Qt making a QPushButton fill layout cell - qt

Question

I need a pushbutton to either fill or not fill the entire space provided by a QGridLayout cell upon the creation of the button (the alignment value is loaded from file). I've simplified my situation with the following code. During run-time, users can set the alignment of the button - either making it fill the entire layout cell or nicely centered. It works so long as the button didn't start off with NULL alignment specified. Yet, I need the ability to start off with a NULL alignment (i.e. the button fills the space of the layout cell). When initially aligning with NULL, what is getting set to make the button lock into a AlignVCenter setting and how can I get the button to return to acting like it was initialized with something other than null alignment?
I'm using Qt 4.8 on Ubuntu 12.04 LTS
#include <QPushButton>
#include <QGridLayout>
#include <QMainWindow>
#include <QApplication>
class MyWidget : public QWidget {
Q_OBJECT
QPushButton* m_pb;
QGridLayout* m_gl;
protected slots:
void pbClicked();
public:
MyWidget(QWidget* parent = 0);
};
MyWidget::MyWidget(QWidget* parent): QWidget(parent)
{
m_pb = new QPushButton(tr("push me"));
connect(m_pb, SIGNAL(clicked()), this, SLOT(pbClicked()));
m_gl = new QGridLayout();
//use (1) to see button expand when button is pressed
//use (2) to show that I can't start off expanded
/*1*/ //m_gl->addWidget(m_pb, 0, 0, Qt::AlignCenter); // creates desired effect
/*2*/ //m_gl->addWidget(m_pb, 0, 0, 0); //does not create desired effect
setLayout(m_gl);
}
void MyWidget::pbClicked(){
//will expand button so long as initial alignment is not NULL
m_gl->setAlignment(m_pb, 0);
}
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
MyWidget* widget = new MyWidget();
QMainWindow window;
window.setCentralWidget(widget);
window.show();
return app.exec();
}
#include "main.moc"

Answer 1

The "desired" behavior that you see is in fact an error, and I will file a bug report for it. Thanks for spotting it - nice corner case.
You need to set the size policy of the button to expanding in both directions. Buttons normally don't want to expand vertically, so if you tried a variant that toggles the alignment, you'd see that it works only horizontally, and that's correct.
This is a simple demonstration that shows the correct behavior that also fulfills your needs.
#include <QPushButton>
#include <QGridLayout>
#include <QApplication>
class AlignButton : public QPushButton {
Q_OBJECT
Qt::Alignment m_alignment;
Q_SLOT void clicked() {
m_alignment ^= Qt::AlignCenter;
parentWidget()->layout()->setAlignment(this, m_alignment);
label();
}
void label() {
setText(QString("Alignment = %1").arg(m_alignment));
}
public:
AlignButton(Qt::Alignment alignment, QWidget * parent = 0) :
QPushButton(parent),
m_alignment(alignment)
{
connect(this, SIGNAL(clicked()), SLOT(clicked()));
setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
label();
}
};
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
QWidget window;
QGridLayout layout(&window);
layout.addWidget(new AlignButton(0), 0, 0, 0);
layout.addWidget(new AlignButton(Qt::AlignCenter), 1, 0, Qt::AlignCenter);
window.setMinimumSize(500, 200);
window.show();
return app.exec();
}
#include "main.moc"