The system bars are the permanent furniture of the vehicle. Most of what an OEM wants to change about them is configuration — and configuration can be overlaid, which means no fork and no re-merge at every platform upgrade.

Step 1 — Find what is configurable#

The config surface

cd ~/aosp
 
# Which bars exist, and their dimensions
grep -rn "config_enable.*SystemBar" packages/apps/Car/SystemUI/res/values/config.xml
 
# The bar layouts themselves
ls packages/apps/Car/SystemUI/res/layout/car_*system_bar*.xml

You are looking at two kinds of thing: boolean and dimension config values, and layout files. Both can be overlaid. Java cannot.

Step 2 — Change which bars exist#

VegaSystemUIRRO/res/values/config.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <bool name="config_enableTopSystemBar">true</bool>
    <bool name="config_enableBottomSystemBar">true</bool>
    <bool name="config_enableLeftSystemBar">false</bool>
    <bool name="config_enableRightSystemBar">false</bool>
</resources>

VegaSystemUIRRO/res/values/dimens.xml

<resources>
    <dimen name="car_top_system_bar_height">64dp</dimen>
    <dimen name="car_bottom_system_bar_height">96dp</dimen>
</resources>

Left and right bars exist because some head units are portrait, where a bottom bar wastes the wrong dimension.

Step 3 — Add a button to the bar#

The bar contents are a layout, so replacing the layout adds a button — no code change.

Start from the real one

cp packages/apps/Car/SystemUI/res/layout/car_bottom_system_bar.xml \
   vendor/oem/vega/overlays/VegaSystemUIRRO/res/layout/

VegaSystemUIRRO/res/layout/car_bottom_system_bar.xml

<!-- Copied from CarSystemUI and extended. Keep the original ids: CarSystemUI
     looks them up by name and will crash if one it expects is missing. -->
<com.android.systemui.car.systembar.CarSystemBarView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res-auto"
    android:id="@+id/car_bottom_bar"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/system_bar_background_color"
    android:orientation="horizontal">
 
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:orientation="horizontal">
 
        <com.android.systemui.car.systembar.CarSystemBarButton
            android:id="@+id/home"
            style="@style/SystemBarButton"
            systemui:categories="android.intent.category.HOME"
            systemui:highlightWhenSelected="true"
            systemui:icon="@drawable/car_ic_home"
            systemui:intent="intent:#Intent;action=android.intent.action.MAIN;category=android.intent.category.HOME;launchFlags=0x14000000;end"/>
 
        <!-- Your addition -->
        <com.android.systemui.car.systembar.CarSystemBarButton
            android:id="@+id/vega_comfort"
            style="@style/SystemBarButton"
            systemui:highlightWhenSelected="true"
            systemui:icon="@drawable/vega_ic_comfort"
            systemui:packages="com.oem.vega.comfort"
            systemui:intent="intent:#Intent;component=com.oem.vega.comfort/.MainActivity;launchFlags=0x10000000;end"/>
 
    </LinearLayout>
</com.android.systemui.car.systembar.CarSystemBarView>

Keep every id CarSystemUI looks up

The Java in CarSystemUI finds views by id — R.id.home, R.id.notifications and others. Removing one from your overlay layout produces a NullPointerException during SystemUI startup, which on a head unit looks like "no system bars at all". Add to the layout; do not remove from it.

Add the icon your layout references

<!-- VegaSystemUIRRO/res/drawable/vega_ic_comfort.xml -->
<vector xmlns:android="http://schemas.android.com/apk/res/android"
    android:width="44dp" android:height="44dp"
    android:viewportWidth="24" android:viewportHeight="24"
    android:tint="?android:attr/colorControlNormal">
    <path android:fillColor="@android:color/white"
        android:pathData="M4,18v-6h16v6h-2v-4H6v4H4zM7,6h10v4H7z"/>
</vector>

Step 4 — Build and verify#

m VegaSystemUIRRO && m -j
emulator -wipe-data -no-snapshot & adb wait-for-device && sleep 45

Verify the overlay applied and SystemUI survived

# 1. The overlay is enabled
adb shell cmd overlay list | grep vega
 
# 2. SystemUI did NOT crash — this is the one that catches a broken layout
adb logcat -b all | grep -iE 'CarSystemBar|SystemUI.*(FATAL|Exception)'
 
# 3. Your button is in the view hierarchy
adb shell dumpsys window | grep -i vega_comfort
 
# 4. Look at it
adb shell screencap -p /sdcard/bar.png && adb pull /sdcard/bar.png
 
# 5. It launches your app
adb shell input tap 640 690     # coordinates depend on your bar layout
adb shell dumpsys activity activities | grep -i vega.comfort

Item 2 first. A missing id crashes SystemUI on startup, and the visible symptom is a screen with no bars rather than an error.

Step 5 — The iteration loop#

Layout change to on-screen in under a minute

m VegaSystemUIRRO
adb root && adb remount
adb push $ANDROID_PRODUCT_OUT/product/overlay/VegaSystemUIRRO/VegaSystemUIRRO.apk \
  /product/overlay/VegaSystemUIRRO.apk
adb shell pkill -f com.android.systemui
sleep 5 && adb shell screencap -p /sdcard/bar.png && adb pull /sdcard/bar.png

When an overlay is not enough#

Overlays reach resources. They do not reach behaviour. You need to modify CarSystemUI itself when you want:

New interaction logic — a long-press action, a custom gesture
A new component class the layouts could reference
Changes to when a bar appears or hides
A different notification ranking or filtering rule

If you must fork, fork narrowly

# Copy the single class you need to change, not the app
cp packages/apps/Car/SystemUI/src/com/android/systemui/car/systembar/CarSystemBar.java \
   vendor/oem/vega/systemui/src/...
 
# Then build a variant that substitutes only that class

A fork is a fifteen-year commitment, not a sprint decision

Every platform upgrade must re-merge your changes into a moving target, for the life of the vehicle. Before forking, write down what specifically cannot be expressed as an overlay. If that list is short, the answer is usually to extend the overlay surface upstream instead — a resource that should have been overlayable and is not is a legitimate AOSP contribution.

Troubleshooting#

Symptom	Cause
No system bars at all	Overlay layout removed an id SystemUI looks up
Bar renders but button does nothing	Malformed `systemui:intent` URI
Overlay listed but not applied	Resource or layout name mismatch with the target
Icon missing	Drawable not included in the overlay's `res/`
Changes need a reboot	Restart SystemUI instead of rebooting
Works on emulator, not on target	Different platform version renamed the layout

Next#

Product configuration — the audio topology.

Modify CarSystemUI

What you will build

Before you start