Qt的插件机制

使用Qt插件和编写Qt插件需要注意：

Making an application extensible through plugins involves the following steps:

Define a set of interfaces (classes with only pure virtual functions) used to talk to the plugins.

Use the Q_DECLARE_INTERFACE() macro to tell Qt's meta-object system about the interface.

Use QPluginLoader in the application to load the plugins.

Use qobject_cast() to test whether a plugin implements a given interface.

Writing a plugin involves these steps:

Declare a plugin class that inherits from QObject and from the interfaces that the plugin wants to provide.

Use the Q_INTERFACES() macro to tell Qt's meta-object system about the interfaces.

Export the plugin using the Q_EXPORT_PLUGIN2() macro.

Build the plugin using a suitable .pro file

Qmediaserviceproviderplugin.h

struct Q_MULTIMEDIA_EXPORT QMediaServiceProviderFactoryInterface : public QFactoryInterface
{
...
};
#define QMediaServiceProviderFactoryInterface_iid /
"com.nokia.Qt.QMediaServiceProviderFactoryInterface/1.0"
Q_DECLARE_INTERFACE(QMediaServiceProviderFactoryInterface, QMediaServiceProviderFactoryInterface_iid)
struct Q_MULTIMEDIA_EXPORT QMediaServiceSupportedDevicesInterface
{
...
};
#define QMediaServiceSupportedDevicesInterface_iid /
"com.nokia.Qt.QMediaServiceSupportedDevicesInterface/1.0"
Q_DECLARE_INTERFACE(QMediaServiceSupportedDevicesInterface, QMediaServiceSupportedDevicesInterface_iid)

class Q_MULTIMEDIA_EXPORT QMediaServiceProviderPlugin : public QObject, public QMediaServiceProviderFactoryInterface
{
Q_OBJECT
Q_INTERFACES(QMediaServiceProviderFactoryInterface:QFactoryInterface)
public:
...
};

Qstreamerplayerplugin.h

class QGstreamerServicePlugin : public QMediaServiceProviderPlugin, public QMediaServiceSupportedDevicesInterface
{
Q_OBJECT
Q_INTERFACES(QMediaServiceSupportedDevicesInterface)
public:
...
private:
...
};

Qsteamerplayerplugin.cpp

#include "qstreamerplayerplugin.h"
//Implementation
...
Q_EXPORT_PLUGIN2(qtmedia_gstengine, QGstreamerServicePlugin);

Qstreamerplayerplugin继承并实现了两个抽象类。Q_INTERFACES的作用是将所实现的接口通知元类型系统，而Q_EXPORT_PLUGIN2(PluginName, ClassName)指明了插件名和根类名，表示向外部声明本插件的“身份证。

QPluginLoader可以在运行时载入一个插件，它在构造函数中指明一个包含插件的文件（shared library）并用load()导入，其最重要的功能是可以用instance()直接访问这个插件的root component，而不用人工地分析这个文件。而所谓的root component其实就是插件中使用了Q_EXPORT_PLUGIN2的那个类，instance()的返回值就是指向这个类的实例的QObject指针。

一旦插件被导入，它会一直驻留在内存中直到所有的QPluginLoader实例被unload或应用程序终止。你可以调用unload()来试图释放一个插件，但是只有一个插件的所有实例都被调用unload()，插件才会被真正释放。