Reload widget contents

Hello.
I am having trouble finding a way to update a widget. I'm modifying the fridge magnets example that comes with the Qt SDK.
Basically what I want to do is to be able to update the DragWidget contents by pressing a button. By update, I mean redrawing the draggable labels to reflect changes in the text file that it reads in order to draw the labels. For instance, I want to change a word in the text file, and after i click a button, i want the labels to be redrawn based on the changes in the text file.
In order to do that, what I did was create a new class that would contain both the DragWidget object and the button. I called it wrapwidget, and here's its definition and implementation:

Code:

---

class wrapWidget: public QWidget
{
    Q_OBJECT
public:
    wrapWidget();
 
};
 
wrapWidget::wrapWidget()
{
    QGridLayout *gridlayout= new QGridLayout();
    DragWidget *w = new DragWidget();
    QPushButton *b = new QPushButton("refresh");
    gridlayout->addWidget(w,0,0);
    gridlayout->addWidget(b,1,0);
    setLayout(gridlayout);
 
    connect(b,SIGNAL(clicked()),w,SLOT(draw()));
}

---

Here's the draw() slot definition:

Code:

---

void DragWidget::draw(){
    QFile dictionaryFile(":/dictionary/words.txt");
    dictionaryFile.open(QFile::ReadOnly);
    QTextStream inputStream(&dictionaryFile);
 
    int x = 5;
    int y = 5;
 
    while (!inputStream.atEnd()) {
        QString word;
        inputStream >> word;
        if (!word.isEmpty()) {
            DragLabel *wordLabel = new DragLabel(word, this);
            wordLabel->move(x, y);
            wordLabel->show();
            wordLabel->setAttribute(Qt::WA_DeleteOnClose);
            x += wordLabel->width() + 2;
            if (x >= 245) {
                x = 5;
                y += wordLabel->height() + 2;
            }
        }
    }
}

---

That code was originally in the DragWidget constructor, I moved it out to the draw() method. I tried to set draw() as a slot, and call it by connecting it with the button's clicked() signal. It says there's no such slot though. What am I doing wrong?

Any tips on how I should be doing that?

Did you possibly forget Q_OBJECT in your DragWidget header file?

Can you show the declaration for class DragWidget ?
Probably you didnt add slots: to draw() :rolleyes:

@norobro

Well, apart from moving the label drawing code out of the constructor into the draw() slot, I haven't modified anything in the DragWidget class. In its original form, there's no Q_OBJECT macro in DragWidget declaration, and it compiles and runs as expected. But adding the macro to the DragWidget class, like this:

Code:

---

class DragWidget : public QWidget
{
    Q_OBJECT
public:
    DragWidget(QWidget *parent = 0);
 
public slots:
    void draw();
 
protected:
    void dragEnterEvent(QDragEnterEvent *event);
    void dragMoveEvent(QDragMoveEvent *event);
    void dropEvent(QDropEvent *event);
    void mousePressEvent(QMouseEvent *event);
    void paintEvent(QPaintEvent *event);
};

---

gave off this error:

collect2: ld returned 1 exit status

@aamer4yu

But I did, look at the class declaration, its there.

Did you recompile ?
run qmake again

I did recompile, the 'public slots:' declaration was there all the time, I didn't add it just now.
I ran qmake through Qt Creator, but the same error comes up. This is really weird, I've no idea what I'm doing wrong.

here's the project if anyone's interested in checking out what's the problem

http://www.4shared.com/account/file/...gemagnets.html

Okay, I don't know what manner of esoteric power was holding my computer back, but I added the Q_OBJECT macro to the DragWidget class and this time running qmake and running it again worked fine. Really weird.

