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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 interfaceframework/qface-tutorial \brief Demonstrates step-by-step how to generate a Middleware API based on a QML application. \ingroup qtinterfaceframework-examples \title Qt Interface Framework Generator Tutorial \image qface-tutorial.png This tutorial demonstrates how you can extend a QML application with your own auto-generated 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 back end} \li \l{chapter2}{Extend the interface and add annotations} \li \l{chapter3}{Add a simulation back end 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 back end} \li \l{chapter6}{Develop a production back end 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 qface-tutorial-final.gif \target chapter1 \section1 Chapter 1: Basic Middlware API with the Interface Framework 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-6/qtquick-codesamples.html}{QML examples}, we'll use the Interface Framework Generator to auto-generate the required parts. \target define-speed-property \section2 Interface Definition Language To be able to auto-generate the Middleware API, the Interface Framework 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 interfaceframework/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 interfaceframework/qface-tutorial/chapter1-basics/instrument-cluster.qface \printuntil module The most important part of the module is its interface definition. \quotefromfile interfaceframework/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 Auto-generation Now that our first version of the IDL file is ready, it's time to auto-generate an API from it, using the \l{Qt Interface Framework Generator}{Interface Framework Generator tool}. Similar to \l{Using the Meta-Object Compiler (moc)}{moc}, this auto-generation process is integrated into the build system and is done at compile time. In the following snippets we build a C++ library based on our IDL file: \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter1-basics/frontend/CMakeLists.txt \skipto find_package \printto install First \e find_package needs to be used to get all needed libraries into the CMake build system. A new library is defined with \l {qt_add_library} and, using CMake target_properties, the output name, as well as the output directory are set. As we need to link to this library in the future, it is easier to put the file into the upper directory. By calling the \l {qt_ifcodegen_extend_target} function, the \c autogenerator is called and the previously defined library is extended with the generated files. The input file is specified using the \e IDL_FILES argument. See \l{Build System Integration} for more information. \e qmake: \quotefromfile interfaceframework/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 Interface Framework Generator integration requires the \c CONFIG variable to specify the \c ifcodegen option. This makes sure the Interface Framework Generator is called during the build process, using the QFace file that we specify in \c IFCODEGEN_SOURCES. For more information, see \l{Build System Integration}. \section2 Which Files are Auto-generated The Interface Framework Generator works based on generation templates. These templates define what content should be generated from a QFace file. Using qmake, the template needs to be defined by the \c IFCODEGEN_TEMPLATE variable. If it is not defined, it defaults to the "frontend" template. In CMake the template needs to be specified using the \c TEMPLATE argument of \l {qt_ifcodegen_extend_target} and friends. For more details on these templates, see \l{Use the Generator}. In short, the "frontend" template generates: \list \li a C++ class derived from QIfAbstractFeature 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 library's build folder. Right now, the most important auto-generated file for us, is the resulting C++ class for our defined interface. It looks like this: \quotefile interfaceframework/qface-tutorial/chapter1-basics/frontend/frontend/instrumentcluster.h As you can see, the auto-generated 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 Front End Library with the QML Code For this integration, we use the auto-generated front-end 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 auto-generated types to QML and loads the Instrument Cluster QML code into the QQmlApplicationEngine: \quotefromfile interfaceframework/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 interfaceframework/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 interfaceframework/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 interfaceframework/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 Interface Framework Generator to generate an updated version of the C++ code. After the Interface Framework 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 interfaceframework/qface-tutorial/chapter2-enums-structs/frontend/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 interfaceframework/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 interfaceframework/qface-tutorial/chapter2-enums-structs/instrument-cluster.qface \printuntil readonly SystemType \skipto } \printuntil enum \printuntil } In the auto-generated 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 interfaceframework/qface-tutorial/chapter2-enums-structs/frontend/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 interfaceframework/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 interfaceframework/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 interfaceframework/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 Back End and Annotations with a QML plugin In the previous two