path: root/src/ivicore/doc/src/examples-qface-tutorial.qdoc

/****************************************************************************
**
** Copyright (C) 2021 The Qt Company Ltd.
** Copyright (C) 2019 Luxoft Sweden AB
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt 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.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \example ivicore/qface-tutorial
    \brief Demonstrates step-by-step how to generate a Middleware API based on a QML application.
    \ingroup qtivicore-examples
    \title Qt IVI Generator Tutorial
    \image examples_qface_tutorial.png

    This tutorial demonstrates how you can extend a QML application with your own autogenerated
    Middleware API. We use an existing QML Instrument Cluster application and proceed through the
    following steps:

    \list 1
        \li \l{chapter1}{Integrate a basic interface without a backend}
        \li \l{chapter2}{Extend the interface and add annotations}
        \li \l{chapter3}{Add a simulation backend and corresponding simulation annotations; with a QML plugin}
        \li \l{chapter4}{Add a custom simulation behavior}
        \li \l{chapter5}{Add a simulation server and use it from a Qt Remote Objects Backend}
        \li \l{chapter6}{Develop a production backend that connects to a DBus interface}
    \endlist

    Before we start the actual Middleware integration, let's take a look at the existing Instrument
    Cluster QML code and all the features it supports:
    \list
        \li \c images -- This folder contains all images used in the QML code.
        \li \c Cluster.qml -- The main QML file that assembles all other QML components together.
        \li \c Dial.qml -- The base component to show values like speed or Revolutions per Minute
            (RPM), using a needle.
        \li \c Fuel.qml -- The component to show the actual fuel level.
        \li \c Label.qml -- A small helper component which sets all common settings used to display
            text.
        \li \c LeftDial.qml -- Shows the current speed using the Dial component and as text, as
            well as the current metric in miles per hour (mph) or kilometers per hour (km/h).
        \li \c RightDial.qml -- Shows the current RPM and offers a way to show warning indicators.
        \li \c Top.qml -- The top bar that shows the current date and the current temperature.
    \endlist

    Next, we use our Middleware API to add support for the following features:
    \list
        \li Show the current speed in the left dial.
        \li Show the current RPM in the right dial.
        \li Change between different metrics.
        \li Show the current temperature in the top bar.
        \li Show different warnings on the right dial.
        \li Indicate whether the instrument cluster is connected and show real data.
    \endlist

    The ultimate goal is to connect all of these features together to simulate a real-time driving
    experience like this:

    \image examples_qface_tutorial_final.gif

    \target chapter1
    \section1 Chapter 1: Basic Middlware API with the IVI Generator

    In this chapter we integrate a Middleware API into the existing Instrument Cluster QML code.
    Instead of manually writing all of these parts ourselves, which is done in most basic
    \l{https://doc.qt.io/qt-5/qtquick-codesamples.html}{QML examples}, we'll use the IVI Generator
    to autogenerate the required parts.

    \target define-speed-property
    \section2 Interface Definition Language

    To be able to autogenerate the Middleware API, the IVI Generator needs some input on what to
    generate. This input is given in form of an Interface Definition Language (IDL), QFace, which
    describes the API in a very simple way.

    Let's start to define a very simple interface which provides us with a speed property:

    \quotefromfile ivicore/qface-tutorial/chapter1-basics/instrument-cluster.qface
    \printuntil }

    First, we need to define which module we want to describe. The module acts as a namespace,
    because the IDL file can contain multiple interfaces.

    \quotefromfile ivicore/qface-tutorial/chapter1-basics/instrument-cluster.qface
    \printuntil module

    The most important part of the module is its interface definition.

    \quotefromfile ivicore/qface-tutorial/chapter1-basics/instrument-cluster.qface
    \skipto interface
    \printuntil }

    In this case, we define an interface named \c InstrumentCluster that consists of one property.
    Each property definition must contain at least a type and a name. Most of the basic types are
    built-in and can be found in the \l{QFace IDL Syntax}.

    \section2 Autogeneration

    Now that our first version of the IDL file is ready, it's time to autogenerate API from it,
    using the \l{Qt IVI Generator}{IVI Generator tool}. Similar to
    \l{https://doc.qt.io/qt-5/moc.html}{moc}, this autogeneration process is integrated into the
    qmake Build System and is done on compile time.

