qt-creator/doc/pluginhowto/qtc-arch.qdoc

/*!
    \page qtc-arch.html
    \title 4. Qt Creator Architecture
		
	Every large system has a well defined "system architecture" which if understood well makes it easy for us to find out
	way in it. Qt Creator is no different. In this chapter we will understand the basic architecture of Qt Creator so that we
	can continue our understanding of writing plugins.
	
	\section1 4.1 Nuts and Bolts of Qt Creator
	
	Qt Creator is Nokia's cross-platform IDE. Currently Qt Creator is mainly used for writing Qt/C++ code.
	The core of Qt Creator is basically only a "plugin loader" All functionality is implemented in plugins.
	
	\inlineimage  qtc-pluginmanager-4.png
	
	
	The core "personality"of Qt Creator is implemented in the \bold {Core Plugin (Core::ICore)}. We have already had a brush
	with the core plugin in the previous chapter. In the rest of this document we will refer to "core plugin" as Core.
	The plugin manager \bold{(ExtensionSystem::PluginManager)} provides simple means for plugin cooperation that
	allow plugins to provide hooks for other plugin's extensions.
	
	\section1 4.2 What exactly is a plugin?
	
	At the most fundamental level plugin is a shared library (DLL file on Windows, SO file on Linux, DYLIB file on Mac). From
	a developer's point of view plugin is a module that
	\list 1
	\o Implements the ExtensionSystem::IPlugin interface in a class. This class will be referred to as "Plugin Class" in the
	   rest of the document.
	\o Exports the Plugin Class using the Q_EXPORT_PLUGIN macro
	\o Provides a pluginspec file that provides some meta information about the plugin
	\o Registers one or more objects that might be of some interest to other plugins
	\o Searches for the availability of one or more objects registered by other plugins.
	\endlist
	
	We have already had some experience with the first three aspects listed above, but we have not touched upon the last
	two.
	\section2 4.2.1 What are registered objects?
	
	 objects are those that land up in the \bold {PluginManager's} object pool. The \bold {allObjects()} method in
	\bold {PluginManager} returns the object pool as a list of QObject pointers. Shown below is the code that we can use to list
	all objects in the object-pool in a \bold {QListWidget}.
	
	\code
	#include <extensionsystem/pluginmanager.h>
	
	ExtensionSystem::PluginManager* pm = ExtensionSystem::PluginManager::instance();
	QList<QObject*> objects = pm->allObjects();
	QListWidget* listWidget = new QListWidget;
	
	Q_FOREACH(QObject* obj, objects)
	{
            QString objInfo = QString("%1 (%2)").arg(obj->objectName()).arg(obj->metaObject()->className());
            listWidget->addItem(objInfo);
	}
	
	listWidget->show();
	\endcode
	When such a list widget is constructed and shown; you will see a window as shown below.
	
    
	\inlineimage qtc-objectlist-4.png
	
	
	From the class names it is easy to picture the fact that each of those objects came from different plugins. 
	
	\bold {\underline {A registered object is an instance of QObject (or one of its subclasses) registered by a plugin and is available in the
	object-pool for other plugins to make use of}}.
	
	\section2 4.2.2 How to "register" an object from a plugin?
	There are three ways to register an object from a plugin:
	
	\list
	
	\o \bold {IPlugin::addAutoReleasedObject(QObject*)}
	\o \bold {IPlugin::addObject(QObject*)}
	\o \bold {PluginManager::addObject(QObject*)}
	
	\endlist
	
	
	The \bold {IPlugin::addObject()} and \bold {IPlugin::addAutoReleasedObject()} essentially call the
	\bold {PluginManager::addObject()} method. The \bold {IPlugin} methods are only provided for convenience. It is
	recommended that plugins make use of the \bold {IPlugin} methods for adding objects.
	
	The only difference between \bold {addAutoReleasedObject()} and \bold {addObject()} is that objects added using the
	former method are automatically removed and deleted in the reverse order of registration when the plugin is destroyed.
	At anytime plugins can make use of the \bold {IPlugin::removeObject(QObject*)} method to remove its object from
	the object pool.
	
	\section2 4.2.3 What objects to register?
	
