Android14 Release

Android 14 Automotive OS Release Skill for VHAL

1. Absolute Truth Protocol: The # Properties reference strictly defines supported properties. You MUST use the properties-reference.md file in the current folder to check the supported properties. Do not invent properties, IDs, or types. Unlisted properties are unsupported or vendor-specific (0x2...), do not hallucinate hex values for them.
2. Framework Compliance: ALWAYS use android.car.VehiclePropertyIds.<CONSTANT>. NEVER hardcode raw IDs into application code.
3. Unit Validation: When manipulating float values (e.g. PERF_VEHICLE_SPEED), remind the developer that VHAL units (e.g. m/s) must be converted for UI display (to km/h or mph).
4. Hardware State Interlocks (Dependencies): Always evaluate the physical state of the car before attempting mutations. Do NOT mutate dependent properties (e.g., HVAC_FAN_SPEED) without verifying the prerequisite state (e.g., HVAC_POWER_ON == true). Enforce gear or speed checks (e.g., GEAR_SELECTION == PARK, PERF_VEHICLE_SPEED == 0f) for safety-critical actions like TRUNK_DOOR_MOVE or PARKING_BRAKE_ON.
5. AOSP Permissions Awareness: When generating VHAL code, proactively comment which android.car.permission.CAR_* manifest permission the developer needs to declare to use the requested property.

Implementation Patterns (Best Practices)

1. Unified Result State (Sealed Interface)

Never close a Flow due to missing properties or permissions, as this crashes downstream UI collectors in Automotive apps. Yield states instead:

import android.car.hardware.CarPropertyValue

sealed interface VhalResult<out T> {
    data class Success<T>(val property: CarPropertyValue<T>) : VhalResult<T>
    data class Unavailable(val reason: String) : VhalResult<Nothing>
    object PermissionDenied : VhalResult<Nothing>
}

1b. Lifecycle-Aware UI Collection

When collecting VHAL flows in an Activity or Fragment, ALWAYS use repeatOnLifecycle to prevent the UI from processing foreground sensor updates while backgrounded. This mitigates battery drain and avoids ANRs.

import androidx.lifecycle.Lifecycle
import androidx.lifecycle.lifecycleScope
import androidx.lifecycle.repeatOnLifecycle
import kotlinx.coroutines.launch

// Example UI usage:
lifecycleScope.launch {
    repeatOnLifecycle(Lifecycle.State.STARTED) {
        observeVehicleSpeedSafe(context).collect { result ->
            when (result) {
                is VhalResult.Success -> { /* Update UI */ }
                is VhalResult.Unavailable -> { /* Handle error */ }
                is VhalResult.PermissionDenied -> { /* Request permissions */ }
            }
        }
    }
}

2. IPC Lifecycle & Reactive Observation

Connecting to the VHAL requires strict Car.createCar lifecycle management. CarPropertyManager instances can become stale if the Car Service crashes. Always generate code that handles Car connection lifecycle and uses safe callbackFlow implementations.

import android.car.VehiclePropertyIds
import android.car.hardware.CarPropertyValue
import android.car.hardware.property.CarPropertyManager
import kotlinx.coroutines.channels.awaitClose
import kotlinx.coroutines.channels.Channel
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.callbackFlow
import kotlinx.coroutines.flow.buffer
import kotlinx.coroutines.flow.flowOn
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch

// IMPORTANT: The `CarPropertyManager` instance should be provided via Dependency Injection (e.g., Hilt) if Hilt is inside the project,
// created once per Application or Activity lifecycle. NEVER instantiate or disconnect `Car` inside a sensor flow.
fun observeVehicleSpeedSafe(carPropertyManager: CarPropertyManager): Flow<VhalResult<Float>> = callbackFlow {
    val callback = object : CarPropertyManager.CarPropertyEventCallback {
        override fun onChangeEvent(value: CarPropertyValue<*>) {
            @Suppress("UNCHECKED_CAST")
            trySend(VhalResult.Success(value as CarPropertyValue<Float>))
        }

        override fun onErrorEvent(propId: Int, zone: Int) {
            trySend(VhalResult.Unavailable("Hardware error for zone $zone"))
        }
    }

    try {
        carPropertyManager.registerCallback(
            callback,
            VehiclePropertyIds.PERF_VEHICLE_SPEED,
            CarPropertyManager.SENSOR_RATE_NORMAL
        )
    } catch (e: SecurityException) {
        trySend(VhalResult.PermissionDenied)
        close(e)
    } catch (e: IllegalArgumentException) {
        trySend(VhalResult.Unavailable("Property unsupported to observe"))
        close(e)
    }

    // Must unregister when the flow collector cancels
    awaitClose {
        carPropertyManager.unregisterCallback(callback)
    }
}.buffer(capacity = Channel.CONFLATED) // Inherent channel buffering is safer than .conflate()
.flowOn(Dispatchers.IO)

3. Safe Synchronous Reads

import android.car.VehiclePropertyIds
import android.car.hardware.property.CarPropertyManager

fun readVehicleSpeedSafe(carPropertyManager: CarPropertyManager): Float {
    return try {
        // Explicitly check config exists rather than rely on PropertyNotAvailableException
        val config = carPropertyManager.getCarPropertyConfig(VehiclePropertyIds.PERF_VEHICLE_SPEED)
        if (config == null) return 0f

        // Use primitive zero-allocation getters instead of generic wrapper allocations
        val propValue = carPropertyManager.getFloatProperty(
            VehiclePropertyIds.PERF_VEHICLE_SPEED,
            0
        )
        // VHAL emits speed in meters per second (m/s)
        propValue
    } catch (e: SecurityException) {
        0f // Missing permissions
    } catch (e: IllegalArgumentException) {
        0f // Hardware doesn't support the property
    }
}

5. Actuation Latency & State Flickering (The "Echo" Race Condition)

Vehicle networks (CAN/LIN/Ethernet) have physical latency (100ms - 500ms). When you mutate a property via setProperty, the ECU takes time to actuate. During this window, CarPropertyEventCallback will often emit the old value, causing the UI to flicker (e.g., ON -> OFF -> ON).

When generating UI or ViewModel code that binds to VHAL properties, ALWAYS instruct the developer to implement Optimistic UI with a debounce:

// In a ViewModel or State Holder:
// 1. User toggles a setting
// 2. We instantly update the UI (Optimistic Update)
// 3. We record the timestamp of the mutation
// 4. We suppress incoming VHAL flows older than (mutation_time + 500ms) to prevent "echo" flickering.

ADB & VHAL Debugging Commands (Testing)

Inject/Spoof a VHAL Property Event:

adb root
adb shell cmd car_service inject-vhal-event 0x1120040A 0 1 # Spoofs ABS_ACTIVE to True

Get Current VHAL Property Value:

adb root
adb shell cmd car_service get-property-value 0x11401020 0 # Reads DRIVER_DISTRACTION_WARNING

List all Car Service Properties Available:

adb shell dumpsys car_service --services CarPropertyService --list

Vehicle Area ID Reference (Bitmasks)

val supportedAreaIds = carPropertyManager.getCarPropertyConfig(VehiclePropertyIds.PROPERTY_NAME)?.areaIds

Never attempt to guess area IDs. Always query the vehicle hardware properties directly dynamically.