QML timer not triggered - c++

Question

I have a timer in qml (view in StackView) which I try to start from C++ code, calling javascript function. But my timer is never triggered. What Am I doing wrong? My flow is a.qml -> b.qml (on Button clicked)
File b.qml :
Item {
function connectionConfirmed() {
myTimer.start()
console.log("started timer")
}
Timer {
interval: 1000; running: false; repeat: false
id: myTimer
onTriggered: {
console.log("timer triggered")
}
}
}
file main.qml:
ApplicationWindow {
id: root
visible: true
width: 320
height: 530
StackView {
id: stackView
initialItem: "qrc:/a.qml"
anchors.fill: parent
}
}
file a.qml
MouseArea{
anchors.fill: parent
onClicked: {
stackView.push("qrc:/b.qml")
}
}
C++ part:
connect(&myObjectInstance, &X::somethingHappend, this, [this](){
QQmlComponent component(&engine_, "qrc:/b.qml");
QObject *obj = component.create();
QVariant returnedValue;
QMetaObject::invokeMethod(obj, "connectionConfirmed",
Q_RETURN_ARG(QVariant, returnedValue));
delete obj;
});
Output is:
started timer
When I tried to set running: true timer is triggered, does it mean that I am not able to start the timer from JS function?

Answer 1

The problem is caused because you are assuming that the component created on the C++ side is the same that is created on the QML side. The .qml file is the source code, when you invoke it, they generate a different object each time.
Explanation of the behavior:
connect(&myObjectInstance, &X::somethingHappend, this, [this](){
QQmlComponent component(&engine_, "qrc:/b.qml");
QObject *obj = component.create();
QVariant returnedValue;
QMetaObject::invokeMethod(obj, "connectionConfirmed",
Q_RETURN_ARG(QVariant, returnedValue));
delete obj;
});
In this code you are first creating the component, and then you create the object, call the function written in js, that function has just been executed and print the message, and after that you delete it, so when you delete it, it will never fire. new Timer, for that reason it seemed that the one that printed the first message was the item created in QML, but it is not, the new item printed it.
Calling a C ++ QML function brings these problems because many times you do not access the object you want.
The recommendation indicated by Qt is, on the contrary, to make the connection on the QML side, for this you must export the C ++ object using setContextProperty().
In the following example using its implementation of QML, we create a class that has a signal called mysignal, it is triggered every 5 seconds for the purpose of testing. An object of that class is exported to QML and connected so that it invokes the js function from QML
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QTimer>
class Helper: public QObject{
Q_OBJECT
public:
using QObject::QObject;
signals:
void mysignal();
};
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
Helper helper;
engine.rootContext()->setContextProperty("helper", &helper);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
QTimer timer;
QObject::connect(&timer, &QTimer::timeout, [&helper](){
emit helper.mysignal();
});
timer.start(5000);
return app.exec();
}
#include "main.moc"
b.qml
import QtQuick 2.0
Item {
id: it
function connectionConfirmed() {
myTimer.start()
console.log("started timer")
}
Timer {
interval: 1000; running: false; repeat: false
id: myTimer
onTriggered: {
console.log("timer triggered")
}
}
Connections{
target: helper
onMysignal: it.connectionConfirmed()
}
}
Finally, in a StackView, items are created and destroyed each time a page is changed.