    In the following \c{.pro} file we build a C++ library based on our IDL file:

    \quotefromfile ivicore/qface-tutorial/chapter1-basics/frontend/frontend.pro
    \printto CONFIG += install_ok

    Most of the \c{.pro} file is a standard setup to define a C++ library, using "lib" \c TEMPLATE
    and defining the required file name in the \c TARGET variable. The \c qtLibraryTarget function
    that we use helps to append the "d" postfix on the filename correctly, for a library that
    provides debugging information. In the future, we need to link this file, so we set the
    \c DESTDIR to the upper directory to simplify this.

    \note Windows searches for libraries in the same directory automatically.

    Activating the IVI Generator integration requires the \c CONFIG variable to specify the
    \c ivigenerator option. This makes sure the IVI Generator is called during the build process,
    using the QFace file that we specify in \c QFACE_SOURCES. For more information, see
    \l{qmake integration}.

    To make sure the library we build works on Windows, it's important to add the
    \c QT_BUILD_EXAMPLE_IVI_INSTRUMENTCLUSTER_LIB to the \c DEFINES variable. This way, all symbols
    are exported when building the library, but imported when linking against it. For more
    information, see \l{https://doc.qt.io/qt-5/sharedlibrary.html}{Creating Shared Libraries}.

    \section2 Which Files are Autogenerated

    The IVI Generator works based on generation templates. These templates define what content
    should be generated from a QFace file. If no \c QFACE_FORMAT is defined, this automatically
    defaults to "frontend" template. For more details on these templates, see \l{Use the Generator}.

    In short, the "frontend" template generates:
    \list
        \li a C++ class derived from QIviAbstractFeature for every interface in the QFace file
        \li one module class that helps to register all interfaces to QML and stores global types
            and functions.
    \endlist

    To inspect the C++ code yourself, you can view these files in the your library's build folder.

    Right now, the most important autogenerated file for us, is the resulting C++ class for our
    defined interface. It looks like this:

    \quotefile ivicore/qface-tutorial/chapter1-basics/frontend/instrumentcluster.h

    As you can see, the autogenerated C++ class implements a \c speed property, that we previously
    defined in the QFace file. By using the \c Q_OBJECT and \c Q_PROPERTY macros, the class is now
    ready for use directly in your QML code.

    \section2 Integrate the Frontend Library with the QML Code

    For this integration, we use the autogenerated frontend library from the QML code. For the sake
    of simplicity, we follow the standard Qt example pattern and use a small C++ main function
    which registers our autogenerated types to QML and loads the Instrument Cluster QML code into
    the QQmlApplicationEngine:

    \quotefromfile ivicore/qface-tutorial/chapter1-basics/instrument-cluster/main.cpp
    \skipto #include "instrumentclustermodule.h"
    \printuntil }

    All we need now is the actual integration of the InstrumentCluster QML element and connecting
    the \c speed property to the \c leftDial. This is done by instantiating the element first with
    the \c instrumentCluster ID.

    \quotefromfile ivicore/qface-tutorial/chapter1-basics/instrument-cluster/Cluster.qml
    \skipto import
    \printuntil InstrumentCluster
    \printuntil }
    \codeline

    Lastly, we can create a Binding for the \c LeftDial Item's \c value property to our
    InstrumentCluster API's \c speed property.

    \printuntil }

    \target chapter2
    \section1 Chapter 2: Extend the Interface and add Annotations

    In this chapter we extend our Middleware API with more properties via enums and by defining our
    own structure.

    \section2 Define Speed as a Read-only Property

    \l{define-speed-property}{Previously}, we defined the speed property in our QFace file in the
    following way:

    \quotefromfile ivicore/qface-tutorial/chapter1-basics/instrument-cluster.qface
    \printuntil }

    This property is defined as readable and writable, as we didn't use any extra specifiers.
    However, it's not necessary for our Instrument Cluster example to have a writable \c speed
    property because it's not used to accelerate the car, but just to visualize the current state.

    To define the property as read-only, use the \c readonly keyword.

    \quotefromfile ivicore/qface-tutorial/chapter2-enums-structs/instrument-cluster.qface
    \printuntil readonly
    \skipto }
    \printuntil }

    When we build our app again, the build system recognizes this change and runs the IVI
    Generator to generate an updated version of the C++ code. After the IVI Generator is done,
    open the \c instrumentcluster.h from the build folder and notice that the generated
    \c speed property changed -- it no longer has a setter anymore and is now read-only.

