Chapter 4: Using Custom Property Types

The PieChart type currently has a string-type property and a color-type property. It could have many other types of properties. For example, it could have an int-type property to store an identifier for each chart:

// C++
class PieChart : public QQuickPaintedItem
{
    Q_PROPERTY(int chartId READ chartId WRITE setChartId NOTIFY chartIdChanged)
    ...

public:
    void setChartId(int chartId);
    int chartId() const;
    ...

signals:
    void chartIdChanged();
};

// QML
PieChart {
    ...
    chartId: 100
}

Aside from int, we could use various other property types. Many of the Qt data types such as QColor, QSize and QRect are automatically supported from QML. (See Data Type Conversion Between QML and C++ documentation for a full list.)

If we want to create a property whose type is not supported by QML by default, we need to register the type with the QML engine.

For example, let's replace the use of the property with a type called "PieSlice" that has a color property. Instead of assigning a color, we assign an PieSlice value which itself contains a color:

Like PieChart, this new PieSlice type inherits from QQuickPaintedItem and declares its properties with Q_PROPERTY():

To use it in PieChart, we modify the color property declaration and associated method signatures:

    ...
    ...
    ...

There is one thing to be aware of when implementing setPieSlice(). The PieSlice is a visual item, so it must be set as a child of the PieChart using QQuickItem::setParentItem() so that the PieChart knows to paint this child item when its contents are drawn:

Like the PieChart type, the PieSlice type has to be registered using qmlRegisterType() to be used from QML. As with PieChart, we'll add the type to the "Charts" type namespace, version 1.0:

    ...
    ...

Try it out with the code in Qt's examples/quick/tutorials/extending/chapter4-customPropertyTypes directory.