My next step was storing the created labels in a QList and delete them before calling draw(). Seems to be working now. Here's the code to my modified DragWidget class:

Code:

---

class DragWidget : public QWidget
{
    Q_OBJECT
public:
    DragWidget(QWidget *parent = 0);
 
public slots:
    void draw();
    void store();
    void refreshWidget();
 
protected:
    void dragEnterEvent(QDragEnterEvent *event);
    void dragMoveEvent(QDragMoveEvent *event);
    void dropEvent(QDropEvent *event);
    void mousePressEvent(QMouseEvent *event);
    void paintEvent(QPaintEvent *event);
 
private:
    QList<DragLabel*> labels;
};

---

and the implementation:

Code:

---

DragWidget::DragWidget(QWidget *parent)
    : QWidget(parent)
{
    store();
    draw();
    QPalette newPalette = palette();
    newPalette.setColor(QPalette::Window, Qt::white);
    setPalette(newPalette);
 
    setMinimumSize(400, 100);//qMax(200, y));
    setWindowTitle(tr("Fridge Magnets"));
    setAcceptDrops(true);
}
 
void DragWidget::draw(){
    int x = 5;
    int y = 5;
    foreach(DragLabel* label, labels){
            label->move(x,y);
            label->show();
            label->setAttribute(Qt::WA_DeleteOnClose);
            x += label->width() + 2;
            if (x >= 245) {
                x = 5;
                y += label->height() + 2;
            }
    }
}
 
void DragWidget::refreshWidget(){
    store();
    draw();
}
 
 
void DragWidget::store(){
    foreach(DragLabel* label, labels){
        label->deleteLater();
    }
    QFile dictionaryFile("../words.txt");
    dictionaryFile.open(QFile::ReadOnly);
    QTextStream inputStream(&dictionaryFile);
 
    labels.clear();
 
    while (!inputStream.atEnd()) {
        QString word;
        inputStream >> word;
        if (!word.isEmpty()) {
            DragLabel *wordLabel = new DragLabel(word, this);
            labels.append(wordLabel);
        }
    }
}

---

and the connect statement now calls refreshWidget() as a slot instead of draw(). It works fine now if you don't drag any labels before clicking the button. If i drag a label out of place and click the button, the program crashes. Any idea why that happens?

Glad that you got it running. I was typing the following (slow typist) while you posted. I'll take a look at your next problem.

You have a stray comma in the wrapwidget.h that you uploaded:

Code:

---

#define WRAPWIDGET_H,

---

I removed the comma, uncommented Q_Object in dragwidget.h, reran qmake and it compiled.

Here's the project, the button works but dragging a label before clicking the button crashes the program

http://www.4shared.com/file/23766778...magnets_2.html

I think it's because of the drop, but I'm not really sure what to do to fix this behaviour. I tried to append the new label created by the drop to the QList, but that didn't solve the problem.

When you drag an item a new QLabel is created in DragWidget::dropEvent. That seems to invalidate your QList "labels" resulting in a segfault.

Try this:

Code:

---

void DragWidget::store(){
   QList <QObject *>  newList = this->children();
   foreach(QObject * label, newList) label->deleteLater();
//    foreach(DragLabel* label, labels){
//        label->deleteLater();
//    }

---

It worked! I was so sure the error was on dropEvent()!
Could you elaborate more on why the segfault was happening? Also, you declared a QObject* list, could it have been a DragWidget* list instead?

How could I go about retrieving a particular set of objects instead of all the children?

Sorry for the delay in responding. Right after my last post I lost my internet connection. Called my ISP and as usual their first reaction was to blame the customer's equipment, in this case my 7 year old modem. So I bought a new modem and they still couldn't get me connected. Had to send out a service tech and of course the problem was on their end.

Code:

---

void DragWidget::mousePressEvent(QMouseEvent *event)
{
. . .
    if (drag->exec(Qt::MoveAction | Qt::CopyAction, Qt::CopyAction) == Qt::MoveAction)
        child->close();
. . .

---

When you move a "magnet" a new DragLabel is created and the above statement deletes the old one which leaves you with a dangling pointer in your list "labels". This results in a segfault when you iterate through the list because you are trying to delete a memory location that has already been deallocated. Try commenting out "child->close()" and use your foreach statement in DragWidget::store().

I tried using a DragLabel list first and got compile errors thus the QObject * list. You make a good point- you might not want to delete all of an object's children. I did find this in the docs so in this case the following should find all of the DragLabel children:

Code:

---

QList<DragLabel *> newList = this->findChildren<DragLabel *>();

---

Hope this makes sense.

Sorry to hear you had trouble with your ISP, I'm all too familiar with your situation. Nice to have you back on the internets ;)

I see why the application was crashing now. Thanks a bunch for your help!