Chapter 6: Writing an Extension Plugin

Currently the PieChart and PieSlice types are used by app.qml, which is displayed using a QDeclarativeView in a C++ application. An alternative way to use our QML extension is to create a plugin library to make it available to the QML engine. This allows app.qml to be loaded with the QML Viewer (or some other QML runtime application) instead of writing a main.cpp file and loading our own C++ application.

To create a plugin library, we need:

  • A plugin class that registers our QML types
  • A project file that describes the plugin
  • A qmldir file that tells the QML engine to load the plugin

First, we create a plugin class named ChartsPlugin. It subclasses QDeclarativeExtensionPlugin and registers our QML types in the inherited registerTypes() method. It also calls Q_EXPORT_PLUGIN2 for Qt's plugin system.

Here is the ChartsPlugin definition in chartsplugin.h:

#include <QDeclarativeExtensionPlugin>

class ChartsPlugin : public QDeclarativeExtensionPlugin
    void registerTypes(const char *uri);

And its implementation in chartsplugin.cpp:

#include "piechart.h"
#include "pieslice.h"
#include <qdeclarative.h>

void ChartsPlugin::registerTypes(const char *uri)
    qmlRegisterType<PieChart>(uri, 1, 0, "PieChart");
    qmlRegisterType<PieSlice>(uri, 1, 0, "PieSlice");

Q_EXPORT_PLUGIN2(chartsplugin, ChartsPlugin);

Then, we write a .pro project file that defines the project as a plugin library and specifies with DESTDIR that library files should be built into a "lib" subdirectory:

CONFIG += qt plugin
QT += declarative

MOC_DIR = tmp

HEADERS += piechart.h \
           pieslice.h \

SOURCES += piechart.cpp \
           pieslice.cpp \

symbian {
maemo5: include($$QT_SOURCE_TREE/examples/maemo5pkgrules.pri)

Finally, we add a qmldir file that is automatically parsed by the QML engine. In this file, we specify that a plugin named "chapter6-plugin" (the name of the example project) can be found in the "lib" subdirectory:

plugin chapter6-plugins lib

Now we have a plugin, and instead of having a main.cpp and an executable, we can build the project and then load the QML file in the QML Viewer:

qmlviewer app.qml

(On Mac OS X, you can launch the "QMLViewer" application instead.)

Notice the "import Charts 1.0" statement has disappeared from app.qml. This is because the qmldir file is in the same directory as app.qml: this is equivalent to having PieChart.qml and PieSlice.qml files inside the project directory, which could both be used by app.qml without import statements.


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