Chapter 4: Using Custom Property Types

Files:

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 QDeclarativeItem
 {
     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
 }

We can also use various other property types. QML has built-in support for the types listed in the QML Basic Types documentation, which includes the following:

bool, unsigned int, int, float, double, qreal
QString, QUrl, QColor
QDate, QTime, QDateTime
QPoint, QPointF, QSize, QSizeF, QRect, QRectF
QVariant

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

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:

 import Charts 1.0
 import QtQuick 1.0

 Item {
     width: 300; height: 200

     PieChart {
         id: chart
         anchors.centerIn: parent
         width: 100; height: 100

         pieSlice: PieSlice {
             anchors.fill: parent
             color: "red"
         }
     }

     Component.onCompleted: console.log("The pie is colored " + chart.pieSlice.color)
 }

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

 class PieSlice : public QDeclarativeItem
 {
     Q_OBJECT
     Q_PROPERTY(QColor color READ color WRITE setColor)

 public:
     PieSlice(QDeclarativeItem *parent = 0);

     QColor color() const;
     void setColor(const QColor &color);

     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget = 0);

 private:
     QColor m_color;
 };

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

 class PieChart : public QDeclarativeItem
 {
     Q_OBJECT
     Q_PROPERTY(PieSlice* pieSlice READ pieSlice WRITE setPieSlice)
     ...
 public:
     ...
     PieSlice *pieSlice() const;
     void setPieSlice(PieSlice *pieSlice);
     ...
 };

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 QDeclarativeItem::setParentItem() so that the PieChart knows to paint this child item when its contents are drawn:

 void PieChart::setPieSlice(PieSlice *pieSlice)
 {
     m_pieSlice = pieSlice;
     pieSlice->setParentItem(this);
 }

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" module, version 1.0:

 int main(int argc, char *argv[])
 {
     ...
     qmlRegisterType<PieSlice>("Charts", 1, 0, "PieSlice");
     ...
 }

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