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Details

Type: Suggestion
Resolution: Done
Priority: P2: Important
Fix Version/s: 5.6.0 Beta
Affects Version/s: 5.4.0
Component/s: QML: Declarative and Javascript Engine
Labels:
None

Commits:
8e7d1a91196197eee4e45bbfa9886ab935e2b67c

Description

A common problem for controls is that when the business logic is implemented in a C++ "base" type and the visual parts in a QML "sub" type, attached properties provided by the C++ type (hidden implementation detail) cannot be accessed by the public QML type name.

Example: a generic ScrollIndicator that can be attached to any Flickable.

C++ base type:

// qquickabstractscrollindicator_p.h
class QQuickScrollIndicatorAttached : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QQuickAbstractScrollIndicator *horizontal ...)
    Q_PROPERTY(QQuickAbstractScrollIndicator *vertical ...)
};

class QQuickAbstractScrollIndicator : public QQuickItem
{
    Q_OBJECT

public:
    explicit QQuickAbstractScrollIndicator(QQuickItem *parent = Q_NULLPTR);

    static QQuickScrollIndicatorAttached *qmlAttachedProperties(QObject *object);
};

QML_DECLARE_TYPEINFO(QQuickAbstractScrollIndicator, QML_HAS_ATTACHED_PROPERTIES)

// qtquickcontrolsplugin.cpp
qmlRegisterType<QQuickAbstractScrollIndicator>(uri, 2, 0, "AbstractScrollIndicator");

QML type:

// qmldir
ScrollIndicator 2.0 ScrollIndicator.qml

// ScrollIndicator.qml
AbstractScrollIndicator {
    id: control
    implicitWidth: ...
    implicitHeight: ...
    indicator: Rectangle { ... }
    background: Rectangle { ... }
}

Application code cannot use ScrollIndicator.vertical (QML engine throws "Non-existent attached object" error), but the attached property must be referenced using the C++ base type name ie. AbstractScrollIndicator. This makes naming very difficult to achieve optimal API for such controls.

Flickable {
    // ScrollIndicator.vertical: ScrollIndicator { ... } // "Non-existent attached object"
    // vs.
    AbstractScrollIndicator.vertical: ScrollIndicator { ... } // :(
}