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

[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.

[Tymer]

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

[SGaist]

No, not at all. Either:

1. Remove the widget from MainWindow and connect the one in main.cpp
2. 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