creating QWidget Plugins

[Spooky]

I want to create QWidget plugins for an application, that implement a certain interface. Those plugins are simply custom views that implement that interface and the main application loads these widgets into the main view.

However, I am not sure how to do this efficiently. I know that the Plugin itself has to be of the form

class MyPlugin : public QObject, public MyPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( MyPluginInterface )
    ...
};

class MyPlugin : public QObject, public MyPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( MyPluginInterface )
    ...
};

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I read that, if I want to actually create a QWidget from this plugin, the interface should simply contain

class MyPluginInterface
{
    ...
    QWidget* createWidget(QWidget* parent) = 0;
    ...
};

class MyPluginInterface
{
    ...
    QWidget* createWidget(QWidget* parent) = 0;
    ...
};

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And thus there would be another class

class MyWidget : public QWidget
{
    ...
};

class MyWidget : public QWidget
{
    ...
};

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But how would I let the actual Widget itself interact with the application? Wouldn't I need to make a wrapper function in the plugin for every single function of the interface, so that it is passed to the Widget object?

[high_flyer]

But how would I let the actual Widget itself interact with the application? Wouldn't I need to make a wrapper function in the plugin for every single function of the interface, so that it is passed to the Widget object?

No.

//in your application code
MyWidget *pWidget = m_pPlugin->createWidget();
if(pWidget){
  pWidget->someMethod();
}

//in your application code
MyWidget *pWidget = m_pPlugin->createWidget();
if(pWidget){
  pWidget->someMethod();
}

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[Spooky]

No.

//in your application code
MyWidget *pWidget = m_pPlugin->createWidget();
if(pWidget){
  pWidget->someMethod();
}

//in your application code
MyWidget *pWidget = m_pPlugin->createWidget();
if(pWidget){
  pWidget->someMethod();
}

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Well, yeah, I guess it was kind of a stupid question . I am simply a bit confused on how to implement this the "correct" way. So if I want to create plugins that are independent views in the main application, with which the main application interfaces through a certain interface (i.e. ViewInterface). Would I need to do something like this?

class ViewPlugin : public QObject, public ViewPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( ViewPluginInterface )
    ...
};

class ViewPlugin : public QObject, public ViewPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( ViewPluginInterface )
    ...
};

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class ViewPluginInterface
{
    ViewWidget* createWidget( QWidget* parent ) = 0;
};

class ViewPluginInterface
{
    ViewWidget* createWidget( QWidget* parent ) = 0;
};

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class ViewWidget : public ViewInterface
{
    ...
};

class ViewWidget : public ViewInterface
{
    ...
};

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class ViewInterface : public QWidget
{
    ...
};

class ViewInterface : public QWidget
{
    ...
};

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In that case, the createWidget function would be the only function of the plugin interface. And I am not sure the inheritance of QWidget works this way?

[high_flyer]

In that case, the createWidget function would be the only function of the plugin interface.

The code you post would work, but make little sense as a plugin - since in what way the fact the widget came from a plugin would be different then using new MyWidget() in your application?

It makes sens to use a plugin, when the objects the various plugins provide conform to one interface.
So in your application you don't need to know which class implements the interface, and just use the interface methods - that is the whole idea (very simplified) of using a plugin.
So if you can contain the functionality of all you plugin provided widgets in a fixed set of methods, then you expose those methods through the interface and use an interface pointer to call them, without caring which class currently implemtns it.

class MyPluginInterface
{
    MyWidget* createWidget(QWidget* parent) = 0;
    void someMethod() = 0;
};

class MyPluginInterface
{
    MyWidget* createWidget(QWidget* parent) = 0;
    void someMethod() = 0;
};

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//in your applicaiton
MyWidget *pView = m_pPlugin->createWidget();
if(pView){
  pView->someMethod(); 
}

//in your applicaiton
MyWidget *pView = m_pPlugin->createWidget();
if(pView){
  pView->someMethod(); 
}

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This brings us back to your previous post.
And yes, you will have to expose all the methods shared by your plugin widgets through the interface.
This is what the interface is for.

[Spooky]

So if you can contain the functionality of all you plugin provided widgets in a fixed set of methods, then you expose those methods through the interface and use an interface pointer to call them, without caring which class currently implemtns it.

Yes, that is the plan at least.

class MyPluginInterface
{
    MyWidget* createWidget(QWidget* parent) = 0;
    void someMethod() = 0;
};

class MyPluginInterface
{
    MyWidget* createWidget(QWidget* parent) = 0;
    void someMethod() = 0;
};

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//in your applicaiton
MyWidget *pView = m_pPlugin->createWidget();
if(pView){
  pView->someMethod(); 
}

//in your applicaiton
MyWidget *pView = m_pPlugin->createWidget();
if(pView){
  pView->someMethod(); 
}

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This brings us back to your previous post.
And yes, you will have to expose all the methods shared by your plugin widgets through the interface.
This is what the interface is for.

Hm, sorry, I don't understand your example . In this example, wouldn't the MyPlugin and the MyWidget class have to implement the MyPluginInterface? And wouldn't the implementation of someMethod() be empty in MyPlugin and implemented in MyWidget, while the createWidget() function would be the other way around?

