Add override behaviors to QQmlListProperty

[ChangeLog][QQmlListProperty] When overriding a
QQmlListProperty in a derived QML type, the default behavior is to
append the derived class elements to the base class ones. This
introduces a macro to allow replacing the base
type contents either always or if the property is not the default one.

Fixes: QTBUG-77529
Change-Id: Ib1abbf52e341c043344c347c612928b47856fb3e
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

3 files changed, 110 insertions, 0 deletions

diff --git a/src/qml/qml/qqmllist.cpp b/src/qml/qml/qqmllist.cpp
index 717170d362..a0d34cbce7 100644
--- a/src/qml/qml/qqmllist.cpp
+++ b/src/qml/qml/qqmllist.cpp
@@ -429,6 +429,83 @@ QML list properties are type-safe - in this case \c {Fruit} is a QObject type th
 */
 
 /*!
+    \macro QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_APPEND
+    \relates QQmlListProperty
+
+    This macro defines the behavior of the list properties of this class to Append.
+    When assigning the property in a derived type, the values are appended
+    to those of the base class. This is the default behavior.
+
+    \code
+    class FruitBasket : QObject {
+        \Q_OBJECT
+        QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_APPEND
+        Q_PROPERTY(QQmlListProperty<Fruit> fruit READ fruit)
+
+        public:
+        // ...
+        QQmlListProperty<Fruit> fruit();
+        // ...
+    };
+    \endcode
+
+    \sa QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE_IF_NOT_DEFAULT
+    \sa QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE
+*/
+
+/*!
+    \macro QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE_IF_NOT_DEFAULT
+    \relates QQmlListProperty
+
+    This macro defines the behavior of the list properties of this class to
+    ReplaceIfNotDefault.
+    When assigning the property in a derived type, the values replace those of
+    the base class unless it's the default property.
+    In the case of the default property, values are appended to those of the base class.
+
+    \code
+    class FruitBasket : QObject {
+        \Q_OBJECT
+        QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE_IF_NOT_DEFAULT
+        Q_PROPERTY(QQmlListProperty<Fruit> fruit READ fruit)
+
+        public:
+        // ...
+        QQmlListProperty<Fruit> fruit();
+        // ...
+    };
+    \endcode
+
+    \sa QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_APPEND
+    \sa QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE
+*/
+
+/*!
+    \macro QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE
+    \relates QQmlListProperty
+
+    This macro defines the behavior of the list properties of this class to Replace.
+    When assigning the property in a derived type, the values replace those
+    of the base class.
+
+    \code
+    class FruitBasket : QObject {
+        \Q_OBJECT
+        QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE
+        Q_PROPERTY(QQmlListProperty<Fruit> fruit READ fruit)
+
+        public:
+        // ...
+        QQmlListProperty<Fruit> fruit();
+        // ...
+    };
+    \endcode
+
+    \sa QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_APPEND
+    \sa QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE_IF_NOT_DEFAULT
+*/
+
+/*!
 \fn template<typename T> QQmlListProperty<T>::QQmlListProperty()
 \internal
 */
diff --git a/src/qml/qml/qqmllist.h b/src/qml/qml/qqmllist.h
index 313d7449fe..199f10196a 100644
--- a/src/qml/qml/qqmllist.h
+++ b/src/qml/qml/qqmllist.h
@@ -50,6 +50,10 @@ QT_BEGIN_NAMESPACE
 class QObject;
 struct QMetaObject;
 
+#define QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_APPEND Q_CLASSINFO("QML.ListPropertyAssignBehavior", "Append")
+#define QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE_IF_NOT_DEFAULT Q_CLASSINFO("QML.ListPropertyAssignBehavior", "ReplaceIfNotDefault")
+#define QML_LIST_PROPERTY_ASSIGN_BEHAVIOR_REPLACE Q_CLASSINFO("QML.ListPropertyAssignBehavior", "Replace")
+
 #ifndef QQMLLISTPROPERTY
 #define QQMLLISTPROPERTY
 template<typename T>
diff --git a/src/qml/qml/qqmlobjectcreator.cpp b/src/qml/qml/qqmlobjectcreator.cpp
index 5fa0eb281e..11d3dd7b5d 100644
--- a/src/qml/qml/qqmlobjectcreator.cpp
+++ b/src/qml/qml/qqmlobjectcreator.cpp
@@ -782,6 +782,35 @@ void QQmlObjectCreator::setupBindings(bool applyDeferredBindings)
                 void *argv[1] = { (void*)&_currentList };
                 QMetaObject::metacall(_qobject, QMetaObject::ReadProperty, property->coreIndex(), argv);
                 currentListPropertyIndex = property->coreIndex();
+
+                // manage override behavior
+                const QMetaObject *const metaobject = _qobject->metaObject();
+                const int qmlListBehavorClassInfoIndex = metaobject->indexOfClassInfo("QML.ListPropertyAssignBehavior");
+                if (qmlListBehavorClassInfoIndex != -1) { // QML.ListPropertyAssignBehavior class info is set
+                    const char *overrideBehavior =
+                            metaobject->classInfo(qmlListBehavorClassInfoIndex).value();
+                    if (!strcmp(overrideBehavior,
+                                "Replace")) {
+                        if (_currentList.clear) {
+                            _currentList.clear(&_currentList);
+                        }
+                    } else {
+                        bool isDefaultProperty =
+                                (property->name(_qobject)
+                                 == QString::fromUtf8(
+                                         metaobject
+                                                 ->classInfo(metaobject->indexOfClassInfo(
+                                                         "DefaultProperty"))
+                                                 .value()));
+                        if (!isDefaultProperty
+                            && (!strcmp(overrideBehavior,
+                                        "ReplaceIfNotDefault"))) {
+                            if (_currentList.clear) {
+                                _currentList.clear(&_currentList);
+                            }
+                        }
+                    }
+                }
             }
         } else if (_currentList.object) {
             _currentList = QQmlListProperty<void>();