chapters, we wrote a Middleware API using a QFace file and used the Interface Framework Generator to auto-generate a C++ API in the form of a library. Now, in this chapter, we extend this further by introducing a simulation back end and using annotations to define default values for our simulation. \section2 Separation between the Front End and Back End Both QtInterfaceFramework and the Interface Framework Generator enable you to write code that separates the front end from the back end. This allows you 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 front end provides the API as a library; the back end provides an implementation of this API. This implementation is based on QtInterfaceFramework's \l{Dynamic Backend System} which enables us to switch between such back ends at runtime. \image feature-backend.png \section2 Add a Simulation Back End For our Instrument Cluster, we'd like to add such a back end 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 back ends are called "simulation backend". To add this type of back end, once again, we use the Interface Framework 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: \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter3-simulation-backend/backend_simulator/CMakeLists.txt \skipto find_package \printto target_link_libraries Similar to the front-end library, first the used components are imported using \e find_package. As we want to build a plugin (dynamic library) which is loaded at runtime instead of linking against it, we use the \l {qt_add_plugin} function instead. One important aspect here is that the library name ends with "_simulation", which is a way to tell QtInterfaceFramework that this is a simulation back end. When a "production" back end is available, it is preferred over the "simulation" one. For more information, see \l{Dynamic Backend System}. As before, the Interface Framework Generator is called by using the \l{qt_ifcodegen_extend_target} function, this time setting "backend_simulator" as the \c TEMPLATE. \e qmake: \quotefromfile interfaceframework/qface-tutorial/chapter3-simulation-backend/backend_simulator/backend_simulator.pro \printto DESTDIR \skipto QT \printuntil CONFIG \skipto IFCODEGEN_TEMPLATE \printto CONFIG += install_ok Just like for the front-end 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 QtInterfaceFramework that this is a simulation back end. When a "production" back end is available, it is preferred over the "simulation" one. For more information, see \l{Dynamic Backend System}. Enabling the Interface Framework Generator is also done in the same way as we did earlier: by using the same \c IFCODEGEN_SOURCE variable, but defining \c IFCODEGEN_TEMPLATE 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 front end and back-end separation, we need to have the back end implement a defined interface class, that is known to the front end. This interface is aptly called "backend interface" and is automatically generated as part of the front-end library. Because this class provides signals and slots and uses QObject for its base class, you need to link to the front-end library when you inherit from it. As this is needed for the back-end plugin, we need to add the following lines in addition: \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter3-simulation-backend/backend_simulator/CMakeLists.txt \skipto target_link_libraries \printto install By defining the front-end library named \e libIc_chapter3 as a target link library the include path gets updated accordingly. \e qmake: \quotefromfile interfaceframework/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.If.InstrumentCluster.InstrumentCluster" . There is no simulation backend implementing "Example.If.InstrumentCluster.InstrumentCluster" . No suitable ServiceObject found. \endcode This message indicates that QtInterfaceFramework 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, QtInterfaceFramework searches for its back-end plugins in the \c interfaceframework folder. To make sure our simulation back end ends up in such a folder, we add the following changes in our build system file: \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter3-simulation-backend/backend_simulator/CMakeLists.txt \skipuntil qt_add_plugin \printuntil set_target_properties \e qmake: \quotefromfile interfaceframework/qface-tutorial/chapter3-simulation-backend/backend_simulator/backend_simulator.pro \skipto DESTDIR \printuntil DESTDIR You might wonder how creating an \c interfaceframework folder in the upper directory solves the 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 front-end 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. \e CMake: Using CMake, the location of our front-end library is automatically added as a \e RUNPATH to the the binary and no further steps are needed. \e qmake: In qmake we can ease the setup by using a relative \e RPATH instead of the \c LD_LIBRARY_PATH and annotate our plugin with the information for the linker, where it might find the needed libraries, relative to the plugin's location: \quotefromfile interfaceframework/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 auto-generated Middleware APIs. Although this works fine, in bigger projects it's common to use a QML Plugin instead and be able to use the \c qml executable for development. Although the code for doing this is not complex, the Interface Framework 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 build system file to generate a QML plugin looks like this: \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter3-simulation-backend/imports/CMakeLists.txt \skipto qt_ifcodegen_import_variables \printto install Unlike all our previous generator calls we don't extend a