如何使用C ++ API获取QML对象的id属性

Question

I need to parse a QML tree and get id s of all QML objects along the way which have it. I noticed that id s don't behave like normal properties (see the example below) – value returned from obj->property call is an invalid QVariant .

My question is – is there a way to retrieve object's id , even in some hacky (but reproductible) way?

Simplified example:

main.qml:

import QtQuick 2.9
import QtQuick.Window 2.2

Window {
    visible: true

    Item {
        id: howToGetThis
        objectName: "item"
    }
}

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QTimer>
#include <QDebug>

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

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    QTimer::singleShot(1000, [&]() {
        auto item = engine.rootObjects()[0]->findChild<QObject*>("item");
        qDebug() << item->property("objectName");
        qDebug() << item->property("id");
    });

    return app.exec();
}

Output:

QVariant(QString, "item")
QVariant(Invalid)

Answer 1

I think what you need is:

QString QQmlContext::nameForObject(QObject *object)

You can find the description here: https://doc.qt.io/qt-5/qqmlcontext.html#nameForObject

Returns the name of object in this context, or an empty string if object is not named in the context. Objects are named by setContextProperty(), or by ids in the case of QML created contexts .

---

Based on comments received, a common pitfall is to call nameForObject using the wrong QQmlContext . (When that happens, you just the empty string.) To help with that, here is a more complete example:

  QQuickItem* const focus_item = my_QQuickWindow->activeFocusItem();
  if (!focus_item) {
    fprintf(stderr, "no item has focus");
  } else {
    // There are many contexts in a hierarchy. You have to get the right one:
    QQmlContext* const context = qmlContext(focus_item);
    if (!context) {
      // Unsure if this branch of code is even reachable:
      fprintf(stderr, "item is not in any context?");
    } else {
      const QString focus_item_id = context->nameForObject(focus_item);
      fprintf(stderr, "focus item: %s\n", focus_item_id.toStdString().c_str());
    }
  }