	Plugins can register just about any object. Normally objects that provide some sort of functionality used by other
	plugin(s) are registered. Functionalities in Qt Creator are defined by means of interfaces. Listed below are some interfaces
    
	\list
	     
	\o \bold {Core::INavigationWidgetFactory}
	\o \bold {Core::IEditor}
	\o \bold {Core::IOptionsPage}
	\o \bold {Core::IWizard}
	  
	\endlist
	
	\bold{\underline { C++ developers normally assume interfaces to be classes with all its functions are public pure 
	virtual functions. In Qt Creator interfaces are subclasses of QObject that offer one or more
	methods are pure virtual}}.
	
	If a plugin has objects that implement an interface, then such an object has to be registered. For example if a plugin
	implements the \bold{INavigationWidgetFactory} interface in an object and registered it, the Core will automatically use that
	object to show the widget provided by it as navigation widget. Take a look at the code snippet below. We provide a
	simple \bold{QTableWidget} as navigation widget via an implementation of \bold {Core::INavigationWidgetFactory}.
	
	\code
    #include <coreplugin/inavigationwidgetfactory.h>

    class NavWidgetFactory : public Core::INavigationWidgetFactory
    {
    public:
        NavWidgetFactory();
        ~NavWidgetFactory();
        Core::NavigationView createWidget();
        QString displayName();
    };

    #include <QTableWidget>

    NavWidgetFactory::NavWidgetFactory() { }
    NavWidgetFactory::~NavWidgetFactory() { }

    Core::NavigationView NavWidgetFactory::createWidget()
    {
        Core::NavigationView view;
        view.widget = new QTableWidget(50, 3);
    }

    QString NavWidgetFactory::displayName()
    {
        return "Spreadsheet";
    }

    bool MyPlugin::initialize(const QStringList& args, QString *errMsg)
    {
        Q_UNUSED(args);
        Q_UNUSED(errMsg);
        // Provide a navigation widget factory.
        // Qt Creator's navigation widget will automatically
        // hook to our INavigationWidgetFactory implementation, which
        // is the NavWidgetFactory class, and show the QTableWidget
        // created by it in the navigation panel.
        addAutoReleasedObject(new NavWidgetFactory);
        return true;
    }
	\endcode
	
	The effect of the above code is
	
	\inlineimage qtc-codeeffect-4.png
	
	\section2 4.2.4 Becoming aware of registered objects
	
	Whenever the \bold {PluginManager::addObject()} is used to add an object, it \bold{(PluginManager)} emits the
	\bold {objectAdded(QObject*)} signal. This signal can be used within our applications to figure out the objects that got
	added.
	
	Obviously a plugin will begin receiving the signal only after it makes a connection to it. That happens only after the
	plugin is initialized; which also means that the plugin will receive the \bold {objectAdded()} signal only for objects added
	after the plugin was initialized.
	
	Usually the slot that is connected to the objectAdded() signal will look for one or more known interfaces. Suppose that
	your plugin is looking for the INavigationWidgetFactory interface, the slot connected to objectAdded() will be like the
	one shown below.
	
	\code
    void Plugin::slotObjectAdded(QObject * obj)
    {
        INavigationWidgetFactory *factory = Aggregation::query<INavigationWidgetFactory>(obj);

        if(factory)
        {
        // use it here...
        }
    }
	\endcode
	
	\section2 4.2.5 Searching for objects
	
	Sometimes a plugin might want to search for an object in the application that offers some functionality. We already
	know by now that
	
	\list
	\o \bold {PluginManager::allObjects()} returns the object pool as a \bold {QList<QObject*>}
	\o Connecting to \bold {PluginManager::objectAdded()} signal helps in known objects as they get registered
	\endlist
	
	Using both of the above mentioned methods you can look for objects. Lets now understand yet another way to find
	objects.

