[SOLVED] QLabel won't update pixmap from inside function

SGaist

Are you doing anything else with these labels ? e.g. configuration ?

Tymer

@SGaist Just the original loop in the constructor of myWidget. The next time my code touches them is the changeImage().

Tymer

Back at work today, I remembered that the QLabels are in a QHorizontalLayout. AFAIK this shouldn't affect anything, but it's worth bringing up.

SGaist

Just to rule out a QPixmap problem.

What happens if you do

QImage img(120, 120, QImage::Format_ARGB32);
img.fill(Qt::red);
pointerArray[activePointer]->setPixmap(QPixmap::fromImage(img));
activePointer++;

?

Tymer

@SGaist Thank you for your continued help.

I tried that, no dice. When I debug, after running the line with setPixmap, I get pixmap (120x120) QVariant (QPixmap) as a property of the label that would be changed.

I'm also noticing that in a different part of a project, the layout doesn't want to update when I run a function. The only similarity is that both of them are labels in QLayouts.

SGaist

Can you try with the following widget:

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

class QLabel;

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);
    ~Widget();

public slots:
    void onClicked();

private:
    QVector<QLabel*> _labels;
    int _currentLabel;
    QColor _color;
};

#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include <QString>
#include <QLabel>
#include <QPushButton>
#include <QHBoxLayout>
#include <QVBoxLayout>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , _currentLabel(0)
    , _color(Qt::blue)
{
    QHBoxLayout *buttonLayout = new QHBoxLayout;
    QImage img(120, 120, QImage::Format_ARGB32);
    img.fill(Qt::red);
    for (int i = 0 ; i < 4 ; ++i) {
        QLabel *label = new QLabel;
        label->setPixmap(QPixmap::fromImage(img));
        buttonLayout->addWidget(label);
        _labels << label;
    }

    QPushButton *button = new QPushButton(tr("Test"));

    QVBoxLayout *layout = new QVBoxLayout(this);
    layout->addLayout(buttonLayout);
    layout->addWidget(button);

    connect(button, &QPushButton::clicked, this, &Widget::onClicked);
}

Widget::~Widget()
{

}

void Widget::onClicked()
{
    QImage img(120, 120, QImage::Format_ARGB32);
    img.fill(_color);

    _labels[_currentLabel]->setPixmap(QPixmap::fromImage(img));
    ++_currentLabel;
    if (_currentLabel == _labels.count()) {
        _currentLabel = 0;
        _color = QColor(qrand() % 255, qrand() % 255, qrand() % 255);
    }
}

main.cpp

#include "widget.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    Widget w;
    w.show();

    return a.exec();
}

Tymer

That's working perfectly for me. Maybe this will give me the clue I need to make it work...

Tymer

Ah, that one works because it is from within the same widget. I added a push button to myWidget and it was working. Unfortunately, the button that I need to push is in a different window.

~~ EDIT: The new button on myWidget works perfectly. I thought maybe I could circumvent the problems of the changeImage button by making it a slot for "emit buttonclicked," which I connected to the new button on myWidget. It had the same problem as before, where it does nothing and prints "end of array" after 11 clicks. BUT strangely, if I press the button on myWidget, it resets the value of activePointer. I'm not sure why this is.

Essentially, I can push the changeImage button 11 times until it prints "end of array," but then I can push the myWidget pushbutton 11 times and it will change the image 10 times and start printing "end of array" after that. Curious. ~~

EDIT EDIT: It looks like right now it's only changing the value of activePointer locally. Whoops. Problem still exists, however.

SGaist

The fact that the button is inside another widget should not have any impact here. Can you reproduce your bug using may sample as base ?

Tymer

I'll post the full code, just to be thorough. The error is exactly the same. When I debugged, it showed that my signal had reached the slot...But there was no change to the image.

widget1.h

#ifndef WIDGET1_H
#define WIDGET1_H

#include <QWidget>
#include "/absolute/path/to/widget.h"

namespace Ui {
class widget1;
}

class widget1 : public QWidget
{
    Q_OBJECT

    Widget* widget;

public:
    explicit widget1(QWidget *parent = 0);
    ~widget1();

signals:
    void clicked();
    void buttonPush();

private slots:
    void on_pushButton_clicked();

private:
    Ui::widget1 *ui;
};

#endif // WIDGET1_H

widget1.cpp

#include "widget1.h"
#include "ui_widget1.h"

widget1::widget1(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::widget1)
{
    ui->setupUi(this);

    widget = new Widget;

    connect(this, SIGNAL(buttonPush()),widget,SLOT(onClicked()));
}

widget1::~widget1()
{
    delete ui;
}

void widget1::on_pushButton_clicked()
{
    emit buttonPush();
}

An instance of widget1 was created in main, then shown. I created a ui file in designer, with all default values.

SGaist

Is it me or are you not showing widget at all ?

Tymer

My main.cpp:

#include "widget.h"
#include "widget1.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    Widget w;
    widget1 w1;
    w1.show();
    w.show();


    return a.exec();
}

SGaist

Ok, you are not showing the widget you have connected. Your Widget w in your main.cpp is not the same as the one you have instantiated in your widgte1 constructor.

Tymer

So I should make my widget a singleton, more or less? How would I go about that?

SGaist

No, not at all. Either:

Remove the widget from MainWindow and connect the one in main.cpp
Remove the one from main.cpp and show the one from MainWindow

Tymer

How would I connect to the instance in main?

SGaist

By using the static version of QObject::connect

Tymer

I know this is really basic, but I'm having a bit of a brain fart. I want to make a pointer to the instance I create in main, but I don't want my class to know too much about what's going on in main. How do I do this so that I can create a connection to the object in main from a different widget?

Tymer

Right now I have:

main.cpp


myWidget* w = new myWidget;
actionTestWindow* t = new actionTestWindow(w, new QWidget);

actionTestWindow.cpp

actionTestWindow::actionTestWindow(myWidget* Widget, QWidget *parent) :
    QWidget(parent),
    ui(new Ui::actionTestWindow)
    {
        ui->setupUi(this);

        Widget = new myWidget;

The actionTestWindow is no longer showing, and I think it has something to do with how I insert the QWidget parameter.

SGaist

Roll back to before you modified that constructor. Just call '''Widget->show();'''