在drawable目录下新建drawable resource

<?xml version="1.0" encoding="utf-8"?><!-- android:shape指定形状类型，默认为rectangle -->
<shape xmlns:android="http://schemas.android.com/apk/res/android"
        android:shape="rectangle">         
        <gradient
      android:startColor="#00ffffff"
      android:centerColor="#30ffffff"
      android:endColor="#00ffffff"
      android:angle="0"
      android:type="linear"
      android:centerX="0.5"
      android:centerY="0.5"
      android:gradientRadius="100"
        />
 
</shape>