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



  • 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.


  • Moderators

    Hi and welcome to devnet

    Did you try already repaint()?



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


  • Lifetime Qt Champion

    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.



  • @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.


  • Lifetime Qt Champion

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



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


  • Lifetime Qt Champion

    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 ?



  • 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.


  • Lifetime Qt Champion

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



  • 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.


  • Lifetime Qt Champion

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



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



  • 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.


  • Lifetime Qt Champion

    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++;
    

    ?



  • @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.


  • Lifetime Qt Champion

    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();
    }
    


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



  • 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.


  • Lifetime Qt Champion

    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 ?



  • 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.


  • Lifetime Qt Champion

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



  • 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();
    }
    

  • Lifetime Qt Champion

    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.



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


  • Lifetime Qt Champion

    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


  • How would I connect to the instance in main?


  • Lifetime Qt Champion

    By using the static version of QObject::connect



  • 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?



  • 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.


  • Lifetime Qt Champion

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



  • Alright, finally got this problem SOLVED!

    I pass in an instance of myWidget as a parameter of actionTestWindow in main. It's important to note that you need to specify the QWidget as parent = 0 or it will not show. The relevant code looks something like this:

    main.cpp

    QWidget* parent = new QWidget;
    parent = 0;
    myWidget* w = new myWidget;
    actionTestWindow* a = new actionTestWindow(w,parent);
    
    w->show();
    a->show();
    

    actionTestWindow.h

    class actionTestWindow : public QWidget {
    myWidget* m;
    
    public:
    actionTestWindow(myWidget* mW, QWidget* parent = 0);
    

    actionTestWindow.cpp

    actionTestWindow::actionTestWindow(myWidget* mW, QWidget* parent)
    {
    this->m = mW;
    /*use this->m to reference myWidget*/
    }
    

    **note: irrelevant or standard code was mostly emitted.


  • Lifetime Qt Champion

    And there you have a memory leak.

    Why don't you just show the myWidget you had originally created in Widget ?



  • @SGaist It's a pretty complicated project. The actionTestWindow is pretty much just what it sounds like...A test window. The point is to make sure that I can successfully pass the signals I need and get the reactions I need for when I connect it to a more complicated process. myWidget needs to be shown from main as soon as the program starts running. Having it wait to show until the process that it displays info from runs would be a problem. The code will be passed around to many other people to be changed for future projects, and we're trying to avoid making them dig through code to find things as much as possible. It needs to be easy to just change a class slightly and change the instantiation slightly in order to make a change.

    Can you point out the exact memory leak? I was under the impression that you can pass in instances of classes as parameters. It's a pretty common thing to do in Java, and I've at least heard of it being done in C++. There is a lot of code omitted. Destructors, importantly.


  • Lifetime Qt Champion

    You have

    QWidget* parent = new QWidget;
    parent = 0; << the original parent is now lost and not destroyed
    myWidget* w = new myWidget;
    

    Since it needs to be the first widget, then create it in main. Connect your MainWindow to it also in main.

    You should rather avoid making that test widget known to any and every widget of your software. Doing so you'll avoid tight coupling and maintenance hell.



  • Changed it to QWidget* parent = 0;. No more memory leak, everything works as expected.


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