HVAC Control

"QtAA HVAC Control"

Building and deploying the example

See specific steps relating to building and deploying Qt for Android Automotive examples.


This example application utilizes the Qt IF Generator Extensions for Android Automotive. It uses Qt Interface Framework, and Qt for Android Automotive, to handle Android specific data types, and the Android Automotive back end.

This solution is based on the generated code of the back end and the front end components, with the interface defined in an Interface Definition Language (IDL) with QFace and the YAML files for annotations. The UI is defined in the Qt Design Studio project, and communicates with the front end by a Plugin (based on the generated code).

Sub-projects for different Communication Models

There are two options for communication between the back end and the service giving access to vehicle properties:

  • Java Native Interface (JNI): By using the basic communication option on Android environments, the back end calls the Vehicle Properties Java API directly from native code through JNI
  • Qt Remote Objects (QtRO): By using the cross-domain communication framework (uses the abstract Unix domain socket configuration with QtRORegistry by default) of Qt, the back end calls another Android service (called hvac_service in this example) which calls the Android Automotive Vehicle Properties API from the service's own separate process

Both configurations use the same interface definitions (.qface and .yaml files) and the UI definition (Qt Design Studio project).

Using the Back End API

The HVAC back end is generated by Qt Interface Framework Generator, based on a QFace API definition (ifvehiclefunctions.qface). To make the application able to access this back end, certain code needs to be added in QML application and C++ implementation.

The HVAC back end can be accessed through the ClimateControl plugin. This plugin is built based on the imports subproject, it exports to the QtIf.Android.VehicleFunctions module as a ClimateControl component. The module needs to be imported to the QML document and then the plugin component instantiated, as shown in the example below:

import QtIf.Android.VehicleFunctions 1.0

In the case of the Qt Remote Objects target, the Remote Objects connection needs to be enabled in the backing service's main() function as follows:

    QAndroidService app(argc, argv);
    return app.exec();

The setup described above enables the use of a connection between the backing Android service (called hvac_service in this example) and the QML application.

Note: In this example, the connection to the back end is defined in a distinct QML file, ClimateControlFacade.qml, which separates the UI implementation and the communication to the backing service. Such an approach facilitates the creation of the ClimateControl stub implementation for Qt Design Studio.

We start by declaring the ClimateControl component.

    ClimateControl {
        id: climateControl

Now, we can exploit the properties defined in the QFace API definition.

    Connections {
        target: climateControl
        function onPowerOnChanged(powerOn) { facade.powerOn = powerOn }
        function onAcOnChanged(acOn) { facade.acOn = acOn }
        function onMaxAcOnChanged(maxAcOn) { facade.maxAcOn = maxAcOn }
        function onAutoOnChanged(autoOn) { facade.autoOn = autoOn }
        function onFanSpeedChanged(fanSpeed) { facade.fanSpeed = fanSpeed }

Passing the values to the back end is done in a similar way.

    Connections {
        target: facade
        function onPowerOnChanged() { climateControl.powerOn = facade.powerOn }
        function onAcOnChanged() { climateControl.acOn = facade.acOn }
        function onMaxAcOnChanged() { climateControl.maxAcOn = facade.maxAcOn }
        function onAutoOnChanged() { climateControl.autoOn = facade.autoOn }

Then, we use the defined ClimateControlFacade in the QML file.

    ClimateControlFacade {
        id: climateControl

And connect the UI element to the facade.

        AirKnob {
            id: airKnob
            x: 0
            y: 0
            value: climateControl.fanSpeed
            onValueChanged: climateControl.fanSpeed = value
            automode: climateControl.autoOn
            active: climateControl.powerOn
            Connections {
                target: climateControl
                onFanSpeedChanged: airKnob.value = climateControl.fanSpeed

Important: As the QML bindings are unidirectional we have to use the signal handler mechanism. Otherwise, each user interaction would disconnect the binding.

Interacting with the HVAC Control App

  • Open the app in the emulator.
  • From the emulator's Extended controls > Car data select the VHal Properties tab.
  • Select a property to change. For example: Fan Speed.
  • Make a change in the app's UI side and observe the sate of the selected property in the car data window.

    Note: A property's state can also be observed in the logs of the app either using the relevant Qt IDE log tab or directly with the ADB logcat tool.

See also Qt for Android, Qt Interface Framework, and Qt IF Generator Extensions.

Available under certain Qt licenses.
Find out more.