Qt Quick: Cannot render QQuickPaintedItem's children - c++

Question

I'm trying to render some QQuickPaintedItems and their children, but I got only the parents rendered. Please tell me what's wrong with my code.
Graph.h (my custom QQuickPaintedItem):
#include <QQuickPaintedItem>
#include "voltrule.h"
class Graph : public QQuickPaintedItem
{
Q_OBJECT
public:
explicit Graph(QQuickItem *parent = 0);
void paint(QPainter *painter);
private:
VoltRule *_rule;
};
Graph::Graph(QQuickItem *parent) :
QQuickPaintedItem(parent)
{
_rule = new VoltRule(this);
_rule->setParentItem(this);
}
void Graph::paint(QPainter *painter)
{
QRect rect(10, 20, 100, 200);
QPen pen(Qt::green);
painter->setPen(pen);
painter->drawRect(rect);
}
VoltRule.h
#include <QQuickPaintedItem>
class VoltRule : public QQuickPaintedItem
{
Q_OBJECT
public:
explicit VoltRule(QQuickItem *parent = 0);
void paint(QPainter *painter);
signals:
public slots:
};
VoltRule.cpp
#include "voltrule.h"
#include <QPainter>
VoltRule::VoltRule(QQuickItem *parent) :
QQuickPaintedItem(parent)
{
setFlag(QQuickItem::ItemHasContents);
}
void VoltRule::paint(QPainter *painter)
{
QRect rect(10, 20, 100, 200);
QPen pen(Qt::white);
painter->setPen(pen);
painter->drawRect(rect);
}
main.qml
ApplicationWindow {
width: 1367
height: 766
Graph{
anchors.fill:parent
}
}
Thanks in advance

Answer 1

jbh answered the question; just setting a size in the VoltRule constructor solves the problem:
VoltRule::VoltRule(QQuickItem *parent) :
QQuickPaintedItem(parent)
{
setSize(QSizeF(100, 200));
}
It looks a bit strange because the child is drawing white on top of the parent's green lines, erasing just part of the parent's lines.
Here is code that sets the size based on the parent's size. Resize the window and you will see the child updates without an explicit "update()" call:
VoltRule.h
class VoltRule : public QQuickPaintedItem
{
Q_OBJECT
public:
explicit VoltRule(QQuickItem *parent = 0);
void paint(QPainter *painter);
private:
QQuickItem* parentItem();
void onParentChanged();
void onParentWidthChanged();
void onParentHeightChanged();
};
VoltRule.cpp
QQuickItem* VoltRule::parentItem()
{
return qobject_cast<QQuickItem*>(parent());
}
VoltRule::VoltRule(QQuickItem *parent) :
QQuickPaintedItem(parent)
{
connect(this, &QQuickItem::parentChanged, this, &VoltRule::onParentChanged);
onParentChanged();
}
void VoltRule::paint(QPainter *painter)
{
QRect rect(.1*width(), .1*height(), .8*width(), .8*height());
QPen pen(Qt::blue);
painter->setPen(pen);
painter->drawRect(rect);
}
void VoltRule::onParentChanged()
{
// disconnect signals from previous parent, if there was one
disconnect();
if (const auto obj = parentItem()) {
connect(obj, &QQuickItem::widthChanged, this, &VoltRule::onParentWidthChanged);
connect(obj, &QQuickItem::heightChanged, this, &VoltRule::onParentHeightChanged);
onParentWidthChanged();
onParentHeightChanged();
}
}
void VoltRule::onParentWidthChanged()
{
if (const auto obj = parentItem()) {
setWidth(obj->width());
}
}
void VoltRule::onParentHeightChanged()
{
if (const auto obj = parentItem()) {
setHeight(obj->height());
}
}