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

Tymer

@SGaist I actually just tried it with them on the stack, it seems to work now, not sure why it wouldn't before. Unfortunately, that didn't fix my issue.

Thank you for all the help.

SGaist

How are you calling changeImage ?
Are you calling setUpdatesEnabled(false); somewhere ?

Tymer

Not calling changeImage() anywhere, just setPixmap(). I actually make a point of running updatesEnabled() before the pixmap is changed.

SGaist

updatesEnabled is a getter that will tell you whether updates are enabled or not. They are by default.

If you don't call changeImage anywhere, how can you expect your labels to get updated with whitex ?

Tymer

I don't understand changeImage, I haven't read anything about it until now. Can you send some documentation?

EDIT: I just remembered that I didn't mention this outside of my tags. I'm using Qt 4.8.7.

SGaist

It's the void myWidget::changeImage()that you wrote in your first post

Tymer

Shoot, I'm sorry, my brain is all over the place today, and the function names are changed from the originals. In another class I have a connection:

connect(this, SIGNAL(changeImageBtnClicked()),[myWidget pointer], SLOT(changeImage()));

I know this connection works, because if I press the button enough times it starts printing the "end of array" repeatedly.

Again, apologies for the confusion. Thank you for your help and patience.

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.