Implementing the Qt plug-in interface

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Article Metadata

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Tested with

Devices(s): Nokia 5800 XpressMusic

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Compatibility

Platform(s): S60 3rd Edition FP1, FP2
S60 5th Edition

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Article

Keywords: QPluginLoader, Q_DECLARE_INTERFACE, Q_INTERFACES, Q_EXPORT_PLUGIN2

Created: tepaa (20 May 2009)

Last edited: hamishwillee (11 Oct 2012)

Overview

Qt applications can be extended with Qt plug-ins. This code snippet demonstrates how to implement the plug-in on top of the plug-in interface defined in the client application (the interface was described in the code snippet Defining a Qt plug-in interface).

The plug-in has to implement all methods defined in the plug-in interface. Additional methods and member variables can also be implemented. However, note that the client application only sees methods and data that are defined into the common interface exampleplugininterface.h.

The defining of a plug-in interface and plug-in usage are described in additional snippet articles.

Important

• When using Qt libraries from the (qt_libs_armv5_udeb.sisx) on a device, be sure to build both the plug-in and the loading application in DEBUG(UDEB) mode. This is because Qt uses a build key for each plug-in to ensure that only compatible plug-ins are loaded, and the build mode [debug is part of that build key.
• In Symbian we have to define the EPOCALLOWDLLDATA variable as true for the plug-ins (library) because Qt macros have initialised global data; see plug-in project files for this.

Project .pro file

TEMPLATE        = lib
 
CONFIG         += plugin
 
# Define path where compiler can found your defined interface for the plugins
INCLUDEPATH    += ../../
 
HEADERS         = yourplugin.h
SOURCES         = yourplugin.cpp
 
TARGET          = $$qtLibraryTarget(yourplugin)
 
symbian: {
# Load predefined include paths (e.g. QT_PLUGINS_BASE_DIR) to be used in the pro-files
load(data_caging_paths)
 
# EPOCALLOWDLLDATA have to set true because Qt macros has initialised global data
TARGET.EPOCALLOWDLLDATA=1
 
# Defines plugin files into Symbian .pkg package
pluginDep.sources = yourplugin.dll
pluginDep.path = $$QT_PLUGINS_BASE_DIR/exampleplugins
DEPLOYMENT += pluginDep
}
 
target.path += $$[QT_INSTALL_PLUGINS]/exampleplugins
INSTALLS += target

Header (yourplugin.h)

The header for your plug-in that implements your own ExamplePluginInterface plug-in interface.

// Your defined interface for the plugins
#include "exampleplugininterface.h"
 
class YourPlugin : public ExamplePluginInterface
    {
    // That is because we use Qt signal/slot feature
    // in "void someSlot(QString value)"
    Q_OBJECT
 
    // This macro tells Qt which interfaces the class implements
    // This is used when implementing plugins
    Q_INTERFACES(ExamplePluginInterface)
 
    public: // From ExamplePluginInterface
        ~LinkPlugin();
        QString sayHello();
 
    public slots: // From ExamplePluginInterface       
        void someSlot(QString value);
    };

Source (yourplugin.cpp)

#include <QtCore/qplugin.h>
#include "yourplugin.h"
 
YourPlugin::~YourPlugin()
    {
    }
 
QString YourPlugin::sayHello()
    {
    return QString("Hello from YourPlugin: "+data);
    }
 
void YourPlugin::someSlot(QString value)
    {
    data = value;
    }
 
// This Qt macro exports the plugin class YourPlugin with the 
// name "exampleplugins"
// There should be exactly one occurrence of this YourPlugin 
// macro in a Qt plugin's source code.
Q_EXPORT_PLUGIN2(exampleplugins, YourPlugin);

See also

• Defining a Qt plug-in interface
• Loading and initialising a Qt plug-in dynamically

• For more information about Qt plug-ins, see How to Create Qt Plugins.

Postconditions

The Qt plug-in interface is implemented.