QQmlInfo: print ancestor of object if it has no QML engine

This results in the following message for objects without a QML engine:

QML AttachedObject (parent or ancestor of Attached): Binding loop
detected for property "a"

for this QML file, named AttachedObject.qml:

import QtQuick 2.0
import org.qtproject.Test 1.0

Item {
    Attached.a: Attached.a
}

This, in turn, allows the warning to be emitted via the
QQmlEngine::warnings signal, since QQmlEnginePrivate::warning() is now
passed a valid engine. This solves the awkward situation where a
binding loop warning can not be detected at all by auto tests involving
attached C++ objects (as message handlers do not receive these messages
either).

Change-Id: I07589974207bd5448d22a6086a52b9230d23e298
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 31c892118ce822ca2e7ded99ff261187ce4cf597)

1 files changed, 16 insertions, 3 deletions

diff --git a/src/qml/qml/qqmlinfo.cpp b/src/qml/qml/qqmlinfo.cpp
index 7204d5ccd2..8852f609e9 100644
--- a/src/qml/qml/qqmlinfo.cpp
+++ b/src/qml/qml/qqmlinfo.cpp
@@ -212,11 +212,24 @@ QQmlInfo::~QQmlInfo()
             QObject *object = const_cast<QObject *>(d->object);
 
             if (object) {
-                engine = qmlEngine(d->object);
+                // Some objects (e.g. like attached objects created in C++) won't have an associated engine,
+                // but we can still try to look for a parent object that does.
+                QObject *objectWithEngine = object;
+                while (objectWithEngine) {
+                    engine = qmlEngine(objectWithEngine);
+                    if (engine)
+                        break;
+                    objectWithEngine = objectWithEngine->parent();
+                }
 
-                d->buffer.prepend(QLatin1String("QML ") + QQmlMetaType::prettyTypeName(object) + QLatin1String(": "));
+                if (!objectWithEngine || objectWithEngine == object) {
+                    d->buffer.prepend(QLatin1String("QML ") + QQmlMetaType::prettyTypeName(object) + QLatin1String(": "));
+                } else {
+                    d->buffer.prepend(QLatin1String("QML ") + QQmlMetaType::prettyTypeName(objectWithEngine)
+                        + QLatin1String(" (parent or ancestor of ") + QQmlMetaType::prettyTypeName(object) + QLatin1String("): "));
+                }
 
-                QQmlData *ddata = QQmlData::get(object, false);
+                QQmlData *ddata = QQmlData::get(objectWithEngine ? objectWithEngine : object, false);
                 if (ddata && ddata->outerContext) {
                     error.setUrl(ddata->outerContext->url());
                     error.setLine(qmlConvertSourceCoordinate<quint16, int>(ddata->lineNumber));