Create qml object dynamically from c++ object (by using setContextProperty) - c++

Question

I am trying to create a qml object dynamically in c++ using the object of c++ class. Below is the minimal code for my approach. Upon execution of this code and after clicking, the application is crashing(see the comment in main.qml).
I have pasted the code below and It can be downloaded here.
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "scene.h"
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
scene sc(engine);
QQmlContext* context = engine.rootContext();
context->setContextProperty("sc", &sc);
if (engine.rootObjects().isEmpty())
return -1;
return app.exec();
}
scene.h
#ifndef SCENE_H
#define SCENE_H
#include <QObject>
#include <QQmlApplicationEngine>
#include <QQmlComponent>
class scene : public QObject
{
Q_OBJECT
public:
explicit scene(QQmlApplicationEngine& engine, QObject *parent = nullptr);
QQmlApplicationEngine& engine;
public slots:
void create_rect_object();
};
#endif // SCENE_H
scene.cpp
#include "scene.h"
scene::scene(QQmlApplicationEngine &engine, QObject *parent) : engine(this->engine),QObject(parent)
{
}
void scene::create_rect_object()
{
QQmlComponent component(&engine, QUrl::fromLocalFile("myrect.qml"));
QObject *object = component.create();
object->setProperty("width", 200);
object->setProperty("height", 150);
object->setProperty("color", "blue");
}
main.qml
import QtQuick 2.11
import QtQuick.Window 2.11
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
Rectangle{
anchors.fill: parent
color: "red"
MouseArea{
anchors.fill: parent
onClicked: {
console.log("Before click");
sc.create_rect_object(); // application is crashing here
console.log("after click");
}
}
}
}
myrect.qml
import QtQuick 2.0
Rectangle {
id:id_rec
width: 100
height: 100
color: "green"
x:0
y:0
}
Update
The object to be created is not the child of the root of main window but child of one of the item inside the chain of children items of root of mainwindow. The pseudo structure looks like below.
main.qml
Window {
customitem1{
id:id_ci1
}
customitem2{
id:id_ci1
}
}
customitem1.qml
Item {
customitem3{
id:id_ci3
}
customitem3{
id:id_ci4
}
}

Answer 1

[UPDATED]
You have two errors for crashing and one for not showing rectangles
1.Your scene's Constructor member initializer list is falsy which causes the app crash
(TIP : use different naming for members of the class by prefixing them with m_ e.g: m_engine for READABILITY and not get confused)
//Correct WAY
class Something
{
private:
int m_value1;
double m_value2;
char m_value3;
public:
//################# YOUR CASE ###############################
Something(int number) : m_value1(number), m_value2(2.2), m_value3('c') // directly initialize our member variables
{
// No need for assignment here
}
//#############################################################
Something() : m_value1(1), m_value2(2.2), m_value3('c') // directly initialize our member variables
{
// No need for assignment here
}
void print()
{
std::cout << "Something(" << m_value1 << ", " << m_value2 << ", " << m_value3 << ")\n";
}
}
and it should be like this :
scene::scene(QQmlApplicationEngine &engine, QObject *parent) : engine(engine),QObject(parent)
instead of
scene::scene(QQmlApplicationEngine &engine, QObject *parent) : engine(this->engine),QObject(parent)
2.The url of myrect.qml which you get from local file that isn't found at runtime caused the app crash aslo and the one of remedies is to load it from your qrc file
QQmlComponent component(&engine, QUrl("qrc:/myrect.qml"));
3.And you'll notice after clicking you got no rectangles that's because the rectangles getting created doesn't have a parent and by changing your create_rect_object()(In this example the parent is the invisible root of our window contentItem) you'll get some rectangles :)
//A QQuickWindow always has a single invisible root item containing all of its content.
//To add items to this window, reparent the items to the contentItem or to an existing item in the scene.
//http://doc.qt.io/qt-5/qquickwindow.html#contentItem-prop
void scene::create_rect_object()
{
QQmlComponent component(&engine, QUrl("qrc:/myrect.qml"));
QObject *object = component.create();
QQuickItem *item = qobject_cast<QQuickItem*>(object);
// Set the parent of our created qml rect
item->setParentItem((QQuickItem*)((QQuickWindow *) engine.rootObjects()[0])->contentItem());
//Set some random position and color
item->setProperty("color", QColor::fromRgb(QRandomGenerator::global()->generate()));
item->setX(20+qFloor(QRandomGenerator::global()->generateDouble()*20));
item->setY(20+qFloor(QRandomGenerator::global()->generateDouble()*20));
}
Finding QML Objects from C++
For finding objects and using them as parentItem, you have to set the objectName of your qml object
Rectangle {
...
objectName : "rect_1"
...
}
and in C++
QObject* obj = dynamic_cast<QObject*>(engine.rootObjects()[0]).findChild("rect_1");