android 在drawable描边只描一边

 <?xml version="1.0" encoding="utf-8"?>  
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">  
     <item  
         android:left="-2dp"  
          android:right="-2dp"  
         android:top="-2dp">  
          <shape>  
             <stroke android:width="1dp"  
                     android:color="@color/black"/>  
          </shape>  
      </item>