QLabel setText() only updates when adjusting window size

shawnlong

Hello all,

I am still new to Qt, but as I am starting to get the hang of things, I ran into a weird issue. I tried to write a simple program with two buttons and a counter, where the label adds 1 or subtracts 1 when you click the corresponding button. The thing is, after clicking the button, the label only changes if I adjust the size of the window while the app is open. I tried using label->update() but it did not work. Any help or a better way to make this simple app would be much appreciated.

Running on Mac OS X 10.15.4
Qt Creator 4.12.0, Based on Qt 5.14.2 (Clang 10.0 (Apple))

Source Code (mainWindow.cpp)
#include "mainwindow.h"
#include <QVBoxLayout>
#include <QPushButton>
#include <QHBoxLayout>
#include <QTextEdit>
#include <QSplitter>
#include <QTreeView>
#include <QListView>
#include <QFrame>
#include <QLabel>
#include <QString>
#include <QSignalMapper>

MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
{
QString labeltext = "Counter: "+QString::number(MainWindow::counter);
QVBoxLayout *rightVertBox = new QVBoxLayout();
QHBoxLayout *mainbox = new QHBoxLayout();
addButton = new QPushButton("Add 1", this);
subtractButton = new QPushButton("Subtract 1", this);
label = new QLabel(labeltext,this);
QWidget *centralWidget = new QWidget();
QWidget *rightWrapper = new QWidget();

mainbox->addWidget(label);
    rightVertBox->addWidget(addButton);
    rightVertBox->addWidget(subtractButton);
        rightWrapper->setLayout(rightVertBox);
mainbox->addWidget(rightWrapper);

connect( addButton, SIGNAL ( clicked() ), this, SLOT ( buttonAdd() ) );
connect( subtractButton, SIGNAL ( clicked() ), this, SLOT ( buttonSubtract() ) );

centralWidget->setLayout(mainbox);
setCentralWidget(centralWidget);

}

MainWindow::~MainWindow()
{
}

void MainWindow::buttonAdd()
{
MainWindow::counter++;
QString labeltext = "Counter: "+QString::number(MainWindow::counter);
label->setText(labeltext);
label->update();
MainWindow::update();
}

void MainWindow::buttonSubtract()
{
MainWindow::counter--;
QString labeltext = "Counter: "+QString::number(MainWindow::counter);
label->setText(labeltext);
label->update();
}

Source Code (mainwindow.h)
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QPushButton>
#include <QLabel>

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();

private slots:
void buttonAdd();
void buttonSubtract();
private:
int counter = 0;
QLabel *label;
QPushButton *addButton;
QPushButton *subtractButton;
};
#endif // MAINWINDOW_H

sierdzio

Your code is OK. I've run it on my side and it works great. This must be some platform or GPU issue. Try on a different operating system, or with different Qt version. Make sure you have all the updates installed.

Some other comments:

connect( addButton, SIGNAL ( clicked() ), this, SLOT ( buttonAdd() ) );
connect( subtractButton, SIGNAL ( clicked() ), this, SLOT ( buttonSubtract() ) );

I strongly recommend using the new connection syntax, like this:

connect( addButton, &QPushButton::clicked, this, &MainWindow::buttonAdd);

It is checked at compile time and generally more powerful than the old syntax.

QLabel *label;
QPushButton *addButton;
QPushButton *subtractButton;

Here I recommend initializing the pointers in headers files, like:

QLabel *label = nullptr;
QPushButton *addButton = nullptr;
QPushButton *subtractButton = nullptr;

This way you won't get garbage values there even if you forget to initialize in constructor. Or you can use QPointer.

sierdzio