    \quotefromfile ivicore/qface-tutorial/chapter2-enums-structs/frontend/instrumentcluster.h
    \skipto class Q_EXAMPLE
    \printuntil Q_PROPERTY
    \dots
    \skipto };
    \printuntil };

    \section2 Extend the Interface

    To reach our goal to provide a full simulation for the Instrument Cluster, we need to add more
    properties to our QFace file: \c rpm, \c fuel and \c temperature:

    \quotefromfile ivicore/qface-tutorial/chapter2-enums-structs/instrument-cluster.qface
    \printuntil readonly real temperature
    \skipto }
    \printuntil }

    You might have noticed that we use a different type for the \c fuel and \c temperature
    properties. We use \c real here, as we would like to show the temperature as a floating point
    number, and the current fuel level as a value between 0 and 1.

    \section2 Define a New Enum Type

    One useful feature is to be able to switch between the metric and the imperial system, so we
    need to define a property for the system we currently use. Using a boolean property would work,
    but doesn't offer a nice API, so we define a new enum type in the QFace file and use it as the
    type for our new \c system property:

    \quotefromfile ivicore/qface-tutorial/chapter2-enums-structs/instrument-cluster.qface
    \printuntil readonly SystemType
    \skipto }
    \printuntil enum
    \printuntil }

    In the autogenerated code, this results in an enum which is part of the module class, making it
    possible for the same enum to be used by multiple classes which are part of the same module:

    \quotefile ivicore/qface-tutorial/chapter2-enums-structs/frontend/instrumentclustermodule.h

    \section2 Add a New Structure

    To display warnings on the Instrument Cluster's right dial, we'd like to use a structure that
    stores color, icon, and text for the warning; instead of using 3 independent properties.
    Similar to defining an interface, we can use the \c struct keyword in our QFace file:

    \quotefromfile ivicore/qface-tutorial/chapter2-enums-structs/instrument-cluster.qface
    \skipto struct
    \printuntil }

    Using this new structure as a type for a property, works in the same way as when using an enum.
    The QFace file should now look like this:

    \quotefile ivicore/qface-tutorial/chapter2-enums-structs/instrument-cluster.qface

    \section2 Integrate the New Properties

    Like in the previous chapter, actually integrating the newly introduced properties involves
    creating Bindings. The \c rpm property can be directly connected to the \c rightDial Item's
    \c value property; the same is done for the top Item's \c temperature property. To control
    which unit is displayed in the left Dial, the \c leftDial Item provides \c metricSystem, a bool
    property. As we used an enum in our QFace file, we need to convert the value first by testing
    the \c sytemType property for the "Metric" value.

    \quotefromfile ivicore/qface-tutorial/chapter2-enums-structs/instrument-cluster/Cluster.qml
    \skipto LeftDial
    \printuntil }
    \codeline

    These enums are part of the module class, which is also exported to QML as
    \c InstrumentClusterModule. To trigger a warning in the \c rightDial Item, we use 3 bindings to
    connect to the 3 member variables in the structure:

    \printuntil }

    \target chapter3
    \section1 Chapter 3: Add a Simulation Backend and Annotations with a QML plugin

    In the previous two chapters, we wrote a Middleware API using a QFace file and used the IVI
    Generator to autogenerate a C++ API in the form of a library. Now, in this chapter, we extend
    this further by introducing a simulation backend and using annotations to define default values
    for our simulation.

    \section2 Separation between the Frontend and Backend

    Both QtIvi and the IVI Generator enable you to write code that separates the frontend from the
    backend -- to split an API from its actual implementation. Already, Qt uses this concept in a
    lot of areas, most prominently in the underlying window system technology on various Qt
    platforms like XCB on Linux and Cocoa on macOS.

    The same separation is done for our Middleware API, where the frontend provides the API as
    a library; the backend provides an implementation of this API. This implementation is based on
    QtIvi's \l{Dynamic Backend System} which enables us to switch between such backends at runtime.

    \image feature-backend.png

    \section2 Add a Simulation Backend

    For our Instrument Cluster, we'd like to add such a backend to provide actual values. For now,
    we'd like to just have some simulation behavior as we can't connect it easily to a real car.
    This is why such backends are called "simulation backend". To add this type of backend, once
    again, we use the IVI Generator to do the heavy lifting for us and generate one. This work
    is done in a similar way to when we generated a library with the "frontend" template. But now,
    we are using the "backend_simulator" template:

    \quotefromfile ivicore/qface-tutorial/chapter3-simulation-backend/backend_simulator/backend_simulator.pro
    \printto DESTDIR
    \skipto QT
    \printuntil CONFIG
    \skipto QFACE_FORMAT
    \printto CONFIG += install_ok

    Just like for the frontend library, the project file builds a \c lib and defines the library
    name using \c qtLibraryTarget to also support the Windows debug postfix. One important aspect
    here is that the library name ends with "_simulation", which is a way to tell QtIvi that this
    is a simulation backend. When a "production" backend is available, it is preferred over the
    "simulation" one. For more information, see \l{Dynamic Backend System}.