previously defined target, but import the generated code into CMake and pass it to the \l {qt_add_qml_module} function. The \l {qt_ifcodegen_import_variables} function will call the generator and export variables starting with \e CLUSTER as prefix to the current CMake scope. Those variables reference auto-generated code, but also expose other information like the QML import URI. In the next call, the variables are used to define a QML Module with the correct URI and version (as specified in our IDL file). By using the \e OUTPUT_DIRECTORY variable we can make sure that the correct folder structure is generated and we can import the QML plugin directly from within the build folder. \note Instead of generating a QML plugin, the new QML type registration can be used, which was introduced in \b 6.3. In order to use this new mechanism the frontend CMakeLists.txt has to be extended like this: \badcode qt_ifcodegen_extend_target(libIc_chapter3 IDL_FILES ../instrument-cluster.qface PREFIX CLUSTER TEMPLATE frontend ) qt_add_qml_module(libIc_chapter3 OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/../imports/${CLUSTER_URI_PATH}" URI ${CLUSTER_URI} VERSION ${CLUSTER_VERSION} IMPORTS QtInterfaceFramework/auto ) \endcode Please see \l {QML Type Registration} for more information. \e qmake: \quotefromfile interfaceframework/qface-tutorial/chapter3-simulation-backend/imports/imports.pro \printto target.path All lines until \c IFCODEGEN_SOURCES should be familiar. We use \c CONFIG to build a plugin, then define the settings for the linker to link against our front-end library. Then, we use \c IFCODEGEN_TEMPLATE to define "qmlplugin" as the generation template. Instead of adding \c ifcodegen 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 auto-generated by the Interface Framework 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 hard-coded registration. So, we can now remove the linking step in the \c instrument-cluster build system file and change our main file accordingly: \quotefromfile interfaceframework/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 back end. 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 back end 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 temperature defaults to 0. Let's change this to a temperature in spring, 15 degrees Celsius, with the following YAML fragment: \quotefromfile interfaceframework/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 Interface Framework 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 interfaceframework/qface-tutorial/chapter4-simulation-behavior/backend_simulator/backend_simulator/instrumentclustermodule.json But how is this JSON file related to the actual simulation back-end code? The auto-generated simulation back-end code uses QIfSimulationEngine, that reads the JSON file and provides its data to a QML simulation file. A default QML file is also auto-generated and loaded from the QIfSimulationEngine. This default QML file provides the behavior of what should happen in the the simulation back end. 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 QIfSimulationEngine allows us to override which JSON file should be loaded into the engine, when we set the \c QTIF_SIMULATION_DATA_OVERRIDE environment variable. Since there can be multiple engines run by different back ends, we need to define which engine we're referring to. In the auto-generated 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 km/h instead of mph: \badcode QTIF_SIMULATION_DATA_OVERRIDE=example.if.instrumentclustermodule=/kmh.json \endcode \section2 Define a QML Behavior Before we define our custom behavior, let's see what's been auto-generated for us. There are two QML files: The first is \c instrumentcluster_simulation.qml and rather simple. It defines an entry point that instantiates 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 QIfSimulationEngine 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 interfaceframework/qface-tutorial/chapter4-simulation-behavior/backend_simulator/backend_simulator/InstrumentClusterSimulation.qml First, there's a \c settings property, that's initialized with the return value from the \l{IfSimulator::findData}{IfSimulator.findData} method, which takes the \l{IfSimulator::simulationData}{IfSimulator.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 front end class. But, apart from the front end, these properties are also writable to make it possible to change them to provide a useful simulation. Each time a front-end instance connects to a back end, 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 setters and getters, for properties or methods. For more details, see \l{QIfSimulationEngine}. Inside the QML \c initialize() function, we call \c{IfSimulator.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 front-end instance connects to the back end. Lastly, the base implementation is called to make sure that the \c initializationDone signal is sent to the front end. Similarly, a setter function is defined for each property; they use the \c{IfSimulator.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{IfSimulator.