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// Copyright (C) 2020 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page pluginmanager.html
\title The Plugin Manager, the Object Pool, and Registered Objects
Usually, plugins do not need to access the plugin manager directly.
They interact with it mostly indirectly through the ExtensionSystem::IPlugin interface.
There are occasions though, where using the plugin manager API is necessary.
Plugins need to access the plugin manager's object pool to extend
some aspects of \QC, for example to add pages to the options dialog. They
can also utilize the object pool to provide extension points for other plugins.
\section1 Plugin Manager
The plugin manager handles all the details regarding finding plugins, reading their
description files, resolving plugin dependencies, loading and initializing all plugins
in the right order, and passing on command line arguments.
In addition, the plugin manager manages an \e{object pool}, where objects can be registered
and retrieved depending on different criteria.
Most interaction of plugins with the plugin manager should be done through the
ExtensionSystem::IPlugin interface, but the following tables summarize some functions
and signals that can be useful for plugins.
See the ExtensionSystem::PluginManager reference documentation for the complete list.
\table
\header
\li Function
\li Description
\row
\li instance()
\li Returns the singleton plugin manager instance, for example for connecting to signals.
\row
\li addObject()
\li Registers an object in the object pool.
Also available through ExtensionSystem::IPlugin::addObject().
\row
\li removeObject()
\li Unregisters an object from the object pool.
Also available through ExtensionSystem::IPlugin::removeObject().
\row
\li allObjects()
\li Returns an unfiltered list of all objects that are registered
in the object pool.
\row
\li getObject<T>()
\li Returns one object of type T that is registered in the object pool.
This respects \l{Aggregation}.
\row
\li getObjectByName(const QString &name)
\li Returns one object with the given object name that is registered in the object pool.
\endtable
\table
\header
\li Signal
\li Description
\row
\li objectAdded(QObject *object)
\li Sent after an object is registered in the object pool.
\row
\li aboutToRemoveObject(QObject *object)
\li Sent just before an object is unregistered from the object pool.
\row
\li initializationDone()
\li Sent when plugins are running and all delayed initialization is done.
\endtable
\target object pool
\section1 Object Pool and Registered Objects
Plugins can register objects to a common \e pool that is managed by
the plugin manager. Objects in the pool must derive from QObject, there are no other
prerequisites.
All objects of a specified type can be retrieved from the object pool
via the \l{ExtensionSystem::PluginManager::getObject()}{getObject()} function.
It is aware of Aggregation::Aggregate, and uses the Aggregation::query()
function instead of \c qobject_cast to determine the matching objects.
An unfiltered list of all objects that are registered in the object pool
can be retrieved via the \l{ExtensionSystem::PluginManager::allObjects()}
{allObjects()} function.
It is also possible to retrieve an object with a specific object name with
\l{ExtensionSystem::PluginManager::getObjectByName()}{getObjectByName()}
(see QObject::objectName()).
Whenever the state of the object pool changes, a corresponding
\c objectAdded() or \c aboutToRemoveObject() signal is emitted
by the plugin manager.
A common use case for the object pool is that a plugin (or the application) provides
an \e{extension point} for other plugins, which is a class that can
be implemented and added to the object pool to be retrieved by the providing plugin.
It is also possible to use the object pool to provide access to an object without actually
linking against the providing plugin library.
*/
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