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/****************************************************************************
**
** Copyright (C) 2018 Pelagicore AG
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the QtIvi module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL-QTAS$
** Commercial License Usage
** Licensees holding valid commercial Qt Automotive Suite licenses may use
** this file in accordance with the commercial license agreement provided
** with the Software or, alternatively, in accordance with the terms
** contained in a written agreement between you and The Qt Company. For
** licensing terms and conditions see https://www.qt.io/terms-conditions.
** For further information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page backend-system.html
\title Dynamic Backend System
\nextpage The Qt IVI Attribute System
\contentspage Concepts
\section1 What's a Feature
Modern automotive systems are very complex and are developed in a tight timeframe. Because of that,
it often makes sense to reuse parts of previously developed systems. At the same time, the main
development is done by independent companies (tier-1). To make it possible to reuse code from
previous project, but at the same time also make it possible to use parts the tier-1 can offer, it
makes sense to split the APIs into two layers: a frontend and a backend. In QtIvi, the frontend API
is called a \b feature, as usually a specific class is responsible for a specific feature area,
e.g. QIviClimateControl, for controlling the climate feature area.
\section1 What's a Backend
To function correctly, every feature needs to have a connected backend. This backend needs to
implement the corresponding feature backend interface. Only then a valid connection between the
feature and its backend can be created.
Usually every feature has exactly one backend interface class, which needs to be implemented by the
backend for this feature to work. Every backend interface is derived from QIviFeatureInterface,
which provides generic functions and signals needed by every feature, e.g. for error handling.
The backend implementations are grouped together and are implemented inside a Qt plugin. This makes
it easy to provide multiple backends at the same time and switch the backend at runtime. Those
backend plugins are loaded through \l{qtivicore - the Glue}{qtivicore}. A plugin can provide
implementations for multiple features. There is no need to create a separate plugin for every
feature. QtIvi also distinguishes between two types of backends: production and simulation. While
on a production system, you only want to have production backends running. But during the
development phase, it might be useful to have a simulation backend available, which can be used for
frontend development until the backend services are in an usable state. QtIvi uses a simple naming
scheme to identify whether a plugin provides simulation or production backends. Every simulation
plugin needs to have either "simulation" or "simulator" in its name. In addition it is also supported
to set the "simulation" key in the plugin metadata. This is especially useful for static
plugins. Please see \l {How to Create Qt Plugins} for more details about Qt's plugin system.
\image backendtypes.jpg "Backend types"
\section1 ServiceObjects
To keep the features very flexible and to make it possible to change the backends at runtime, we
introduced a concept called ServiceObject. A QIviServiceObject is a handle, which is used by the
feature to connect to the correct backend interface. It provides methods to query the available
backend interfaces that the ServiceObject is implementing. Plugins are automatically wrapped by
ServiceObjects. This makes it possible to share the ServiceObject between multiple features and to
explicltly select which backend should be used for your feature instance.
\section1 qtivicore - the Glue
The qtivicore module provides all the classes that are needed to glue the parts together. In
addition to providing the base classes like QIviAbstractFeature or QIviServiceObject, it also
provides the QIviServiceManager, responsible for loading the needed backend plugins.
\image feature_backend.jpg "Feature backend relation"
\section1 QIviServiceManager
The QIviServiceManager is the central part of qtivicore, keeping book on all the available backends
and their exported interfaces. For this, the manager scans through all available plugins and their
accompanying metadata. This gives the QIviServiceManager the ability to only load the plugins,
which are actually needed by a Feature in order to reduce the startup time. All these information
is collected in the manager in form of a model, which enables the user to pick and choose the
plugin he wants to use.
\section1 How a Feature Finds its Backend
Usually every Feature is using the so called auto discovery mode. From QML, you can set the
QIviAbstractFeature::discoveryMode property; from the C++ side, this can be started using
QIviAbstractFeature::startAutoDiscovery(). This will ask the QIviServiceManager for all the
available backends implementing the required interface for your feature. The manager will then
choose the first matching backend and will connect the feature to it. QIviAbstractFeature will
first ask for production backends and only if none are available, fall back to a simulation
backend. This behavior can be controlled using the QIviAbstractFeature::discoveryMode (defaults to
QIviAbstractFeature::AutoDiscovery). The resulting backend type can be retrieved via
QIviAbstractFeature::discoveryResult. After the feature has successfully loaded a backend, the
QIviAbstractFeature::serviceObject property holds the loaded ServiceObject and
QIviAbstractFeature::isValid returns \c true.
\section2 Manual Assignment
If a feature does not want not use the auto discovery mechanism, it can simply set the
discoveryMode to QIviAbstractFeature::NoAutoDiscovery. After that, the feature won't search for a
backend itself anymore, so the user needs to assign a ServiceObject manually.
\section2 DiscoveryModels
For features like QIviClimateControl the auto discovery is fitting, as there is usually a 1:1
mapping between a feature and a backend providing the implementation for the feature. For more
generic interfaces like a media player, this might not be sufficient: you could control a built-in
media player backend with this, but you might also want to control the media player running on your
mobile phone over bluetooth. For this to work, you first would need to discovery the available
devices and then pass the ServiceObject of the selected device to the media player interface. The
discovery of the available mobile phones can be done using a DiscoveryModel. This provides you with
a ServiceObject for every device found. The concept of a discovery model is not limited to mobile
phones, it can be used for all backends that are not hard-wired to the system, like internet
services or controlling multiple rearseat systems.
\section2 Detailed connection order
\list 1
\li A ClimateControl element is created in QML.
\li ClimateControl will call QIviAbstractFeature::startAutoDiscovery on its completion.
\li QIviAbstractFeature::startAutoDiscovery will ask QIviServiceManager for all backends.
\li QIviServiceManager searches for all available plugins and the interfaces they are implementing
(this happens only once).
\li QIviAbstractFeature will accept the first QIviServiceObject and connect to the
corresponding interface.
\li The ClimateControl element is ready to be used.
\endlist
*/
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