Cannot modify Qml propriety from Cpp

mikecurl91

I've this main.cpp code:
@int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv); //preparo l'applicazione
QQmlApplicationEngine engine(QUrl("qml/main.qml")); //carico l'interfaccia
QObject *radice = engine.rootObjects().value(0); //identifico la radice
QQuickWindow *applicationwindow = qobject_cast<QQuickWindow *>(radice); //acquisisco in C++ l'ogg QML ApplicationWindow
if ( !applicationwindow )
{
qWarning("Error: Your root item has to be a Window.");
return -1;
}

QObject *button = radice->findChild<QObject*>("buttonOpenCloseCom",Qt::FindChildrenRecursively);
if (button)
{
    button->setProperty("text", "Open");
    qDebug()<<"find!";
}

applicationwindow->show();
return app.exec();

}@

and this main.qml file:
@import QtQuick 2.1
import QtQuick.Controls 1.0
import QtQuick.Controls.Styles 1.0
import QtQuick.Layouts 1.0

ApplicationWindow
{
id: applicationwindow;
/Interface config/
title: appname
property int margin: 11
property string appname: "Controller 0.1"
property int coordsFontSize: 25
property string buttonComText: "Open/Close"
//width: mainLayout.implicitWidth + 2 * margin
//height: mainLayout.implicitHeight + 2 * margin
width: 1024
height: 768
minimumWidth: 1024
minimumHeight: 768

RowLayout
{
    id: mainLayout
    anchors.fill: parent
    anchors.margins: margin
    Layout.fillWidth: true

    ColumnLayout
    {
        id: leftColumn
        anchors.bottom: parent.bottom
        anchors.bottomMargin: 0
        anchors.top: parent.top
        anchors.left: parent.left
        Layout.fillWidth: false
        Layout.preferredWidth: parent.width/2
        //Layout.preferredHeight: parent.height/2

        GroupBox
        {
            id: comGroupBox
            title: "COM Layer"
            Layout.fillWidth: true

            RowLayout
            {
                id: rowlayout1
                anchors.top: parent.top
                anchors.fill: parent
                anchors.margins: 20


                spacing: 5

                ComboBox
                {
                    id: comboComs
                    model: [ "COM1", "COM2", "COM3" ]
                    function test()
                    {
                        console.log("TEST OK");
                    }
                onClipChanged: {console.log("azz")}
                }

                Button
                {
                    id: buttonOpenCloseCom
                    objectName:  buttonOpenCloseCom
                    text:applicationwindow.buttonComText
                    enabled: true
                }

            }
        }
}

}@

I need to change the text of the button that has the objectName proprety "buttonOpenCloseCom", but the code defined in main.cpp doesn't work, and I'm not able to understand why. I've followed examples like "this":http://qt-project.org/doc/qt-5.1/qtqml/qtqml-cppintegration-interactqmlfromcpp.html but it seem that the button object is not found by the findChild method... infact the qDebug print is not performed...
Can anyone help me?

sierdzio

I guess you need to set that property after even loop is started in app.exec().

Otherwise, the code seems to be correct (although there is no need to modify buttonOpenCloseCom, when you can just modify buttonComText in ApplicationWindow).

mikecurl91

Hi, thanks for the answer.
I've changed the code:
@int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv); //preparo l'applicazione
QQmlApplicationEngine engine(QUrl("qml/main.qml")); //carico l'interfaccia
QObject *radice = engine.rootObjects().value(0); //identifico la radice
QQuickWindow *applicationwindow = qobject_cast<QQuickWindow *>(radice); //acquisisco in C++ l'ogg QML ApplicationWindow
if ( !applicationwindow )
{
qWarning("Error: Your root item has to be a Window.");
return -1;
}
applicationwindow->show();
return app.exec();

QObject *button = radice->findChild<QObject*>("buttonOpenCloseCom",Qt::FindChildrenRecursively);
if (button)
{
    button->setProperty("text", "Open");
    qDebug()<<"found!";
}
else
{
    button->setProperty("text", "Open");
    qDebug()<<"not found!";
}

}@

but there is no output.
The problem persists!

I've also tried to change the buttonComText in ApplicationWindows, but the result is the same.

wspilot

Try this:
objectName: "buttonOpenCloseCom"

mikecurl91

There is yet the objectName proprety.
I've tried this solution... but doesn't work.
[quote author="wspilot" date="1376930837"]Try this:
objectName: "buttonOpenCloseCom"[/quote]

wspilot

In your code it is still an identifier/var or whatever, but not a string
objectName: buttonOpenCloseCom
not
objectName: “buttonOpenCloseCom”

mikecurl91

Excuseme but in the code:
@
Button
{
id: buttonOpenCloseCom
objectName: buttonOpenCloseCom
text:applicationwindow.buttonComText
enabled: true
}
@

objectName: “buttonOpenCloseCom” there is![/quote]

what is the problem you talk about?!

sierdzio

Put it in quotes: ". Not just string but "string".

wspilot

@ Button
{
id: buttonOpenCloseCom
objectName: "buttonOpenCloseCom"
text:applicationwindow.buttonComText
enabled: true
}@

mikecurl91

I've understand what you mean, and I've tried with the correction, but the result is the same as before.
Debugging i've see that the root object is correctly the "applicationwindow",
but when I try to find the child, it fails and return a null pointer...
in "this":http://qt-project.org/doc/qt-5.1/qtqml/qtqml-cppintegration-interactqmlfromcpp.html#loading-qml-objects-from-c reference it used QQuickView, and not QQuickWindow to handle the root object...
Could it be a problem?
If yes, how can I resolve it? any suggestions?

mranger90

The problem is that you make the change after the return() statement.
Nothing after that will be executed.

mikecurl91

I've modified the code:
@int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv); //preparo l'applicazione
QQmlApplicationEngine engine(QUrl("qml/main.qml")); //carico l'interfaccia
QObject *radice = engine.rootObjects().value(0); //identifico la radice
QQuickWindow *applicationwindow = qobject_cast<QQuickWindow *>(radice); //acquisisco in C++ l'ogg QML ApplicationWindow
if ( !applicationwindow )
{
qWarning("Error: Your root item has to be a Window.");
return -1;
}
QObject button = radice->findChild<QObject>("buttonOpenCloseCom",Qt::FindChildrenRecursively);
if (button)
{
button->setProperty("text", "Open");
qDebug()<<"found!";
}
else
{
button->setProperty("text", "Open");
qDebug()<<"not found!";
}

applicationwindow->show();
return app.exec();

}@

but is the same as before...
[quote author="mranger90" date="1376947874"]The problem is that you make the change after the return() statement.
Nothing after that will be executed.[/quote]

sierdzio

One drawback of QML is that it's mightily hard to explain certain things... In order for this to work (IMO) you need to do that property manipulation outside of "main.cpp":https://github.com/sierdzio/closecombatfree/blob/master/src/ccfmain.cpp so for example in a QObject that gets initialised somewhere later. Same as with QWidgets or console apps in Qt: you need to have a working event loop for moc to work.

One possible alternative would be to modify the property using Qt::QueuedConnection, or the infamous singleshot QTimer.