res/drawable/shadow_horizontal.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <!-- android:centerColor="@android:color/transparent" -->

    <gradient
        android:angle="90"
        android:centerX="50.0%"
        android:centerY="50.0%"
        android:endColor="@android:color/transparent"
        android:startColor="#90a2a2a2"
        android:type="linear" />

</shape>
1	<?xml version="1.0" encoding="utf-8"?>
2	<shape xmlns:android="http://schemas.android.com/apk/res/android"
3	android:shape="rectangle" >
4
5	<!-- android:centerColor="@android:color/transparent" -->
6
7	<gradient
8	android:angle="90"
9	android:centerX="50.0%"
10	android:centerY="50.0%"
11	android:endColor="@android:color/transparent"
12	android:startColor="#90a2a2a2"
13	android:type="linear" />
14
15	</shape>