Best way to access a cpp structure in QML - qt

Question

I need to pass structures between cpp and QML. If i use property i should create an individual set and get functions, My structure contains minimum 5 members so i felt it's not good to use set and get for all those members.
Following is an example of what i am trying to do :
MyClass.h
#include <QObject>
#include <QDebug>
using namespace std;
struct MyStruct {
Q_GADGET
int m_val;
QString m_name1;
QString m_name2;
QString m_name3;
QString m_name4;
Q_PROPERTY(int val MEMBER m_val)
Q_PROPERTY(QString name1 MEMBER m_name1)
Q_PROPERTY(QString name2 MEMBER m_name2)
Q_PROPERTY(QString name3 MEMBER m_name3)
Q_PROPERTY(QString name4 MEMBER m_name4)
};
class MyClass:public QObject
{
Q_OBJECT
Q_PROPERTY(MyStruct mystr READ getMyStruct
WRITE setMyStruct NOTIFY myStructChanged)
public:
explicit MyClass(QObject *parent = nullptr);
MyStruct strObj;
// Edit: changed get function
MyStruct getMyStruct() const
{
return strObj;
}
// Edit: Added set function
void setMyStruct(myStruct val)
{
mystr = val;
emit myStructChanged();
}
signals:
void myStructChanged();
}
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QDebug>
#include <QObject>
#include "MyClass.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
MyClass classObj;
engine.rootContext()->setContextProperty("classObj",&classObj);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
return app.exec();
}
Main.qml
import QtQuick 2.6
import QtQuick.Controls 2.2
import QtQuick.Window 2.3
ApplicationWindow {
id: applicationWindow
visible: true
width: 600
height: 400
title: qsTr("My App")
MainForm{
id : mainform
Component.onCompleted: {
console.log("name===="+classObj.mystr.name1)
//EDIT added more code to explain the use case.
classObj.myStr.name1 = "abc" //Calls setter
classObj.mystr.name2 = "ans" // Calls setter
}
}
}
If i print just (classObj.myVariant) i am getting QVariant(MyStruct) but when i tried to access any parameter like classObj.myVariant.name1 i am getting "undefined" and also how to set a variant from QML?
[UPDATE] - Should also add MyStruct to Q_DECLARE_METATYPE as below: Q_DECLARE_METATYPE(MyStruct)

Answer 1

You need meta data to access C++ objects from QML.
For non QObject derived, this is achieved by using the Q_GADGET macro, and exposing the members as properties:
struct MyStruct {
Q_GADGET
int m_val;
QString m_name1;
QString m_name2;
QString m_name3;
QString m_name4;
Q_PROPERTY(int val MEMBER m_val)
Q_PROPERTY(QString name1 MEMBER m_name1)
Q_PROPERTY(QString name2 MEMBER m_name2)
Q_PROPERTY(QString name3 MEMBER m_name3)
Q_PROPERTY(QString name4 MEMBER m_name4)
};

Answer 2

your struct or simple class must have Q_GADGET as minimum
you should declare properties in order to access from qml
you must declare your struct/class by Q_DECLARE_METATYPE()
you must register it using qRegisterMetaType<>() somewhere before loading qml file by engine such as main.cpp
so you will have something like this:
//review carefully
struct MyStruct {
Q_GADGET //<-- 1.
Q_PROPERTY(QString str1 MEMBER m_str1) //<-- 2.
public: //<-- important
QString m_str1;
};
Q_DECLARE_METATYPE(MyStruct) //<-- 3.
and use somewhere:
class Controller : public QObject
{
Q_OBJECT
public:
explicit Controller(QObject *parent = nullptr);
Q_INVOKABLE MyStruct setNewConfig(QString v); //<-- e.g.
//...
}
main.cpp
//...
qmlRegisterType<Controller>("AppKernel", 1, 0, "Controller");
qRegisterMetaType<MyStruct>(); //<-- 4.
//...
engine.load(url);
//...
so it is usable in qml
main.qml
//...
Controller {
id: con
}
FileDialog {
id: fileDialog
nameFilters: ["Config file (*)"]
onAccepted: {
var a = con.setNewConfig(file);
console.log(a.str1); //<-- yeah! it is here
}
}
//...
NOTE 1: Be careful, it seems that nested classes/struct not supported by Qt meta
NOTE 2: You can expose struct just like a class. Inherit from QObject and use Q_OBJECT. See this article from Evgenij Legotskoj
NOTE 3: Above instructions makes struct/class known to qml and you can access properties/members, but is not instantiable in qml. see Qt document
NOTE 4: Be aware that qmlRegisterType<>() method is marked as "obsolete" in Qt 5.15+. Keep yourself updated ;)