Files:
Suppose we want PieChart to have a "clearChart()" method that erases the chart and then emits a "chartCleared" signal. Our app.qml would be able to call clearChart() and receive chartCleared() signals like this:
import Charts 1.0
import QtQuick 1.0
Item {
width: 300; height: 200
PieChart {
id: aPieChart
anchors.centerIn: parent
width: 100; height: 100
color: "red"
onChartCleared: console.log("The chart has been cleared")
}
MouseArea {
anchors.fill: parent
onClicked: aPieChart.clearChart()
}
Text {
anchors { bottom: parent.bottom; horizontalCenter: parent.horizontalCenter; bottomMargin: 20 }
text: "Click anywhere to clear the chart"
}
}

To do this, we add a clearChart() method and a chartCleared() signal to our C++ class:
class PieChart : public QDeclarativeItem
{
...
public:
...
Q_INVOKABLE void clearChart();
signals:
void chartCleared();
...
};
The use of Q_INVOKABLE makes the clearChart() method available to the Qt Meta-Object system, and in turn, to QML. Note that it could have been declared as as a Qt slot instead of using Q_INVOKABLE, as slots are also callable from QML. Both of these approaches are valid.
The clearChart() method simply changes the color to Qt::transparent, repaints the chart, then emits the chartCleared() signal:
void PieChart::clearChart()
{
setColor(QColor(Qt::transparent));
update();
emit chartCleared();
}
Now when we run the application and click the window, the pie chart disappears, and the application outputs:
The chart has been cleared
Try out the example yourself with the updated code in Qt's examples/tutorials/extending/chapter2-methods directory.