How get the initial value from Qml Checkbox to c++? - qt

Question

I have qtquick frontend and c++ backend. In Qml I have a checkbox with random initial value. I have a signal emitted onCheckedChanged which is never received. I believe, it is because the component is created before connect statements are made. When user interacts, I can catch those events but I miss the initial value. I cannot make the connections sooner because the QMl engine has to first create the components so I can have a reference to them to make the signal slot connection. So how to find out the initial value? Do I have to make a timer which will emit the value few seconds after startup? Is there a better way?
here is minimum example (when the initial value is true, the slot is never triggered)
main.qml
import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12
ApplicationWindow {
width: 640
height: 480
visible: true
title: qsTr("Hello World")
signal checkboxStateChanged(enabled: bool)
CheckBox{
checked: Math.random() > 0.5
text: "value"
onCheckedChanged: checkboxStateChanged(checked)
}
}
backend.h
#include <QObject>
#include <QDebug>
class Backend: public QObject{
Q_OBJECT
public:
Backend(){}
public slots:
void logChecked(bool checked){
qDebug()<<checked;
}
};
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QObject>
#include "backend.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
const QUrl url(QStringLiteral("qrc:/main.qml"));
QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
&app, [url](QObject *obj, const QUrl &objUrl) {
if (!obj && url == objUrl)
QCoreApplication::exit(-1);
}, Qt::QueuedConnection);
engine.load(url);
const auto root=engine.rootObjects();
QObject *window = root[0];
Backend b;
QObject::connect(window,SIGNAL(checkboxStateChanged(bool)),
&b,SLOT(logChecked(bool)));
return app.exec();
}

Answer 1

I am not sure about the approach you are trying but a cleaner method would be to bind the QML checked value to the c++ class. Then you can handle everything in your backend class which is a lot easier to debug and maintain. So using your example,
#include <QObject>
#include <QDebug>
class Backend: public QObject{
Q_OBJECT
// Add a Q_PROPERTY to bind in QML
Q_PROPERY(Qt::CheckState checked READ getChecked WRITE setChecked NOTIFY checkedChanged)
signals:
void checkedChanged();
public:
Backend() { m_checked = <random_value>;}
// implement the q_property methods
void setChecked(const Qt::CheckState value) {
if (m_checked != value) {
m_checked = value;
emit checkedChanged();
}
}
Qt::CheckState getChecked() const { return m_checked; }
public slots:
void logChecked(bool checked){
qDebug()<<checked;
}
private:
m_checked;
};
Then in QML:
import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12
import QtQml 2.12 // Binding
import Backend 1.0
ApplicationWindow {
width: 640
height: 480
visible: true
title: qsTr("Hello World")
signal checkboxStateChanged(enabled: bool)
CheckBox{
id: qmlCheckBox
checked: backend.checked // C++ to QML binding
text: "value" // you can bind this too
}
// QML to C++ binding
Binding {
target: backend
property: "checked"
value: qmlCheckBox.checked
}
}
In main.cpp:
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QObject>
#include "backend.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
// Register backend class for qml
qmlRegisterType<Backend>("Backend", 1, 0, "Backend");
engine.rootContext()->setContextProperty("backend", new Backend);
const QUrl url(QStringLiteral("qrc:/main.qml"));
QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
&app, [url](QObject *obj, const QUrl &objUrl) {
if (!obj && url == objUrl)
QCoreApplication::exit(-1);
}, Qt::QueuedConnection);
engine.load(url);
return app.exec();
}