	Suppose that you wanted to look for objects that implement the \bold {INavigationWidgetFactory} interface and show it in a
	\bold {QListWidget}. You can make use of the \bold {PluginManager::getObjects<T>()} method for this purpose. The following code
	snippet explains this
	
	\code
    ExtensionSystem::PluginManager* pm = ExtensionSystem::PluginManager::instance();
    QList<Core::INavigationWidgetFactory*> objects = pm->getObjects<Core::INavigationWidgetFactory>();
    QListWidget* listWidget = new QListWidget();

    Q_FOREACH(Core::INavigationWidgetFactory* obj, objects)
    {
        QString objInfo = QString("%1 (%2)").arg(obj->displayName()).arg(obj->metaObject()->className());
        listWidget->addItem(objInfo);
    }
	\endcode
	
	When the list widget is shown you will notice that the navigation widgets are shown in the same order as they are
	shown in the navigation combo box. Take a look at the screenshot below.
	
	\inlineimage qtc-nevigationwidget-4.png
	
	
	\section1 4.3 Aggregations
	
	Aggregations are provided by the \bold {Aggregation} namespace. It adds functionality for "glueing" \bold {QObjects} of different
	types together, so you can "cast" between them. Using the classes and methods in this namespace you can bundle
	related objects into a single entity. Objects that are bundled into an aggregate can be "cast" from the aggregate into the
	object class type.
	
	\section2 4.3.1 Aggregations - the old fashioned way
	
	Suppose that you wanted an object that provided implementations of two interfaces. Normally we would go about
	coding the object like this.
	
	\code
    class Interface1
    {
        ....
    };
    Q_DECLARE_INTERFACE("Interface1", "Interface1");

    class Interface2
    {
        ....
    };
    Q_DECLARE_INTERFACE("Interface2", "Interface2");

    class Bundle : public QObject,public Interface1,public Interface2
    {
        Q_OBJECT(Interface1 Interface2)
        ....
    };
    Bundle bundle;
	
	\endcode
	
	Now we can think of \bold {bundle} as an object that provides \bold {Interface1} and \bold {Interface2} implementations. We can
	make use of casting operators on the bundle object to extract \bold{Interface1} and \bold {Interface2}.
	
	\code
    Interface1* iface1Ptr = qobject_cast<Interface1*>(&bundle);
    Interface2* iface2Ptr = qobject_cast<Interface2*>(&bundle);
	\endcode
	
	\section2 4.3.2 Aggregations - the Qt Creator way
	
	Qt Creator's Aggregation library offers a cleaner way to define interfaces and bundle them into a single object. Instances
	of Aggregation::Aggregate can be created and objects can be added to it. Each of the objects added to the aggregation
	can implement an interface. The following code snippet shows how to create an aggregation.
	
	\code
	
    #include <aggregation/aggregate.h>

    class Interface1 : public QObject
    {
        Q_OBJECT
        
    public:
        Interface1() { }
        ~Interface1() { }
    };

    class Interface2 : public QObject
    {
        Q_OBJECT
        
    public:
        Interface2() { }
        ~Interface2() { }
    };

    Aggregation::Aggregate bundle;
    bundle.add(new Interface1);
    bundle.add(new Interface2);
	
	\endcode
	
	The aggregation instance "bundle" now conceptually contains implementations of two interfaces. To extract the
	interfaces we can make use of the following code
	
	\code
    Interface1* iface1Ptr = Aggregation::query<Interface1>( &bundle );
    Interface2* iface2Ptr = Aggregation::query<Interface2>( &bundle );
	\endcode
	
	With aggregation you can also several objects of the same interface into a single bundle. For example
	
	\code
    Aggregation::Aggregate bundle;
    bundle.add(new Interface1);
    bundle.add(new Interface2);
    bundle.add(new Interface1);
    bundle.add(new Interface1);
    QList<Interface1*> iface1Ptrs = Aggregation::query_all<Interface1>( &bundle );
	\endcode
	
	Another key advantage of Aggregation is that, you can delete any one of the objects in the bundle to delete the whole
	bundle. Example
	
	\code
    Aggregation::Aggregate* bundle = new Aggregation::Aggregate;
    bundle->add(new Interface1);
    bundle->add(new Interface2);
    Interface1* iface1Ptr = Aggregation::query<Interface1>(bundle);
    delete iface1Ptr;
    // deletes the bundle and all objects in it
    // same as delete bundle
	\endcode
	
	The use of aggregation will become clearer when we deal with real plugin examples in the coming chapters.
*/