Android: transperent gradient

I have a li开发者_运维问答st activity...

 <?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
<gradient 
    android:startColor="#fafafa"
    android:centerColor="#e1ebeb"
    android:endColor="#fafafa"
    android:angle="180"
     />

This is the gradient what I need for every row. But there is no selector....How to get selector ? How to make this color transparent ? Thanks...