car/res/drawable/car_action_button_activated_ring_foreground.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2018 The Android Open Source Project

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
-->
<selector
    xmlns:android="http://schemas.android.com/apk/res/android">
  <item
      android:state_activated="true">
    <shape
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:shape="oval">
      <stroke
          android:color="@color/car_tint"
          android:width="@dimen/car_action_bar_activation_ring_stroke_width">
      </stroke>
      <size
          android:height="@dimen/car_action_bar_activation_ring_radius"
          android:width="@dimen/car_action_bar_activation_ring_radius"/>
    </shape>
  </item>
  <!--Yes, this is a load-bearing invisible circle.-->
  <!--Android doesn't remeasure foregrounds when state_activated changes, -->
  <!--so we need a default drawable with the same size as the one we actually want to draw-->
  <item>
    <shape
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:shape="oval">
      <stroke
          android:color="#00000000"
          android:width="@dimen/car_action_bar_activation_ring_stroke_width">
      </stroke>
      <size
          android:height="@dimen/car_action_bar_activation_ring_radius"
          android:width="@dimen/car_action_bar_activation_ring_radius"/>
    </shape>
  </item>
</selector>