    Enabling the IVI Generator is also done in the same way as we did earlier: by using the same
    \c QFACE_SOURCE variable, but defining \c QFACE_FORMAT to "backend_simulator", to use the
    correct generation template. In addition, we need to add 'plugin' to the \c CONFIG variable,
    to make this library a Qt plugin which can be easily loaded at runtime.

    \section2 Link Settings and Locating Plugins

    Trying to build the project file just as it is, right now, would result in compilation and
    linking errors. This is because: to do the frontend and backend separation, we need to have the
    backend implement a defined interface class, that is known to the frontend. This interface is
    aptly called "backend interface" and is automatically generated as part of the frontend
    library. Because this class provides signals and slots and uses QObject for its base class, you
    need to link to the frontend library when you inherit from it. As this is needed for the
    backend plugin, we need to add the following lines in addition:

    \quotefromfile ivicore/qface-tutorial/chapter3-simulation-backend/backend_simulator/backend_simulator.pro
    \skipuntil CONFIG
    \printuntil INCLUDEPATH

    Now the project should build fine and create the plugin in your build folder; or the plugin
    folder if you don't use a shadow build. When you start the Instrument Cluster again, you should
    see the following message:

    \badcode
    There is no production backend implementing "Example.IVI.InstrumentCluster.InstrumentCluster" .
    There is no simulation backend implementing "Example.IVI.InstrumentCluster.InstrumentCluster" .
    No suitable ServiceObject found.
    \endcode

    This message indicates that QtIvi is still unable to find the simulation plugin we just created.
    Here, you need to know a little bit more about Qt's Plugin System, especially how it it finds
    plugins.

    Qt searches for it's plugins in multiple directories, the first one is the plugin folder,
    \c plugins, which comes with your Qt installation. Within the plugins folder, every plugin type
    has it's own sub-folder, such as \c platforms, for the platform plugins used to talk to the
    underlying platform API and the windowing system.

    Similarly, QtIvi searches for its backend plugins in the \c qtivi folder. To make sure our
    simulation backend ends up in such a folder, we add the following \c DESTDIR definition.

    \quotefromfile ivicore/qface-tutorial/chapter3-simulation-backend/backend_simulator/backend_simulator.pro
    \skipto DESTDIR
    \printuntil DESTDIR

    You might wonder how creating a \c qtivi folder in the upper directory solves this problem of
    finding the plugin as it's not part of the system plugins folder. But Qt supports searching in
    multiple folders for such plugins and one of those folders is the path to where the executable
    itself is located.

    Alternatively, we could add an additional plugin path using the QCoreApplication::addLibraryPath()
    function or using the \c QT_PLUGIN_PATH environment variable. For more information, see
    \l{https://doc.qt.io/qt-5/plugins-howto.html}{How to create Qt Plugins}.

    Now everything is in place, but because our plugin links against the frontend library, we need
    to make sure the library can be found by the dynamic linker. This can be achieved by
    setting the \c LD_LIBRARY_PATH environment variable to our library folder. But this results
    in the problem, that every user would need to set this variable to be able to use our
    application.
    To make things easier for the user, we rather use a relative RPATH instead and annotate our
    plugin with the information for the linker, where it might find the needed libraries, relative
    to the plugin's location:

    \quotefromfile ivicore/qface-tutorial/chapter3-simulation-backend/backend_simulator/backend_simulator.pro
    \skipto INCLUDEPATH
    \printuntil QMAKE_RPATHDIR

    \section2 Export the QML Types in a QML Plugin

    In the first chapter, we extended our \c main.cpp to register all types of our autogenerated
    Middleware APIs. Although this works fine, in bigger projects it's common to use a QML Plugin
    instead and be able to use qmlscene for development. Although the code for doing this is
    not complex, the IVI Generator supports this as well and makes it even easier.

