How to write, load and unload plug-ins in QT

Qt Provides a class QPluginLoader To load static and dynamic libraries , stay Qt in ,Qt Consider both dynamic library and static library as a plug-in , Use QPluginLoader To load and unload these libraries . Because in the process of developing the project , To develop a plug-in system , I used Qt This class library of .

One Writing a plug-in

Write a Qt The plug-in of requires the following steps  

1. Declare a plug-in class ,

2. Define a class , Implement the interface defined by this plug-in class , This class must be defined from QObject Integrated .

3. Use Q_INTERFACESQ_INTERFACES() inform Qt The existence of the interface of the meta system

3. Use macros Q_EXPORT_PLUGIN2() To export the interface of the plug-in

4. Write a plug-in class .pro file

Here is an example of a plug-in

class FilterInterface
{
public:
     virtual ~FilterInterface() {}

     virtual QStringList filters() const = 0;
     virtual QImage filterImage(const QString &filter, const QImage &image,
                                QWidget *parent) = 0;
};

Be careful ： The interface class must be a pure virtual function

Q_DECLARE_INTERFACE(FilterInterface,"com.trolltech.PlugAndPaint.BrushInterface/1.0") 

In order to enable the program to query whether a plug-in implements a given interface at runtime , You must use macros Q_DECLARE_INTERFACE(). Its first parameter is the name of the interface , The second parameter is a string that determines the interface . Convenience , Used JAVA Package naming method . If we change the interface later , You must use a new string to confirm the interface . otherwise , The application may crash , So including a version number is a good way . 

#include <QObject>
#include <QStringList>
#include <QImage>

#include <plugandpaint/interfaces.h>

class ExtraFiltersPlugin : public QObject, public FilterInterface
{
     Q_OBJECT
     Q_INTERFACES(FilterInterface)

public:
     QStringList filters() const;
     QImage filterImage(const QString &filter, const QImage &image,
                        QWidget *parent);
};

Implement this interface class . The class that implements the interface must start from QObject Derived , And you must implement the virtual function defined in the interface class . 

 Q_EXPORT_PLUGIN2(PlugExtraFilters,  ExtraFiltersPlugin )

 Because the application function uses main() Function as the entry point , Plug ins must use macros Q_EXPORT_PLUGIN2() To specify which class provides plug-ins .

 This line of code can appear in the .cpp Any line of the file . The first parameter is the name of the plug-in , The second parameter is the plug-in class 

 Two   The use of plug-in 

QPluginLoader loader(pluginsDir.absoluteFilePath(fileName));

         QObject *plugin = loader.instance();

         if (plugin) {

             populateMenus(plugin);

             pluginFileNames += fileName;

         }

 

    QPluginLoader Class provides a function loader To load an instance of the plug-in , If the DLL is not a plug-in , Or compiled Qt Wrong version Library , Back to 

QObject The pointer to the object is null .】

 Use unload Function to uninstall the plug-in .

 3、 ... and  .pro Compilation of documents 

TEMPLATE      = lib

 CONFIG       += plugin 

 HEADERS       = extrafiltersplugin.h

 SOURCES       = extrafiltersplugin.cpp

 TARGET        =  pnp_extrafilters

 DESTDIR       = ../../plugandpaint/plugins 

TEMPLATE The type of lib.CONFIG The type of plugin