Extending Qt IVI
Qt IVI provides a pattern for extending Qt with more features in a way that suites the automotive use-case.
A key aspect is the separation between the frontend that defines the API, from the backend that implements the functionality. This separation makes it possible to have multiple implementations of the same API which can be used to interface various hardware configurations, but also to provide stubbed or simulated implementations for early development and testing purposes.
The Big Picture
Qt IVI consists of three types of building blocks. The core module provides base classes and common code for all the more specific modules and API definitions like a climate API or a mediaplayer API. Each of these APIs is the second building block called Feature, which defines the API used by the application developers. The last block consists of a backend interface and one or more Backends implementing it to either connect to the underlying vehicle or do a simulation of it.
A feature is a set of classes for working with a specific function. It defines the interface towards application developers, but it does not implement the complete functionality. Instead, it defines a backend interface to be implemented by a backend providing the actual function.
For easy deployment, Qt IVI extensions should be built as Qt modules. This makes it easy to install and find headers, shared libraries, and plugin modules from app projects.
By using the module system the developer can easily enable the inclusion of his module in the following way:
QT += <module>
In addition, your module is properly set up to work with cmake, qdoc, and auto test."
When creating a new Qt IVI module, it is recommended that you pick a name such as OemFeatureName, where Oem is the name of the car maker or platform owner, and FeatureName is the name of the feature(s) of the module. In addition to the name, a reverse domain name prefix is needed for prefixing backend interface names, for example com.example
Features define the backend interface to be implemented by the backends providing the functionality. The backend interface is commonly based on the QObject class. It is important to keep this API as unbiased as possible, as there might be multiple backend implementations relying on different technological solutions.
Some key rules to keep in mind are:
- Keep everything asynchronous to avoid blocking the main loop.
- Avoid call-to-signal sequences and try to keep signals independent from calls, For example, when a value change is signalled, it must not be due to a call to change the value from the feature (and visa versa). In other words a call to change a value is not required to always result in a value changed signal.
- Avoid stateful APIs whenever possible as the backend may be replaced at any time.
It is common to provide a stubbed backend implementation and a testing backend with each feature.
A backend class is derived from the (commonly QObject-derived) backend interface class specified by a feature. Instances of the backend class are called service objects.
Backends are implemented as an ordinary Qt C++ plugin project that depends on Qt IVI Core and the corresponding feature module.
The backends are loaded by Qt IVI Core when the features request them. Each backend has to provide a Qt IVI plugin that exposes a factory to the Core. This is what is used to load and create backends. The plugin interface is called QIviServiceInterface.
In most cases, the backend loading is handed over to the Qt IVI Core, however, QIviServiceManager can be used in this case to manually search for plugins with a specific BackendInterface. The discovery and loading of the backends takes place in this class.
The QIviServiceManager class can also be used to register backends which are part of the same application and shouldn’t go into a plugin. This is especially useful for autotest as you need to control the backend and the feature at the same time.
The reference APIs provided as a part of Qt IVI introduce some common concepts. If implementing the same functionality, it is recommended to use these already defined patterns and it is encouraged to add API additions back to the Qt IVI repository to create more common ground for future APIs.
Zones is a standard way to provide a single API for multiple points in the vehicle. For instance, climate control commonly has a driver and passenger zones, and might also have a rear seat zone. The same goes for wheels, doors, mirrors, windows and more.
A common pattern is to combine zones with property attributes to handle small differences in capabilities between zones; for example, no steering wheel heater for the passenger side of the car.
The top level interface can provide vehicle wide settings. For example, whether recirculation is to be used in a climate control API, while the zoned interface provides per-zone functions (for example, the desired temperature).
Building a zoned feature requires the backend interface to be derived from QIviZonedFeatureInterface. This provides the backend an interface for enumerating the available zones. This interface also includes the necessary QIviZonedFeatureInterface::initialize method to initialize any property attributes.
Some properties need additional associated information to be useful from the application. By exposing the properties as QIviProperty elements instead of a set of methods, the interface for the application developers is made smaller and more homogenous.
Each QIviProperty element is initialized with getter and setter functions, a notification signal for value changes, as well as a getter function and a notification signal for a QIviPropertyAttribute. The latter describes the former by providing available values for enumerated types, minimum and maximum values for ranged numerical values and availability. This means that the application can adapt to the abilities of the backend while accessing the backend through an interface shared by all backend implementations.
© 2018 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.