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

Tymer

SOLUTION IN POST 32

Hi all. I've been working with Qt now for a few weeks, and I've finally run into a problem I have not been able to solve.

I've tried absolutely everything I know, and I've come to the conclusion that this issue is over my head. I've tried running repaint(), update() and this->update(); and everything else that I could think of. Pixmap works outside of the function (in the constructor) but not inside a function. Here is code (only relevant is pasted, please indicate if you would like more):

myWidget.h

#define NUM_POINTERS 10


QLabel* pointerArray[NUM_POINTERS];
QPixmap circle;
QPixmap* triangle;
QPixmap* whitex;
int activePointer;

myWidget.cpp

activePointer = 0;

QPixmap circle (":/Resources/greencircle.png");
this->whitex = new QPixmap(":/Resources/white_x.png");
this->triangle = new QPixmap(":/Resources/redtriangle.png");

//create an array of pointers to the label1-10 objects
pointerArray[0] = ui->label1;
pointerArray[1] = ui->label2;
pointerArray[2] = ui->label3;
...
pointerArray[9] = ui->label10;

for (int i = 0;i < 10; i++)
{
    pointerArray[i]->setPixmap(circle);
}

void myWidget::changeImage()
{
  updatesEnabled();
  if (activePointer < 10){
      pointerArray[activePointer]->setPixmap(*this->whitex);
      activePointer++;
      update();
}
 else{
     printf("end of array\n");
     fflush(stdout);
 }

}

I get a row of circles printed where I want them, but I won't get any white Xs. The pixmap changes to the whitex, but it will not update. It does not crash, it continues adding to activePointer until the end of the array. I have tried pointerArray[activePointer]->update(); with no luck.

I also asked this question on Stack Overflow and answered the following questions:

-The paths are all correct. I copied them directly by right clicking on the resource and selecting "copy URL."
-I can't declare it on the stack, because then it will not work inside the function.
-I'm sure that the setPixmap is executing, because the debugger shows the pixmap changing to the 50x50 image (iirc it also says it is the whitex). I even tried having no pixmap set, and even then it wouldn't set the pixmap from inside the function.

Thanks in advance.

koahnig

Hi and welcome to devnet

Did you try already repaint()?

Tymer

@koahnig I've tried repaint() with no success. I just tried it again to be sure, still no luck.

SGaist

HI and welcome to devnet,

What would be the problem with triangle and whitex on the stack rather that the heap ? By the way you are shadowing circle in your constructor.

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 ?