[high_flyer]

Hm, sorry, I don't understand your example . In this example, wouldn't the MyPlugin and the MyWidget class have to implement the MyPluginInterface?

There is no MyPlugin class.
m_pPlugin in my example is:

MyPluginInterface *m_pPlugin;

MyPluginInterface *m_pPlugin;

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And yes, your class needs to implement the interface, since it will derive from it.

you can do the following instead of what you did:

class ViewPluginWidget : public QWidget, public ViewPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( ViewPluginInterface )
    ...
};

class ViewPluginWidget : public QWidget, public ViewPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( ViewPluginInterface )
    ...
};

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[Spooky]

Ok, I think my confusion came from the fact, that I thought the plugin must somehow inherit from QObject and can't be just QWidget only . However, in that case I don't really need a createWidget function?

I was thinking of doing something like this now:

class ViewPluginInterface : public QWidget // to ensure, that anyone using this interface, is creating it as a QWidget
{
    ...
};
 
Q_DECLARE_INTERFACE( ViewPluginInterface, "..." )

class ViewPluginInterface : public QWidget // to ensure, that anyone using this interface, is creating it as a QWidget
{
    ...
};

Q_DECLARE_INTERFACE( ViewPluginInterface, "..." )

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class SomeCustomViewPluginForTheApp : public ViewPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( ViewPluginInterface )
    ...
};

class SomeCustomViewPluginForTheApp : public ViewPluginInterface
{
    Q_OBJECT
    Q_INTERFACES( ViewPluginInterface )
    ...
};

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And then at some point in the main application

QDir pluginDir( QApplication::applicationDirPath() );
 
if ( !pluginDir.cd( "plugins" ) )
    return;
 
foreach( QString fileName, pluginDir.entryList( QDir::Files ) )
{
    QPluginLoader loader( pluginDir.absoluteFilePath( fileName ) );
    if ( ViewPluginInterface *interface = qobject_cast<ViewPluginInterface*>( loader.instance() ) )
    {
        // this should be a ViewPluginInterface instance, which in turn is a QWidget instance 
        ...
    }
}

QDir pluginDir( QApplication::applicationDirPath() );

if ( !pluginDir.cd( "plugins" ) )
    return;

foreach( QString fileName, pluginDir.entryList( QDir::Files ) )
{
    QPluginLoader loader( pluginDir.absoluteFilePath( fileName ) );
    if ( ViewPluginInterface *interface = qobject_cast<ViewPluginInterface*>( loader.instance() ) )
    {
        // this should be a ViewPluginInterface instance, which in turn is a QWidget instance 
        ...
    }
}

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[high_flyer]

Ok, I think my confusion came from the fact, that I thought the plugin must somehow inherit from QObject and can't be just QWidget only

QWidget is a QObject.

class ViewPluginInterface : public QWidget // to ensure, that anyone using this interface, is creating it as a QWidget
{
    ...
};

class ViewPluginInterface : public QWidget // to ensure, that anyone using this interface, is creating it as a QWidget
{
    ...
};

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You can't do that, since then your interface is not an interface any more.

[Spooky]

Darn, thx . Works now as I intended. However, I made it so that I have a getWidget() function defined in the plugin interface, instead of a createWidget() function, which simply returns this in the implementation of the plugin widget.

class ViewPluginInterface
{
public:
	virtual QWidget* getWidget() = 0;
};
 
Q_DECLARE_INTERFACE( ViewPluginInterface, "..." )

class ViewPluginInterface
{
public:
	virtual QWidget* getWidget() = 0;
};

Q_DECLARE_INTERFACE( ViewPluginInterface, "..." )

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class SomeViewPlugin : public QWidget, public ViewPluginInterface
{
	Q_OBJECT
	Q_INTERFACES( ViewPluginInterface )
 
	...
	QWidget* getWidget();
	...
};

class SomeViewPlugin : public QWidget, public ViewPluginInterface
{
	Q_OBJECT
	Q_INTERFACES( ViewPluginInterface )

	...
	QWidget* getWidget();
	...
};

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...
 
QWidget* SomeViewPlugin::getWidget()
{
	return this;
}
 
...
 
Q_EXPORT_PLUGIN2( someviewplugin, SomeViewPlugin )

...

QWidget* SomeViewPlugin::getWidget()
{
	return this;
}

...

Q_EXPORT_PLUGIN2( someviewplugin, SomeViewPlugin )

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[high_flyer]

However, I made it so that I have a getWidget() function defined in the plugin interface, instead of a createWidget() function, which simply returns this in the implementation of the plugin widget.

What for?
You always have an instance of your plugin from the plugin loader...

[Spooky]

What for?
You always have an instance of your plugin from the plugin loader...

Yes, another stupid thing from me .

However, the way I implemented it now is still not really the way I want it, because there always only exists one instance of a plugin. There should be multiple instances of these 'views' be available in my application (someone else ran into a similar problem here). But then I am back to this version (more or less), where the plugin interface itself does nothing but specify a factory function for a certain type (e.g. a 'ViewWidget').