Why QGraphicsView is not shown - c++

Question

I have the following codes in my Qt project with the following main:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
The class Widget is a QWidget object with the following constructor:
Widget::Widget(QWidget *parent)
: QWidget(parent)
{
m_Scene = new QGraphicsScene(this);
QGraphicsLinearLayout* layout = new
QGraphicsLinearLayout(Qt::Orientation::Vertical);
for(int i = 0; i < 10; i++)
{
std::string name = "m_" + std::to_string(i);
GraphicsTextItem* item = new GraphicsTextItem(nullptr, QString(name.c_str()));
layout->addItem(item);
}
QGraphicsWidget* list = new QGraphicsWidget;
list->setPos(0,0);
list->setLayout(layout);
m_Scene->addItem(list);
QGraphicsView* view = new QGraphicsView(this);
view->setScene(m_Scene);
// Why one of these lines must be uncommented?
//m_Scene->setSceneRect(0, 0, 1920, 768);
//QVBoxLayout *ttopLayout = new QVBoxLayout;
//ttopLayout->addWidget(view);
//setLayout(ttopLayout);
}
GraphicsTextItem is just a QGraphicsWidget for displaying text:
class GraphicsTextItem : public QGraphicsWidget
{
public:
QString m_Name;
QColor m_Color;
public:
GraphicsTextItem(QGraphicsItem * parent = nullptr, const QString& name = QString());
void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
{
Q_UNUSED(option)
Q_UNUSED(widget)
QFont font("Times", 10);
painter->setFont(font);
painter->setPen(m_Color);
painter->drawText(0, 0, m_Name);
}
};
My question is that why my scene is not shown. I must either define a SceneRect or define a layout on my widget?

Answer 1

I made an even shorter MCVE for demonstration:
#include <QtWidgets>
int main(int argc, char **argv)
{
qDebug() << "Qt Version:" << QT_VERSION_STR;
QApplication app(argc, argv);
QWidget qWinMain;
qWinMain.resize(320, 240);
QFrame qFrm(&qWinMain);
qFrm.setFrameStyle(QFrame::Box | QFrame::Raised);
qFrm.setLineWidth(0);
qFrm.setMidLineWidth(1);
qWinMain.show();
return app.exec();
}
compiled and started in cygwin64. This is how it looks:
There is a main window (with window manager decoration).
There is a child QFrame.
The child QFrame is "pressed" into the upper left corner.
How comes?
What QWidget does ensure: Child widgets are rendered (in front) when QWidget is rendered.
What QWidget is not (directly) responsible for: Layouting child widgets.
For this, a layout manager has to be plugged in:
#include <QtWidgets>
int main(int argc, char **argv)
{
qDebug() << "Qt Version:" << QT_VERSION_STR;
QApplication app(argc, argv);
QWidget qWinMain;
qWinMain.resize(320, 240);
QVBoxLayout qVBox(&qWinMain);
QFrame qFrm(&qWinMain);
qFrm.setFrameStyle(QFrame::Box | QFrame::Raised);
qFrm.setLineWidth(0);
qFrm.setMidLineWidth(1);
qVBox.addWidget(&qFrm);
qWinMain.show();
return app.exec();
}
compiled and started again in cygwin64. This is how it looks:
Now, the QFrame qFrm is filling the QWidget qWinMain nicely. Resize events received in qWinMain will be forwarded to the layout manager qVBox which will re-layout the children of qWinMain (i.e. qFrm) again.
I strongly believe OP's GraphicsView is just not visible because it has no minimal size requirement. (It's just to small to be visible.)
Hence, adding a layout manager ensures that the GraphicsView fills the parent widget client area. Resizing the contents of GraphicsView (by m_Scene->setSceneRect(0, 0, 1920, 768);) is yet another option to fix this, albeit the worse one.
Finally, the link to Qt Doc.: Layout Management.
Layout Management
The Qt layout system provides a simple and powerful way of automatically arranging child widgets within a widget to ensure that they make good use of the available space.
Introduction
Qt includes a set of layout management classes that are used to describe how widgets are laid out in an application's user interface. These layouts automatically position and resize widgets when the amount of space available for them changes, ensuring that they are consistently arranged and that the user interface as a whole remains usable.
All QWidget subclasses can use layouts to manage their children. The QWidget::setLayout() function applies a layout to a widget. When a layout is set on a widget in this way, it takes charge of the following tasks:
Positioning of child widgets
Sensible default sizes for windows
Sensible minimum sizes for windows
Resize handling
Automatic updates when contents change:
Font size, text or other contents of child widgets
Hiding or showing a child widget
Removal of child widgets