QQmlEngine retranslate not translating other StackView items - c++

Question

I have a custom class where I need to save the new selected language and change the app language at the same time. An example based on the StackView sample project in QtCreator:
//main.cpp
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QApplication app(argc, argv);
qmlRegisterType<CustomClass>("io.qt.CustomClass", 1, 0, "CustomClass");
QTranslator translator;
translator.load(":/EN.qm");
app.installTranslator(&translator);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
return app.exec();
}
//customclass.h
class CustomClass : public QObject
{
Q_OBJECT
public:
explicit CustomClass(QObject *parent = nullptr) : QObject(parent) {}
Q_INVOKABLE void change(){
QTranslator translator;
QApplication::removeTranslator(&translator);
translator.load(":/CZ.qm");
QApplication::installTranslator(&translator);
//QQmlApplicationEngine * engine = qobject_cast<QQmlApplicationEngine *>(qmlEngine(this));
QQmlEngine *engine = QQmlEngine::contextForObject(this)->engine();
engine->retranslate();
}
};
//main.qml
...
CustomClass{id:test}
...
ItemDelegate {
text: qsTr("Page 1")
width: parent.width
onClicked: {
test.change()
drawer.close()
}
}...
//.pro file
QT += quick gui core
...
TRANSLATIONS = EN.ts CZ.ts
...
HEADERS += \
customclass.h
In this example clicking on Page 1 button should change the language.
My application is based on StackView and when I call the function with this code everything seems to work. The strings are translated. However when I push a new item on the stack (like opening a new section from menu), the strings there are back in the original language before the change. It's like the retranslate changes only the currently visible strings.
Anybody knows where the problem is? I suspect the engine not to be correctly acquired. It is a custom class I need to have registered (qmlRegisterType) to use in qml and I am not sure how to properly get the engine there (since engine is created in the main function).

Answer 1

I ran into the same problem. The solution I found is to use the new operator when creating the QTranslator object. This causes the object to be created in the heap memory instead of the stack. If it is in the stack memory, the QTranslator will be deleted after the function is finished (at least, that is how I understood it. I'm sure people with more C++ experience could probably explain it better).
So then your customclass code would look like this:
//customclass.h
class CustomClass : public QObject
{
Q_OBJECT
public:
explicit CustomClass(QObject *parent = nullptr) : QObject(parent) {}
Q_INVOKABLE void change(){
QTranslator *translator = new QTranslator(qApp);
if (m_previousTranslator) {
QApplication::removeTranslator(m_previousTranslator);
m_previousTranslator->deleteLater();
m_previousTranslator = nullptr;
}
translator->load(":/CZ.qm");
m_previousTranslator = translator;
QApplication::installTranslator(translator);
QQmlEngine *engine = QQmlEngine::contextForObject(this)->engine();
engine->retranslate();
}
private:
QTranslator *m_previousTranslator = nullptr;
};
Note that I added a private variable m_previousTranslator to do some memory management. I haven't tested this example code, but I have used very similar code in my project and it worked

Answer 2

I used a dirty trick to achieve the whole app retranslation without the need to get the engine in the class. It's not prefect but it works. I have added a loop in main like this:
int returnValue = 0;
do
{
QApplication app(argc, argv);
QTranslator translator;
translator.load(":/translation/"+langString+".qm"); //langString might be the "CZ" as in the question example
app.installTranslator(&translator);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/src/qml/main.qml")));
if (engine.rootObjects().isEmpty())
return -1;
returnValue = app.exec();
langString = settings.value("language").toString();
}
while(returnValue == TRANSLATION_RESTART);
return returnValue;
So in my custom class I simply exit the app using:
qApp->exit(TRANSLATION_RESTART);
And I also save the langString value. This way the app is basically restarted with the new language.
UPDATE: While this works it's better to store the engine and app into singleton class that handles translations using the approach mentioned in the question.