第 6 章：编写扩展插件

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 查看器 (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 .

这里是 ChartsPlugin definition in chartsplugin.h :

#include <QDeclarativeExtensionPlugin>
class ChartsPlugin : public QDeclarativeExtensionPlugin
{
    Q_OBJECT
public:
    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:

TEMPLATE = lib
CONFIG += qt plugin
QT += declarative
DESTDIR = lib
OBJECTS_DIR = tmp
MOC_DIR = tmp
HEADERS += piechart.h \
           pieslice.h \
           chartsplugin.h
SOURCES += piechart.cpp \
           pieslice.cpp \
           chartsplugin.cpp
symbian {
    include($$QT_SOURCE_TREE/examples/symbianpkgrules.pri)
    TARGET.EPOCALLOWDLLDATA = 1
}
maemo5: include($$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 查看器 :

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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