QCanvasPainterItem Class

Detailed Description

To write your own painted item, you first create a subclass of QCanvasPainterItem, and then start by implementing its only pure virtual public function: createItemRenderer(), which returns an object that performs the actual painting.

The below code snippet shows the typical structure of a QCanvasPainterItem subclass. See QCanvasPainterItemRenderer for an example of the MyRenderer class.

 class MyItem : public QCanvasPainterItem
 {
     Q_OBJECT
     QML_NAMED_ELEMENT(MyItem) // exposed to QML, instantiate as MyItem { ... }

     // a custom property
     Q_PROPERTY(float value READ value WRITE setValue NOTIFY valueChanged)

 public:
     MyItem(QQuickItem *parent = nullptr)
         : QCanvasPainterItem(parent)
     {
     }

     QCanvasPainterItemRenderer *createItemRenderer() const override
     {
         return new MyRenderer;
     }

     float value() const { return m_value; }
     void setValue(float newValue)
     {
         if (m_value != newValue) {
             m_value = newValue;
             emit valueChanged();
         }
     }

 signals:
     void valueChanged();

 private:
     float m_value = 0.0f;
 };

Using the QtQuick::Item::antialiasing property, it is possible to disable antialiasing. The default value is true. When this is set to false, it disables antialiasing even if the painting code calls QCanvasPainter::setAntialias() with a value bigger than 0. Disabling antialiasing potentially increases the performance.