    From the first chapter, we know that the module name is used for the QML import URI. This is
    important for a QML plugin as the QmlEngine expects the plugin in a specific folder to
    follow the module name, where every section of the module name is a sub-folder. Our project
    file to generate a QML plugin looks like this:

    \quotefromfile ivicore/qface-tutorial/chapter3-simulation-backend/imports/imports.pro
    \printto target.path

    All lines until \c QFACE_SOURCES should be familiar. We use \c CONFIG to build a plugin, then
    define the settings for the linker to link against our frontend library. Then, we use
    \c QFACE_FORMAT to define "qmlplugin" as the generation template. Instead of adding
    \c ivigenerator to \c CONFIG, this time we use
    \l{https://doc.qt.io/qt-5/qmake-test-function-reference.html#load-feature}
    {qmake's load() function} to explicitly load the feature. This enables us to use the \c URI
    variable which is part of the "qmlplugin" generation template. This URI can be used to define
    a \c DESTDIR by replacing all dots with slashes.

    In addition to the folder structure, the QmlEngine also needs a \c qmldir file which indicates
    what files are part of the plugin, and under which URI. For more information, see
    \l{https://doc.qt.io/qt-5/qtqml-modules-qmldir.html}{Module Definition qmldir Files}. Both this
    \c qmldir file and a \c plugins.qmltypes file which provides information about code-completion,
    are autogenerated by the IVI Generator; but they need to be placed next to the library. To do
    so, we add the files to a scope similar to an \c INSTALL target, but add it to the \c COPIES
    variable instead. This makes sure that the files are copied when the plugin is built.

    Now the plugin is ready for use, but our Instrument Cluster application doesn't know where to
    search for it and is still using the old hardcoded registration. So, we can now remove the
    linking step in the \c instrument-cluster.pro file and change our main file accordingly:

    \quotefromfile ivicore/qface-tutorial/chapter3-simulation-backend/instrument-cluster/main.cpp
    \skipto #include
    \printuntil }

    What has changed is that we've now added an additional import path with the \c addImportPath
    function, which points to the "imports" folder next to the binary's location.

    \target chapter4
    \section1 Chapter 4: Add a Custom Simulation

    So far, we've created a Middleware API and integrated it into our Instrument Cluster QML code,
    extended it with a QML plugin, and generated a simulation backend. In the background, quite a
    lot has happened to support us; but on the UI side not much has changed till now. This chapter
    is about bringing our simulation backend to life by defining sane default values and starting
    to simulate a real car ride.

    \section2 Define Default Values

    We start by defining default values for our properties, using annotations in our QFace file.
    An annotation is a special kind of comment which adds extra data to an interface, method,
    property, and so on. For this use case we use the \c config_simulator annotation. For more
    information, see \l{annotations-yaml}{Annotations}.

    Currently, in our Instrument Cluster, the temperatur defaults to 0. Let's change this to a
    temperature in spring, 15 degrees Celsius, with the following YAML fragment:

    \quotefromfile ivicore/qface-tutorial/chapter4-simulation-behavior/instrument-cluster.qface
    \printuntil }

    Compile the plugin again for this temperature change to be reflected in our Instrument Cluster.
    Let's see how this actually works: when starting the IVI Generator, the config_simulator
    annotation was transformed into a JSON file that's now part of the "simulation backend" build
    folder. This JSON file looks like this:

    \quotefile ivicore/qface-tutorial/chapter4-simulation-behavior/backend_simulator/instrumentcluster.json

    But how is this JSON file related to the actual simulation backend code? The autogenerated
    simulation backend code uses QIviSimulationEngine, that reads the JSON file and provides its
    data to a QML simulation file. A default QML file is also autogenerated and loaded from the
    QIviSimulationEngine. This default QML file provides the behavior of what should happen in the
    the simulation backend.

    Later, in the next section, we take a look at the QML file and how we can change it. But first,
    let's see how we can change the default values in a more dynamic way.

    The QIviSimulationEngine allows us to override which JSON file should be loaded into the
    engine, when we set the \c QTIVI_SIMULATION_DATA_OVERRIDE environment variable. Since there can
    be multiple engines run by different backends, we need to define which engine we're referring
    to. In the autogenerated code, the module name is always used as the engine specifier. For this
    chapter, we already prepared a second JSON file which is part of our source directory. Setting
    the environment variable as follows, changes the \c systemType to mph instead of km/h:

    \badcode
    QTIVI_SIMULATION_DATA_OVERRIDE=instrumentcluster=<path-to-file>/miles.json
    \endcode

    \section2 Define a QML Behavior

    Before we define our custom behavior, let's see what's been autogenerated for us. There are two
    QML files: The first is \c instrumentcluster_simulation.qml and rather simple. It defines an
    entry point that istantiates the second file, an \c InstrumentClusterSimulation.qml file. This
    split is done as there can be multiple interfaces defined as part of the same module.

