Right QML <-> C++ declarative approach - c++

Question

I have some data structure updated in c++ layer. I have to display it in qml and save changes from qml layer to c++ structures. I hope there is a declarative approach to do it but I in desperate to find it.
Here is the part of code:
C++ header:
#ifndef NODEINFO_H
#define NODEINFO_H
#include <QObject>
#include <QString>
class NodeInfo : public QObject {
Q_OBJECT
Q_PROPERTY(QString label READ label WRITE setLabel NOTIFY labelChanged)
public:
NodeInfo(QObject *parent = 0);
virtual ~NodeInfo() {}
const QString& label() const;
void setLabel(const QString& val);
signals:
void labelChanged();
private:
QString d_label;
};
#endif // NODEINFO_H
C++ body:
#include "nodeinfo.h"
#include <QDebug>
NodeInfo::NodeInfo(QObject *parent) : QObject(parent), d_label("Test string") {
}
const QString &NodeInfo::label() const {
qDebug() << "NodeInfo::label: getter";
return d_label;
}
void NodeInfo::setLabel(const QString &val) {
qDebug() << "NodeInfo::label: setter - " << val;
d_label = val;
emit labelChanged();
}
main.cpp
#include <QGuiApplication>
#include <QDebug>
#include <QQmlContext>
#include <QQuickView>
#include <QQmlApplicationEngine>
#include "nodeinfo.h"
int main(int argc, char *argv[]) {
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
// qmlRegisterType<NodeInfo>("NodeInfo", 1, 0, "NodeInfo");
NodeInfo nodeDescr;
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("nodeData", &nodeDescr);
const QUrl url(QStringLiteral("qrc:/main.qml"));
engine.load(url);
QObject *root = engine.rootObjects().value(0);
if (QWindow *window = qobject_cast<QWindow *>(root))
window->show();
else
return -1;
return app.exec();
}
Qml code:
import QtQuick 2.15
import QtQuick.Controls 2.12
import QtQuick.Layouts 1.12
ApplicationWindow {
id: root
width: 360
height: 520
visible: true
// property alias a_label: nodeData.label
Column {
anchors.fill: parent
TextInput {
id: simpleTxt
text: nodeData.label
}
Text {
id: txt
text: nodeData.label
}
Button {
text: "writeProp"
onClicked: nodeData.label = simpleTxt.text
}
// Binding {
// target: nodeData
// property: "label"
// value: simpleTxt.text
// }
}
}
So when I'm editing text in TextInput it should automatically set property in c++ code but it do not. Only if I press button.
There is the Binding way as you see in comments and it works but I it's not a true way I hope.
Let's imagine if I have 15-30 or more data fields in my c++ structure and it's full rubbish if I must do 30 Bindings such way or if I need to write signal/slot on each data field and connect them.
But what is right way?
Any ideas appreciated

Answer 1

A simpler solution is to assign the signal associated to the property text:
Text {
id: txt
text: nodeData.label
onTextChanged: {
if(nodeData.label != simpleTxt.text)
nodeData.label = simpleTxt.text
}
}

Answer 2

text: nodeData.label sets binding nodeData.label --> text (one direction). I.e. text is updated whenever nodeData.label is changed. When the user types some text in the field, this binding is destroyed.
So if you want to update nodeData.label when the user changes text, you need to use onTextChaged event.
Text {
onTextChanged: nodeData.label = text
}
One more note: you need to check if the property is really changed before emitting the appropriate changed signal. So the code should be something like this:
void NodeInfo::setLabel(const QString &val) {
qDebug() << "NodeInfo::label: setter - " << val;
if (d_label != val)
{
d_label = val;
emit labelChanged();
}
}
This will prevent your code from endless binding loops.