Qt 4.8: Chapter 1: Creating a New Type

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# Chapter 1: Creating a New Type

Files:

- declarative/tutorials/extending/chapter1-basics/app.qml
- declarative/tutorials/extending/chapter1-basics/piechart.cpp
- declarative/tutorials/extending/chapter1-basics/piechart.h
- declarative/tutorials/extending/chapter1-basics/main.cpp
- declarative/tutorials/extending/chapter1-basics/chapter1-basics.pro

A common task when extending QML is to provide a new QML type that supports some custom functionality beyond what is provided by the built-in QML Elements. For example, this could be done to implement particular data models, or provide elements with custom painting and drawing capabilities, or access system features like network programming that are not accessible through built-in QML features.

In this tutorial, we will show how to use the C++ classes in the Qt Declarative module to extend QML. The end result will be a simple Pie Chart display implemented by several custom QML types connected together through QML features like bindings and signals, and made available to the QML runtime through a plugin.

To begin with, let's create a new QML type called "PieChart" that has two properties: a name and a color. We will make it available in a module called "Charts", with a module version of 1.0.

We want this `PieChart` type to be usable from QML like this:

     import Charts 1.0

     PieChart {
         width: 100; height: 100
         name: "A simple pie chart"
         color: "red"
     }

To do this, we need a C++ class that encapsulates this `PieChart` type and its two properties. Since QML makes extensive use of Qt's meta object system, this new class must:

- Inherit from QObject
- Declare its properties using the Q_PROPERTY macro

Here is our `PieChart` class, defined in `piechart.h`:

     #include <QDeclarativeItem>
     #include <QColor>

     class PieChart : public QDeclarativeItem
     {
         Q_OBJECT
         Q_PROPERTY(QString name READ name WRITE setName)
         Q_PROPERTY(QColor color READ color WRITE setColor)

     public:
         PieChart(QDeclarativeItem *parent = 0);

         QString name() const;
         void setName(const QString &name);

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

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

     private:
         QString m_name;
         QColor m_color;
     };

The class inherits from QDeclarativeItem because we want to override QDeclarativeItem::paint() in order to draw. If the class just represented some data type and was not an item that actually needed to be displayed, it could simply inherit from QObject. Or, if we want to extend the functionality of an existing QObject-based class, it could inherit from that class instead.

The `PieChart` class defines the two properties, `name` and `color`, with the Q_PROPERTY macro, and overrides QDeclarativeItem::paint(). The class implementation in `piechart.cpp` simply sets and returns the `m_name` and `m_color` values as appropriate, and implements `paint()` to draw a simple pie chart. It also turns off the QGraphicsItem::ItemHasNoContents flag to enable painting:

     PieChart::PieChart(QDeclarativeItem *parent)
         : QDeclarativeItem(parent)
     {
         // need to disable this flag to draw inside a QDeclarativeItem
         setFlag(QGraphicsItem::ItemHasNoContents, false);
     }
     ...
     void PieChart::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
     {
         QPen pen(m_color, 2);
         painter->setPen(pen);
         painter->setRenderHints(QPainter::Antialiasing, true);
         painter->drawPie(boundingRect(), 90 * 16, 290 * 16);
     }

Now that we have defined the `PieChart` type, we will use it from QML. The `app.qml` file creates a `PieChart` item and display the pie chart's details using a standard QML Text item:

     import Charts 1.0
     import QtQuick 1.0

     Item {
         width: 300; height: 200

         PieChart {
             id: aPieChart
             anchors.centerIn: parent
             width: 100; height: 100
             name: "A simple pie chart"
             color: "red"
         }

         Text {
             anchors { bottom: parent.bottom; horizontalCenter: parent.horizontalCenter; bottomMargin: 20 }
             text: aPieChart.name
         }
     }

Notice that although the color is specified as a string in QML, it is automatically converted to a QColor object for the PieChart `color` property. Automatic conversions are provided for various other basic types; for example, a string like "640x480" can be automatically converted to a QSize value.

We'll also create a C++ application that uses a QDeclarativeView to run and display `app.qml`. The application must register the `PieChart` type using the qmlRegisterType() function, to allow it to be used from QML. If you don't register the type, `app.qml` won't be able to create a `PieChart`.

Here is the application `main.cpp`:

     #include "piechart.h"
     #include <qdeclarative.h>
     #include <QDeclarativeView>
     #include <QApplication>

     int main(int argc, char *argv[])
     {
         QApplication app(argc, argv);

         qmlRegisterType<PieChart>("Charts", 1, 0, "PieChart");

         QDeclarativeView view;
         view.setSource(QUrl::fromLocalFile("app.qml"));
         view.show();
         return app.exec();
     }

This call to qmlRegisterType() registers the `PieChart` type as a type called "PieChart", in a module named "Charts", with a module version of 1.0.

Lastly, we write a `.pro` project file that includes the files and the `declarative` library:

     QT += declarative

     HEADERS += piechart.h
     SOURCES += piechart.cpp \
                main.cpp

Now we can build and run the application:

Try it yourself with the code in Qt's `examples/tutorials/extending/chapter1-basics` directory.

At the moment, the `app.qml` is run from within a C++ application. This may seem odd if you're used to running QML files with the QML Viewer. Later on, we'll show how to create a plugin so that you can run `app.qml` using the QML Viewer instead.