    \note A QML Engine can only have one entry point. While QIviSimulationEngine has this same
    limitation, if you have a module with multiple interfaces, you want to have multiple simulation
    files -- one per interface. This is why the first QML file merely instantiates the QML files for
    all interfaces that it supports. In the case of our example, it's only one interface.

    The InstrumentClusterSimulation.qml file is very interesting:

    \quotefile ivicore/qface-tutorial/chapter4-simulation-behavior/backend_simulator/InstrumentClusterSimulation.qml

    First, there's a \c settings property, that's initialized with the return value from the
    \l{IviSimulator::findData}{IviSimulator.findData} method, which takes the
    \l{IviSimulator::simulationData}{IviSimulator.simulationData} and a string as input. The
    \c simulationData is the JSON file represented as a JavaScript object.

    The \c findData method helps us to extract only the data that is of interest for this
    interface, \c InstrumentCluster. The properties that follow help the interface to know whether
    the default values are set. The \c LoggingCategory is used to identify the log output from this
    simulation file.

    Afterwards, the actual behavior is defined by instantiating an \c InstrumentClusterBackend Item
    and extending it with more functions. The \c InstrumentClusterBackend is the interface towards
    our \c InstrumentCluster QML frontend class. But, apart from the frontend, these properties are
    also writable to make it possible to change them to provide a useful simulation.

    Each time a frontend instance connects to a backend, the \c initialize() function is called.
    The same applies to the QML simulation: as the \c initialize() C++ function forwards this to
    the QML instance. This also applies to all other functions, like setter and getters, for
    properties or methods. For more details, see \l{QIviSimulationEngine}.

    Inside the QML \c initialize() function, we call \c{IviSimulator.initializeDefault()}, to read
    the default values from the \c simulationData object and initialize all properties. This is
    done only \b once, as we don't want the properties be reset to default when the next frontend
    instance connects to the backend. Lastly, the base implementation is called to make sure that
    the \c initializationDone signal is sent to the frontend.

    Similarly, a setter function is defined for each property; they use the
    \c{IviSimulator.checkSettings()} to read specific constraint settings for the property from
    the \c simulationData and check whether these constraints are valid for the new value. If
    these constraints aren't valid, then \c{IviSimulator.constraint()} is used to provide a
    meaningful error message to the user.

    \section2 Define Our Own QML Simulation

    As mentioned above, the \c InstrumentClusterBackend Item does provide all the properties of our
    QFace file. This can be used to simulate a behavior by changing the properties to the values
    we want. The simplest form for this would be value assignment, but this would be rather static
    not exactly what we'd like to achieve. Instead, we use QML Animation objects to change the
    values over time:

    \quotefromfile ivicore/qface-tutorial/chapter4-simulation-behavior/backend_simulator/simulation.qml
    \skipto NumberAnimation
    \printuntil }

    The code snippet above changes the speed property to 80 over 4000 seconds and simulates an
    accelerating car. Extending this to the other properties, and combining both sequential and
    parallel animations, we can create a full simulation:

    \quotefromfile ivicore/qface-tutorial/chapter4-simulation-behavior/backend_simulator/simulation.qml
    \skipto property var animation
    \printuntil property: "fuel"
    \printuntil property: "fuel"
    \printuntil }
    \printuntil }

    Then, to provide a nice simulation for the \c rpm property, we use a binding which does some
    calculations based on the current speed. The complete simulation file looks like this:

    \quotefromfile ivicore/qface-tutorial/chapter4-simulation-behavior/backend_simulator/simulation.qml
    \skipto import

    The next step is to tell the IVI Generator and the QIviSimulationEngine about our new
    simulation file. Similar to QML files, the best aproach here is to put the simulation file into
    a resource file. In our example, we add a new file called \c simulation.qrc which contains our
    \c simulation.qml using the \c{/} prefix.

    In our QFace file, this location now needs to be added in the form of an annotation:

    \quotefromfile ivicore/qface-tutorial/chapter4-simulation-behavior/instrument-cluster.qface
    \printuntil module
    \dots

    Now, rebuilding the simulation backend embeds the simulation file into the plugin and hands the
    file over to the QIviSimulationEngine, which starts the simulation when loaded.

