#include <QtWidgets>

class MyWidget : public QWidget
{
    Q_OBJECT
public:
    MyWidget(QWidget *parent = 0) : QWidget(parent) 
    {
        QVBoxLayout *vbox = new QVBoxLayout;
        setLayout(vbox);
        QTimer::singleShot(5000, this, SLOT(addWidget()));
    }
    public slots:
        void addWidget()
        {
            layout()->addWidget(new QLineEdit);
        }
protected:
    void resizeEvent(QResizeEvent *e) { 
        if (e->spontaneous()) 
            qDebug("Catch"); 
    }
};

#include "main.moc"

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