QQmlListReference: Allow construction from a QVariant

Currently there is no public API to transform a QVariant holding a
QQmlListProperty into a QQmlListReference. We cannot pass
QQmlListProperty itself as that is templated. The metatype-based casting
is somewhat evil, but not more so than what we already have in the other
ctor.

Change-Id: I2d56499b1fd188613bc71016cb00ec23081d3cea
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

3 files changed, 59 insertions, 0 deletions

diff --git a/src/qml/qml/qqmllist.cpp b/src/qml/qml/qqmllist.cpp
index cffdf7c907..fe0b820a1e 100644
--- a/src/qml/qml/qqmllist.cpp
+++ b/src/qml/qml/qqmllist.cpp
@@ -120,6 +120,42 @@ QQmlListReference::QQmlListReference()
 }
 
 /*!
+\since 6.1
+
+Constructs a QQmlListReference from a QVariant \a variant containing a QQmlListProperty. If
+\a variant does not contain a list property, an invalid QQmlListReference is created. If the object
+owning the list property is destroyed after the reference is constructed, it will automatically
+become invalid.  That is, it is safe to hold QQmlListReference instances even after the object is
+deleted.
+
+The \a engine is required to look up the element type, which may be a dynamically created QML type.
+If it's omitted, only pre-registered types are available.
+*/
+QQmlListReference::QQmlListReference(const QVariant &variant, QQmlEngine *engine)
+    : d(nullptr)
+{
+    const QMetaType t = variant.metaType();
+    if (!(t.flags() & QMetaType::IsQmlList))
+        return;
+
+    QQmlEnginePrivate *p = engine ? QQmlEnginePrivate::get(engine) : nullptr;
+    const int listType = p ? p->listType(t.id()) : QQmlMetaType::listType(t.id());
+    if (listType == -1)
+        return;
+
+    d = new QQmlListReferencePrivate;
+    d->propertyType = t.id();
+    d->elementType = p
+            ? p->rawMetaObjectForType(listType)
+            : QQmlMetaType::qmlType(listType).baseMetaObject();
+
+    d->property.~QQmlListProperty();
+    t.construct(&d->property, variant.constData());
+
+    d->object = d->property.object;
+}
+
+/*!
 Constructs a QQmlListReference for \a object's \a property.  If \a property is not a list
 property, an invalid QQmlListReference is created.  If \a object is destroyed after
 the reference is constructed, it will automatically become invalid.  That is, it is safe to hold
diff --git a/src/qml/qml/qqmllist.h b/src/qml/qml/qqmllist.h
index 3cc353ef30..abf4ec6ebf 100644
--- a/src/qml/qml/qqmllist.h
+++ b/src/qml/qml/qqmllist.h
@@ -197,6 +197,7 @@ class Q_QML_EXPORT QQmlListReference
 {
 public:
     QQmlListReference();
+    explicit QQmlListReference(const QVariant &variant, QQmlEngine *engine = nullptr);
     QQmlListReference(QObject *, const char *property, QQmlEngine * = nullptr);
     QQmlListReference(const QQmlListReference &);
     QQmlListReference &operator=(const QQmlListReference &);
diff --git a/tests/auto/qml/qqmllistreference/tst_qqmllistreference.cpp b/tests/auto/qml/qqmllistreference/tst_qqmllistreference.cpp
index bf5ee92374..c6accb3908 100644
--- a/tests/auto/qml/qqmllistreference/tst_qqmllistreference.cpp
+++ b/tests/auto/qml/qqmllistreference/tst_qqmllistreference.cpp
@@ -179,6 +179,17 @@ void tst_qqmllistreference::qmllistreference()
 
     tt.data.append(&tt);
     QCOMPARE(r.count(), 1);
+
+    const QMetaObject *m = tt.metaObject();
+    const int index = m->indexOfProperty("data");
+    const QMetaProperty prop = m->property(index);
+    const QVariant var = prop.read(&tt);
+
+    QQmlListReference fromVar(var);
+    QVERIFY(fromVar.isValid());
+    QCOMPARE(fromVar.count(), 1);
+    fromVar.append(&tt);
+    QCOMPARE(tt.data.count(), 2);
 }
 
 void tst_qqmllistreference::qmllistreference_invalid()
@@ -805,6 +816,17 @@ void tst_qqmllistreference::engineTypes()
     QVERIFY(ref.listElementType());
     QVERIFY(ref.listElementType() != &QObject::staticMetaObject);
 
+
+    const QMetaObject *m = o->metaObject();
+    const int index = m->indexOfProperty("myList");
+    const QMetaProperty prop = m->property(index);
+    const QVariant var = prop.read(o);
+
+    QQmlListReference fromVar(var, &engine);
+    QVERIFY(fromVar.isValid());
+    QCOMPARE(fromVar.count(), 2);
+    QCOMPARE(fromVar.listElementType(), ref.listElementType());
+
     delete o;
 }