How can I access my control in windows qml type from C++ while using QQmlApplicationEngine?

QtBeginnnnner

here is my qml file ,very simple one. I have tried two ways to get pointer to my button but failed
Window {
id:mainWindow
objectName: "mainWindow"
visible: true
width: 480
height: 320
title: qsTr("Hello World")
Button {
id: button1
x: 0
y: 0
width: 173
height: 88
text: qsTr("Button")
}
}

first I tried :
int main(int argc, char argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
QObject rootObject = engine.rootObjects().first();
QObject * btn = rootObject->findChild<QObject*>("Button");
return app.exec();
}
but failed, btn is NULL.

second , I tried this way( https://forum.qt.io/topic/83225/findchild-always-returns-null-looking-for-qml-objects/3 ):
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
QQuickWindow * rootWindow = (QQuickWindow *)engine.rootObjects().first();
if (rootWindow)
rootWindow->setProperty("width", 1360);
QObject * btn = Q_NULLPTR;
auto list = rootWindow->contentItem()->childItems();
for (auto i : list) {
btn = i->findChild<QObject *>("Button"); //do not find ,btn is NULL
if (btn) break;
}

return app.exec();

}
but failed, btn is NULL.

so how could I access my control defined in qml file ?

dheerendra

@QtBeginnnnner said in How can I access my control in windows qml type from C++ while using QQmlApplicationEngine?:

btn = i->findChild<QObject *>("Button")

Button {
id: button1
objectName : "QtButton"

}

Then do the following. It will work.
btn = i->findChild<QObject *>("QtButton")

Not sure why you are doing this. It is not good practice.

QtBeginnnnner

@dheerendra hi dheerendra, I have modified my code to
Button {
id: idButton1
objectName: "objButton1"
...
text: qsTr("txtButton1")
}

btn = i->findChild<QObject *>("objButton1");

but , still got a null pointer ..
and I'm doing this for preparation to transfer my app frame from wxwidget to QT/QML, any suggestions ?

dheerendra

Try this

Window {
    id:mainWindow
    objectName: "mainWindow"
    visible: true
    width: 480
    height: 320
    title: qsTr("Hello World")
    Button {
        id: button1
        x: 0
        y: 0
        width: 173
        height: 88
        text: qsTr("Button")
        objectName: "pthinks.com"
    }
 }
int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    QObject *rootObj = engine.rootObjects().first();
    qDebug() << Q_FUNC_INFO << rootObj->objectName() << endl;
    foreach (auto o1, rootObj->children()) {
        qDebug()  << o1->objectName() << endl;
    }

    return app.exec();
}

J.Hilk

@QtBeginnnnner
I think you're already too deep down the children tree to find the button,
try this:

QQuickWindow * rootWindow = (QQuickWindow *)engine.rootObjects().first();
auto list = rootWindow->contentItem()->childItems();

for(QQuickItem *item : list)
    if(item->objectName() == "objButton1")
        qDebug() << "Found the button";

QtBeginnnnner

@dheerendra @J-Hilk
Thank you very much.
Finally , two ways to get pointer to my button both works , here is the code.

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Layouts 1.2
import QtQuick.Controls 1.4

Window {
id:idWindow1
objectName: "objWindow1"
visible: true
width: 480
height: 320
title: qsTr("Hello World")

Button {
	id: idButton1
	objectName: "objButton1"
	x: 0
	y: 0
	width: 173
	height: 88
	text: qsTr("txtButton1")
}

}

#include "mainwindow.h"
#include <QApplication>
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQuickWindow>
#include <QPushButton>
#include <QQuickItem>

//method 1 OK
int main(int argc, char *argv[]){
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
if (engine.rootObjects().isEmpty()) return -1;
QQuickWindow * rootWindow = (QQuickWindow *)engine.rootObjects().first();
if (rootWindow)
rootWindow->setProperty("width", 1360);
QObject * btn = Q_NULLPTR;
auto list = rootWindow->contentItem()->childItems();
for (QQuickItem *item : list) {
if (item->objectName() == "objButton1") {
qDebug() << "Found the button";
btn = item;
}
if (btn) break;
}
btn->setProperty("text", "textNew1");

return app.exec();

}

//method 2 OK
int main(int argc, char argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
QObject rootObject = engine.rootObjects().first();
QObject * btn = rootObject->findChild<QObject*>("objButton1");

btn->setProperty( "text", "textNew2" );

return app.exec();

}

GrecKo

Don't do that.

Here are some resources to tell you why :) :
https://doc.qt.io/qt-5/qtquick-bestpractices.html#interacting-with-qml-from-c
http://doc.qt.io/qt-5/qtqml-cppintegration-overview.html#interacting-with-qml-objects-from-c
https://youtu.be/vzs5VPTf4QQ?t=23m20s