    \target chapter5
    \section1 Chapter 5: Add a Simulation Server Combined with QtRemoteObjects

    In this chapter we extend our Instrument Cluster to use an Inter-Process Communication (IPC)
    mechanism and use two processes. At the moment, the simulation is loaded as a plugin that
    causes it to be part of the same service. Although this is good enough for a small example
    application, it's not how it's done in modern multi-process architectures, where multiple
    processes need to be able to access the same value and react to changes. We could write a
    second Application that uses the same Middleware API. However, we can achieve the same thing
    just by starting the Instrument Cluster twice and checking whether the animations are in sync.
    Currently, they're not.

    \image examples_qface_tutorial_unsync.gif

    \section2 Add a QtRemoteObjects Integration

    The IPC for this example is QtRemoteObjects, because the IVI Generator already supports it
    out of the box. To use QtRemoteObjects we generate a second plugin, a "production" backend,
    this time. Production backends are automatically preferred over the simulation backend we
    introduced before.

    This is done with the following project, \c{.pro}, file:

    \quotefromfile ivicore/qface-tutorial/chapter5-ipc/backend_qtro/backend_qtro.pro
    \printto CONFIG += install_ok

    This \c{.pro} file is almost identical to the one we used earlier for our simulation backend.
    For now we highlight what's changed.

    The name of the plugin doesn't end with "_simulation" to indicate that this is a "production"
    backend. The \c QFACE_FORMAT is now changed to "backend_qtro" to generate a backend that uses
    QtRemoteObjects Replicas to connect to a QtRemoteObjects Source that provides the values. In
    addition to a QtRemoteObject-based backend, we also need a QtRemoteObject-based server. This
    part can also be autogenerated using the IVI Generator in a similar fashion:

    \quotefromfile ivicore/qface-tutorial/chapter5-ipc/simulation_server/simulation_server.pro
    \printto RESOURCES

    Because we'd like to generate a server binary, the \c TEMPLATE needs to be set to "app" instead
    of "lib". Similar to the plugin, the server also needs to link against our library to give it
    access to the defined enums, structures, and other types. The template we use to generate a
    simulation server is called "server_qtro_simulator".

    \section2 Reuse the Existing Simulation Behavior

    Now, if you start the server and then the Instrument Cluster, you don't see the simulation
    from our previous chapter anymore. The reason for this, is that the simulation code is part of
    our simulation backend, but this backend is no longer used as we added the
    QtRemoteObjects-based "production" backend.

    Because we used the "server_qtro_simulator" generation template, this can easily be fixed, as
    the generated server code is also using the QIviSimulationEngine and supports to use the same
    simulation file than our simulation backend. We just need to extend the project file in the
    same way as we did before and are also able to use the same resource file for this.

    \quotefromfile ivicore/qface-tutorial/chapter5-ipc/simulation_server/simulation_server.pro
    \skipto RESOURCES
    \printuntil RESOURCES

    In the same way, we can also use the other simulation data JSON file that we defined in the
    previous chapter, by using the same environment variable. We just need to pass it to the
    server instead of our Instrument Cluster application.

    Let's do the final test: starting two Instrument Cluster instances should now show the
    animations in sync:

    \image examples_qface_tutorial_sync.gif

    \target chapter6
    \section1 Chapter 6: Develop a Production Backend with D-Bus

    Previously, we extended our Instrument Cluster code by using QtRemoteObjects as IPC and
    autogenerated a backend for it, as well as a server that provides the simulation. In this
    chapter, we'd like to write our own backend \b manually using D-Bus as IPC.

    We've already prepared a working D-Bus server which provides limited simulation.

    First, let's look at the server code and see what's done there; then write the backend that
    connects to it.

    \section2 D-Bus Server

    As mentioned above, we use D-Bus for this chapter and we already have an XML file that
    describes the D-Bus interface, similar to our QFace file:

    \quotefile ivicore/qface-tutorial/chapter6-own-backend/demo_server/instrumentcluster.xml

    This XML file is used to let qmake generate a base class which is extended by the server with
    actual functionality. For more information, see \l{QtDBus}.

    Our D-Bus server starts on the session bus, on the \c{/} path, and provides an interface named
    "Example.IVI.InstrumentCluster". To simulate some values, we keep it simple and use a timer
    event to change the speed value every 100 milliseconds. Then, we start from 0, once the
    maximum of 250 is reached. Similarly, the \c rpm value is increased to 5000. For all other
    properties, we provide hardcoded values.

