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

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.

Ariel M. Ariel M. · Accepted Answer · 2015-12-01T03:31:23

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.

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

3 Answers

Correct solution for your original problem: