenameTitleDialog( QWidget *parent ) : QDialog( parent )
{
m_confirmButton = new QPushButton;
m_confirmButton->setText( tr( "&Confirm" ) );
m_confirmButton->setDefault( true );
m_confirmButton->setEnabled( false );
connect( m_confirmButton, SIGNAL( clicked() ), this, SLOT( accept() ) );
}

QString RenameTitleDialog::editorText()
{
return m_titleEditor->text();
}

class Task : public QLabel
{
Q_OBJECT
public:
Task( QWidget *parent = NULL, QString m_taskTitle = QString() );

private:
RenameTitleDialog *m_renameTitleDialog;

private slots:
void slotOpenRenameTitleDialog();
void slotRenameTitle();
};

Task::Task( QWidget* parent, QString title ) :
QLabel( parent )
{
setText( title );
m_renameTitleDialog = new RenameTitleDialog( this );
setStyleSheet( "Task{ background-color: yellow; color: black; border: 1px solid black; border-radius: 4px }");
createActions();
}

void Task::createActions()
{
connect( actionRenameTitle, SIGNAL( triggered() ), this, SLOT( slotOpenRenameTitleDialog() ) );
connect( m_renameTitleDialog, SIGNAL( accepted() ), this, SLOT( slotRenameTitle() ) );
}

void Task::slotOpenRenameTitleDialog()
{
m_renameTitleDialog->exec();
}

void Task::slotRenameTitle()
{
QString newTitle = m_renameTitleDialog->editorText();
setText( newTitle );
}@

[joonhwan]

On your OT, there is QSignalMapper class is for you, I think.

[notgary]

Ok, I've compiled the code on Mac OS and the slots are now working perfectly. I don't know what was messed up with my Linux environment (where it was being compiled before), but my code was in fact fine.