[solved]QQmlApplicationEngine: Moving from 5.2 to 5.3

qmlLearner

Hi

I am updating my little QtQuick application from 5.2 to 5.3. I noticed that the main.cpp looks a bit different now with the QQmlApplicationEngine and that the QQuickWindow has disappeared.
My main.cpp in 5.2 looked like the following:

@
#include <QtQml>
#include <qtquick2applicationviewer.h>

#include "XXX.h"

// switch native or generic // if using native add QT+=widget in pro file
#define USE_NATIVE_LOOK 0

#if USE_NATIVE_LOOK
#include <QtWidgets/QApplication>
#define Application QApplication
#else
#include <QtGui/QGuiApplication>
#define Application QGuiApplication
#endif

int main(int argc, char *argv[])
{
Application app(argc, argv);

qmlRegisterType<XXX>("XXX", 1, 0, "XXX");

QQmlApplicationEngine engine;
engine.load(QUrl("qml/qmlEngine08/main.qml"));
QObject *topLevel = engine.rootObjects().value(0);
QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);
if ( !window ) {
    qWarning("Error: Your root item has to be a Window.");
    return -1;
}

window->show();
return app.exec&#40;&#41;;

}
@

what i have so far with in the new 5.3 version:

@
#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include <QtQml>

#include "XXX.h"

int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);

qmlRegisterType<XXX>("XXX", 1, 0, "XXX");

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));

return app.exec(&#41;;

}
@

The application compiles without any errors, but the GUI is not shown. What am I missing?
Thank you for any help!

sierdzio

You are missing the QQuickWindow part, obviously ;-) You are loading the QML code into the engine, but not displaying it.

BTW. I recommend using QQuickView as a better alternative. Requires less code to be written.

qmlLearner

Alright :)
I was a bit confused by the fact that if you create a new QtQuick project, the QQuickWindow part is not there anymore by default, yet it diplays the GUI. So why does it work without a QQuickView if you just launch a newly created project?

I adapted the code as follows (it works now, thank you!):
@
int main(int argc, char *argv[])
{
Application app(argc, argv);

qmlRegisterType<DownloadManager>("DownloadManager", 1, 0, "DownloadManager");

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));

QObject *topLevel = engine.rootObjects().value(0);
QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);
if ( !window ) {
    qWarning("Error: Your root item has to be a Window.");
    return -1;
}

window->show();
return app.exec&#40;&#41;;

}
@

What do you mean by "QQuickView is the better alternative"? I do require the engine for the C++ Classes, right?
Sorry, I don't really have the overview yet :)
Thank you for your superfast help anyway!!!

sierdzio

In QQuickView, the engine is built-in, you only need to specify the input file path and call show(), which is why I use it in my projects. But your approach is also entirely valid.

qmlLearner

I see! I think I am slowly getting it...so a basically have three options:

@
#include <QGuiApplication>
#include <QQmlApplicationEngine>

// option 2:
//#include <QQuickWindow>

// option 3:
//#include <QQuickView>

#include <QtQml>

#include "DownloadManager.h"

// switch native or generic // if using native add QT+=widget in pro file
#define USE_NATIVE_LOOK 0

#if USE_NATIVE_LOOK
#include <QtWidgets/QApplication>
#define Application QApplication
#else
#include <QtGui/QGuiApplication>
#define Application QGuiApplication
#endif

int main(int argc, char *argv[])
{
Application app(argc, argv);

qmlRegisterType<DownloadManager>("DownloadManager", 1, 0, "DownloadManager");

// option 1 (easiest):
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
// note: ApplicationWindow must explicitly be set to: "visible: true"

// option 2:

// QQmlApplicationEngine engine;
// engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
// QObject *topLevel = engine.rootObjects().value(0);
// QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);
// if ( !window ) {
// qWarning("Error: Your root item has to be a Window.");
// return -1;
// }
// window->show();

// option 3: only works if main.qml is NOT ApplicationWindow

// QQuickView *view = new QQuickView;
// view->setSource(QUrl(QStringLiteral("qrc:///main.qml")));
// view->show();

return app.exec&#40;&#41;;

}

@

As my main.qml uses an ApplicationWindow, I can't use QQuickView. For option 1, it turned out, that is crucial to set "visible: true" in the ApplicationWindow. Some more info can be found "here":http://www.qtcentre.org/threads/54649-ApplicationWindow-as-root-object-result-in-error, "here":http://stackoverflow.com/questions/18506784/qml-and-c-with-qt-quick-2-application and "here":http://stackoverflow.com/questions/19159928/how-to-change-title-of-a-qtquick-2-window.

Is that correct?

sierdzio

Nice, thanks for thorough explanation, it also helped me learn something :-D

Muhammad

Thank you guys that was really helpful.