第 2 章：连接到 C++ 方法和信号

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 目录。

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