How can I pass a QML object reference into Qt C++? - qt

Question

I am attempting to create something like jQuery's autocomplete as a widget in QML and Qt C++. Toward that end, I created a C++ AutoCompleteListener child of QObject and then register it with:
qmlRegisterType<AutoCompleteListener>(
"foo.AutoCompleteListener",0,1,"AutoCompleteListener");
Then, I instantiate the listener and the AutoCompleteForm like:
import QtQuick 2.5
import com.foo.AutoCompleteListener 0.1
Item {
AutoCompleteForm { id: autocomplete_form }
AutoCompleteListener { id: listener }
}
How can I pass a reference to the QML object AutoCompleteForm into AutoCompleteListener?
I tried passing the autocomplete_form field into:
Q_INVOKABLE void set_autocomplete_form(QQmlComponent *autocomplete_form);
on the onCompleted signal:
Item {
AutoCompleteForm {
id: autocomplete_form
Component.onCompleted: {
console.log("AutoCompleteForm completed");
listener.set_autocomplete_form(autocomplete_form);
}
}
AutoCompleteListener {
id: listener
Component.onCompleted: {
console.log("AutoCompleteListener completed");
}
}
}
However, the reference is a nullptr even though both AutoCompleteListener and AutoCompleteForm have been instantiated:
Instantiating AutoCompleteListener and parent is QObject(0x0)
qml: AutoCompleteListener completed
qml: AutoCompleteForm completed
Setting autocomplete_form = QObject(0x0)
How can I get a reference to the AutoCompleteForm or AutoCompleteListener's QML parent? I want to avoid crawling down the entire QML hierarchy with something like:
QObject* f = mView.rootObject();->findChild<QObject *>("AutoCompleteForm");
I plan to support having multiple AutoComplete widgets instantiated in parallel so a relative path (../AutoCompleteForm) to manipulate the QML objects seems better than having to crawl through the tree.

Answer 1

How can I pass a QML object reference into Qt C++?
You can't, the language was not designed for that. But you can get references from within C++ using findChild and findChildren. But read below for your real solution.
Also, I think your question is about an XY Problem. You have a problem X and you think Y solves it, so you ask for Y.
Correct solution for your original problem:
I am attempting to create something like jQuery's autocomplete as a
widget in QML and Qt C++.
In order to solve your real problem you need to use property bindings correctly. QML is a declarative language and self obsession with imperative programming makes it difficult to be straightforward.
Use this pattern for QML:
AutocompleteForm{
id: form
text: "Search here..."
suggestedTerms: helper.results
}
AutoCompleteHelper{
id: helper
searchFor: form.text
}
And for C++ implement
class AutoCompleteHelper : public QObject
{
Q_OBJECT
Q_PROPERTY(QString searchFor READ searchFor WRITE setSearchFor NOTIFY searchForChanged)
Q_PROPERTY(QStringList results READ results NOTIFY resultsChanged)
public:
AutoCompleteHelper() {}
virtual ~AutoCompleteHelper() {}
QString searchFor() const { return m_searchFor; }
QStringList results() const { return m_results; }
public slots:
void setSearchFor(QString searchFor)
{
if (m_searchFor == searchFor)
return;
m_searchFor = searchFor;
emit searchForChanged();
// 1. Search for it...
// 2. Some time later fill m_results
// 3. Then: emit resultsChanged()
}
signals:
void searchForChanged();
void resultsChanged();
private:
QString m_searchFor;
QStringList m_results;
};
Then you will see it magically works, because as soon as you change the form.text the binding sets the value in helper.searchFor, which then immediately fires the C++ slot where you can react even instantaneously, and in C++ the emission of resultsChanged() magically fills form.suggestedTerms.
You will find this pattern is extremely efficient so you'll even want to delay it by restarting a timer in C++. Furthermore, it is also beautifully declarative and clean.

Answer 2

How can I get a reference to the AutoCompleteForm or
AutoCompleteListener's QML parent?
The only alternative to rootObject()->findChild() appears to be QQmlProperty::read. There is several of read() function overloads so you can specify the context more precise. You have to provide the name for the object you would like to fetch and make it a property of some root object to start with. I like the general article on this subject of interfacing between QML and C++.
To accomplish precisely what you want or access the parent of certain known property you can try QQmlContext::parentContext together with QQmlProperty::read that accepts the context and see if the empty object name allows to resolve the object then.

Answer 3

You can pass it as a QVariant:
Q_INVOKABLE void QmlLink::pass_object(QVariant v)
{
YourObject* tempObject = (YourObject*) v.value<void *>();
...
}
qml:
my_bound_property.pass_object(the_object);