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 provides 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 interfaceframework/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 interfaceframework/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 interfaceframework/qface-tutorial/chapter4-simulation-behavior/backend_simulator/simulation.qml \skipto import \printuntil /^\}/ The next step is to tell the Interface Framework Generator and the QIfSimulationEngine about our new simulation file. Similar to QML files, the best approach 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 interfaceframework/qface-tutorial/chapter4-simulation-behavior/instrument-cluster.qface \printuntil module \dots Now, rebuilding the simulation back end embeds the simulation file into the plugin and hands the file over to the QIfSimulationEngine, 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 qface-tutorial-unsync.gif \section2 Add a QtRemoteObjects Integration The IPC for this example is QtRemoteObjects, because the Interface Framework Generator already supports it out of the box. To use QtRemoteObjects we generate a second plugin, a "production" back end, this time. Production back ends are automatically preferred over the simulation back end we introduced before. This is done with the following build system files: \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter5-ipc/backend_qtro/CMakeLists.txt \skipto qt_add_plugin \printto install \e qmake: \quotefromfile interfaceframework/qface-tutorial/chapter5-ipc/backend_qtro/backend_qtro.pro \printto CONFIG += install_ok These files are almost identical to the ones we used earlier for our simulation back end. For now we highlight what's changed. The name of the plugin doesn't end with "_simulation" to indicate that this is a "production" back end. The template is now changed to "backend_qtro" to generate a back end that uses \l{Qt Remote Objects Replica}{Qt Remote Objects Replicas} to connect to a \l{Qt Remote Objects Source} that provides the values. In addition to a back end based on QtRemoteObject, we also need a server based on QtRemoteObject . This part can also be auto-generated using the Interface Framework Generator in a similar fashion: \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter5-ipc/simulation_server/CMakeLists.txt \skipto qt_add_executable \printto # Resources: \e qmake: \quotefromfile interfaceframework/qface-tutorial/chapter5-ipc/simulation_server/simulation_server.pro \printto RESOURCES Because we'd like to generate a server binary, the qmake \c TEMPLATE needs to be set to "app" instead of "lib", in CMake we use \l {qt_add_executable} instead. 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 back end, but this back end is no longer used as we added the QtRemoteObjects-based "production" back end. Because we used the "server_qtro_simulator" generation template, this can easily be fixed, as the generated server code is also using the QIfSimulationEngine and supports to use the same simulation file than our simulation back end. 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. \e CMake: \quotefromfile interfaceframework/qface-tutorial/chapter5-ipc/simulation_server/CMakeLists.txt \skipto # Resources: \printto install \e qmake: \quotefromfile interfaceframework/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 qface-tutorial-sync.gif \target chapter6 \section1 Chapter 6: Develop a Production Back End with D-Bus Previously, we extended our Instrument Cluster code by using QtRemoteObjects as IPC and auto-generated a back end for it, as well as a server that provides the simulation. In this chapter, we'd like to write our own back end \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 interfaceframework/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.If.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 hard-coded values. \quotefromfile interfaceframework/qface-tutorial/chapter6-own-backend/demo_server/instrumentcluster.cpp \skipto timerEvent \printuntil } \section2 Write Our own D-Bus Back End Let's start with a build system file for our back end. This is very similar to previous files, but it doesn't use the Interface Framework Generator. Instead, it uses \c DBUS_INTERFACES for qmake to auto-generate some client code which sends and receives messages over D-Bus. In the CMake case the \l {qt_add_dbus_interface} function is used to do the same. Now, we need to define an entry point for our plugin. This plugin class needs to derive from QIfServiceInterface and implement two functions: \list \li \c {QStringList interfaces()} -- that returns a list of all interfaces this plugin supports. \li \c {QIfFeatureInterface *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 interfaceframework/qface-tutorial/chapter6-own-backend/backend_dbus/instrumentcluster_dbus.json We need this list, as it gives QtInterfaceFramework the chance to know which interfaces a back end supports, before instantiating it and loading only the plugins which the application code needs. Our plugin code looks like this: \quotefromfile interfaceframework/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 auto-generated library. In \c insterfaceInstance() we check for the correct string and return an instance of the instrument cluster back-end we implemented. This back end 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 QtInterfaceFramework 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 back-end. 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 back end, we define a fetch function for each property that's implemented like this: \quotefromfile interfaceframework/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 front end 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 interfaceframework/qface-tutorial/chapter6-own-backend/backend_dbus/instrumentclusterbackend.cpp \skipto ::checkInitDone \printto onSpeedChanged It ensures that the \c initializationDone() signal is sent to the front end 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 front end 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 interfaceframework/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 back-end 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 back end should connect to our D-Bus server and look like this: \image qface-tutorial-dbus.gif */