B08c: Exercise: Use Vehicle Properties

Attach Android Studio to Cuttlefish

In the previous exercise you've built and copied the apk to the virtual machine and installed the application by hand on the command line. To speed up your development cycle, you can attach Android Studio to the cuttlefish device via adb over an additional ssh forward.

To do so, open the ssh configuration file and add/edit the following line

LocalForward 5038 localhost:5037 # for adb

Then restart your ssh connection to the cloud instance.

Now reconfigure Android Studio:

• Click on File -> Settings
• Search for adb
• In Adb Server lifecylce management enable Use existing manually managed server and use the port 5038.

# Either add -P 5038 to all adb calls, for instance:
adb -P 5038 install myapp.apk

# or set the environment variable and skipp the port argument:
export ANDROID_ADB_SERVER_PORT=5038
adb install myapp.apk

Preface

The Automotive APIs of Android are not part of core Android APIs for applications. They are located in additional libraries.

When developing a production-ready Android Application, you will be using the Car API provided by Android Jetpack. See Car App.

In this exercise you use the Car API that is exposed in the Android SDK directly. On the page developer.android.com: android.car your find the API reference.

Build the Application

Open the application in the folder app-automotive/ with Android Studio.

Take a look at the app-automotive/app/build.gradle.kts. That file contains the line

useLibrary("android.car")

This means that the uses-library XML tag is added to the Android Manifest file. When the application starts on the device, the Car Java library is loaded into the application and the Car API is available.

Navigate into the file MainActivity.java and look for the function setupUI(). Here you should add your code.

First let the instructor show you the basics of the code setup.

Use Our First Vehicle Property

In this section you should add the necessary code to query the vehicle property INFO_MODEL.

Every vehicle property is protected by an Android permission. Search in AOSP sources in the folders

• packages/services/Car/ and
• hardware/interfaces/automotive/

and find out which permission is needed.

Add this permission in a uses-permission XML tag to the AndroidManifest.xml. An example code line:

<uses-permission android:name="<permission name>" />

<application [...]

Furthermore, every vehicle property has an area type. Search the two directories again and find out what the correct area type is.

Then find out the type of the property. E.g., whether it's an integer, float or string.

After that use the CarPropertyManager to retrieve the value of the property and set it in the TextView.

Code template for setupUI():

variable = [Add the code for the CarPropertyManager here]
textViewModel.setText("Model: " + <variable>);

Rebuild and rerun the application and see the results.

Debugging tips:

• Use logcat -c to clear the log cat buffer. This avoids seeing older and unrelated log messages from a previous instance of your application.
• Use alogview to only see the logcat buffer of your application.

Changing the Vehicle Property

The previous chapter already mentioned that you can query and set vehicle properties in the car service and in the VHAL.

adb shell dumpsys android.hardware.automotive.vehicle.IVehicle/default --set 0x11100102 -s new-model

If you see the error

Error with service 'android.hardware.automotive.vehicle.IVehicle/default' while dumping: FAILED_TRANSACTION

execute adb root to restart the adb daemon on the device with root privileges.

After that you should be able to set the property successfully.

You can query the value of the property that is saved in the VHAL with

adb shell dumpsys android.hardware.automotive.vehicle.IVehicle/default --get 0x11100102

The INFO_MODEL vehicle property is a static property. Therefore, the car services does not get notified when this property is changed in the VHAL. The car service even implements a caching layer for the static properties. See the file and code packages/services/Car/service/src/com/android/car/hal/PropertyHalService.java.

To see the new value of INFO_MODEL in the application, you have to restart the car sevice. Use

adb shell pkill com.android.car

Since the application has implemented the onLifecycleChanged() listener, the application will auto reconnect to the car service. If you start logcat with adb shell logcat -s MainActivity:*, you will see the following output

I MainActivity: onLifecycleChanged: ready=false
I MainActivity: onLifecycleChanged: ready=true
D MainActivity: model=new-model
D MainActivity: currentGear=4
D MainActivity: onChangeEvent: gear=4

Also look at the Activity of your application. It also shows the new model string now.

Changing a Non-Static Vehicle Property

In the previous chapter a static vehicle property was used. Now add the dynamic property CURRENT_GEAR to the application.

Again search for the permission, the area type and the type of the value.

Since the property is non-static, use CarPropertyManager to register a callback that listens for events with property changes.

There are multiple ways to implement the solution. You could either follow the pattern already present in the implementation or use a more idiomatic approach. If you're stuck, don't hesitate to ask for help.

Remember, that updating the view should happen on the UI thread. You may use the following line in your callback code:

runOnUiThread(() -> textViewGear.setText("Gear: " + <variable>));

To inject a new value for the property, use the following command

adb shell dumpsys car_service inject-vhal-event 0x11400401 0 0x10
adb shell dumpsys car_service inject-vhal-event 0x11400401 0 0x20
adb shell dumpsys car_service inject-vhal-event 0x11400401 0 0x40

You should observe how the value on the screen is changing while the app is running.

Further reading

On the page Vehicle hardware abstraction layer you can read more about the hardware abstraction layer.