Qt: Force a repaint to update text from fast button clicks? - c++

Question

I have some very simple code that displays a QPushButton which, when clicked, updates a spinbox with a random number 1 - 100. The problem is I can click the button many times in quick succession and only see one or two updates in the spinbox.
How can I repaint the spinbox for each click on the QPushButton? I've verified that I am firing and catching multiple click signals, but Qt doesn't repaint most of them.
So far I've tried calling repaint(), repaint() on all parent widgets, sendPostedEvents(), and processEvents().
#include <QtWidgets/QWidget>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QSpinBox>
#include <QtWidgets/QLayout>
#include <random>
#include <ctime>
class QtBtnEx : public QWidget
{
Q_OBJECT
public:
QtBtnEx(QWidget *parent = 0);
QPushButton* btn;
QSpinBox* spin;
public slots:
void onClicked();
};
QtBtnEx::QtBtnEx(QWidget *parent)
: QWidget(parent)
{
btn = new QPushButton("button");
spin = new QSpinBox();
btn->setFixedSize(90, 30);
spin->setFixedSize(90, 30);
this->setLayout(new QVBoxLayout());
this->layout()->setAlignment(Qt::AlignCenter);
this->layout()->addWidget(btn);
this->layout()->addWidget(spin);
connect(btn, &QPushButton::clicked, this, &QtBtnEx::onClicked);
}
//Fires for every button click but does not paint for every click
void QtBtnEx::onClicked()
{
srand(time(nullptr));
spin->setValue(rand() % 100);
}

Answer 1

Found my problem; I had the call to srand(time(nullptr)) in the slot code. It was responsible for the delays I was seeing. Pulled it up top and the spinbox refreshes immediately.
#include <QtWidgets/QWidget>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QSpinBox>
#include <QtWidgets/QLayout>
#include <random>
#include <ctime>
class QtBtnEx : public QWidget
{
Q_OBJECT
public:
QtBtnEx(QWidget *parent = 0);
QPushButton* btn;
QSpinBox* spin;
public slots:
void onClicked();
};
QtBtnEx::QtBtnEx(QWidget *parent)
: QWidget(parent)
{
srand(time(nullptr));
btn = new QPushButton("button");
spin = new QSpinBox();
btn->setFixedSize(90, 30);
spin->setFixedSize(90, 30);
this->setLayout(new QVBoxLayout());
this->layout()->setAlignment(Qt::AlignCenter);
this->layout()->addWidget(btn);
this->layout()->addWidget(spin);
connect(btn, &QPushButton::clicked, this, &QtBtnEx::onClicked);
}
void QtBtnEx::onClicked()
{
spin->setValue(rand() % 100);
}