Why isn't the “rectangle” that I want to draw on my Qt widget showing up?

Question

I basically want to display a rectangle on a dialog window widget. Using another question as reference, I tried to adapt the framework of using a QLabel and painting to it (the process overall seems overly complicated).

I started by making a member in the dialog box's class:

QLabel* label; 

In the constructor of the dialog box:

label = new QLabel(this);
label->setGeometry(20, 50, 50, 100);

Just to try and make it work, I gave the dialog box a button to make the "rectangle" created with the label appear on the widget. I connected the "pressed" signal of this button to a slot which does the following:

QPixmap pixmap(50, 100);
pixmap.fill(QColor("transparent"));
QPainter painter(&pixmap);
painter.setBrush(QBrush(Qt::black));
painter.drawRect(20, 50, 50, 100);
label->setPixmap(pixmap);
update(); 

Unfortunately, nothing appears in the widget when I press the button. What am I missing here?

Answer 1

I tried this with PyQt and it generally works, but I'm not 100% sure about the procedure. Maybe you should try calling painter.end() the painter before calling setPixmap(). Also, I'm not sure if one is supposed to draw onto a QPixmap outside of QWidget:paintEvent, it might be safer to draw a QImage and create a QPixmap from it.

from PyQt4 import QtGui

app = QtGui.QApplication([])

class Test(QtGui.QWidget):
    def __init__(self):
       QtGui.QWidget.__init__(self)
       self.bn = QtGui.QPushButton("Paint")
       self.lb = QtGui.QLabel()
       layout = QtGui.QVBoxLayout(self)
       layout.addWidget(self.bn)
       layout.addWidget(self.lb)
       self.bn.clicked.connect(self.handleClick)

    def handleClick(self):
       pixmap = QtGui.QPixmap(50, 100)
       pixmap.fill(QtGui.QColor("transparent"))
       p = QtGui.QPainter(pixmap)
       p.drawRect(0,0,50-1,100-1)
       p.end()       
       self.lb.setPixmap(pixmap)

t = Test()
t.show()
app.exec_()

For simply drawing a rectangle this is certainly very complicated. I don't know what you are planning, be aware that there is QGraphicsView for drawing figures.