    \quotefromfile ivicore/qface-tutorial/chapter6-own-backend/demo_server/instrumentcluster.cpp
    \skipto timerEvent
    \printuntil }

    \section2 Write Our own D-Bus Backend

    Let's start with a \c{.pro} file for our backend. This is very similar to previous \c{.pro}
    files, but it doesn't use the IVI Generator. Instead, it uses \c DBUS_INTERFACES to
    autogenerate some client code which sends and receives messages over D-Bus.

    Now, we need to define an entry point for our plugin. This plugin class needs to derive from
    QIviServiceInterface and implement two functions:

    \list
        \li \c {QStringList interfaces()} -- that returns a list of all interfaces this plugin
            supports.
        \li \c {QIviFeatureInterface *interfaceInstance(const QString &interface)} -- that returns
            an instance of the requested interface.
    \endlist

    Additionally, we also need to provide a list of interfaces we support as plugin metadata, in
    the form of a JSON file which looks like this:

    \quotefile ivicore/qface-tutorial/chapter6-own-backend/backend_dbus/instrumentcluster_dbus.json

    We need this list, as it gives QtIvi the chance to know which interfaces a backend supports,
    before instantiating it and loading only the plugins which the application code needs.

    Our plugin code looks like this:

    \quotefromfile ivicore/qface-tutorial/chapter6-own-backend/backend_dbus/instrumentclusterplugin.cpp
    \skipto #include
    \printto

    In \c interfaces() we use the IID which is defined in \c{instrumentclusterbackendinterface.h}
    from our autogenerated library. In \c insterfaceInstance() we check for the correct string and
    return an instance of the instrument cluster backend we implemented.

    This backend is defined in \c instrumentclusterbackend.h and derives from
    \c InstrumentClusterBackendInterface. In our \c InstrumentClusterBackend class, we need to
    implement all pure virtual functions from InstrumentClusterBackendInterface and derived classes.

    For our example, this isn't complex, as we just need to implement the initialize() function.
    If our XML file would use writable properties or methods, then we'd need to implement those as
    well. We don't need to implement getters for our properties, because QtIvi uses the changed
    signals during the initialization phase to get information about the current state. Although
    the generated D-Bus interface class would provide getters to retrieve the properties from our
    server, it's not recommended to use these when you develop a backend. These getters are
    implemented by using synchronous calls, which means they will block the event loop until an
    answer is received by the client. Since this can lead to performance issues, we recommend to
    use \b asynchronous calls instead.

    In our backend, we define a fetch function for each property that's implemented like this:

    \quotefromfile ivicore/qface-tutorial/chapter6-own-backend/backend_dbus/instrumentclusterbackend.cpp
    \skipto ::fetchSpeed
    \printto ::fetchRpm

    First, we add the property to a list, to know which properties have been fetched successfully.
    Next, we use the \c asyncCall() function to call the getter for the \c speed property and use a
    \c QDBusPendingCallWatcher to wait for the result. Once the result is ready, the lambda removes
    the property again from our \c fetchList, uses the \c onSpeedChanged() function to store the
    value and notifies the frontend about it. Since we don't need the watcher anymore, we delete it
    in the next event loop run using \c deleteLater(), and call the \c checkInitDone() function.

    The \c checkInitDone() function is defined as follows:

    \quotefromfile ivicore/qface-tutorial/chapter6-own-backend/backend_dbus/instrumentclusterbackend.cpp
    \skipto ::checkInitDone
    \printto onSpeedChanged

    It ensures that the \c initializationDone() signal is sent to the frontend once all our
    properties are fetched from the server, and the initialization is complete.

    In addition to retrieving the current state from the server, we also need to inform our frontend
    every time a property changes. This is done by emitting the corresponding change signal when the
    server changes one of its properties. To handle this, we define a slot for each property. This
    slot saves the property in our class an emits the change signal:

    \quotefromfile ivicore/qface-tutorial/chapter6-own-backend/backend_dbus/instrumentclusterbackend.cpp
    \skipto void InstrumentClusterBackend::onSpeedChanged(int speed)
    \printto onRpmChanged

    The same slot is also used during the initialization phase to save and emit the value.

    You might wonder why saving the value is needed at all, if we can just emit the signal. This is
    because the backend plugin is used directly by every instance of the \c InstrumentCluster class
    and every instance calls the \c initialize() function to retrieve the current state. Instead of
    fetching all properties again, the second \c initialize() call just emits values that were
    already saved; and the slots keep them up to date.

    Now, when we start the Instrument Cluster, our backend should connect to our D-Bus server and
    look like this:

    \image examples_qface_tutorial_dbus.gif

*/