<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_focused="true">
        <layer-list>
            <item
                android:bottom="0.5dp"
                android:top="-1dp"
                android:left="-1dp"
                android:right="-1dp">
                <shape>
                <solid
                    android:color="#00F5FF" />
                <stroke
                    android:width="0.5dp"
                    android:color="#00F5FF" />
            </shape>
            </item>
        </layer-list>
    </item>
    <item android:state_pressed="true">
        <layer-list>
            <item
                android:bottom="0.5dp"
                android:top="-1dp"
                android:left="-1dp"
                android:right="-1dp"><shape>
                <solid
                    android:color="#00F5FF" />
                <stroke
                    android:width="0.5dp"
                    android:color="#00F5FF" />
            </shape>
            </item>
        </layer-list>
    </item>
    <item>
        <layer-list>
            <item
                android:bottom="0.5dp"
                android:top="-1dp"
                android:left="-1dp"
                android:right="-1dp">
                <shape>
                  <solid
                      android:color="#00F5FF" />
                  <stroke
                      android:width="0.5dp"
                      android:color="#949494" />
                </shape>
            </item>
        </layer-list>
    </item>
</selector>

概要：

实现单边框主要是layer-list  item中bottom,top,left,right四种属性！