ImageView设置点击效果需要注意两点，第一个设置android:clickable="true"，第二个 <item android:drawable="@drawable/button_n"/>属性必须放到最后才能生效

<?xml version="1.0" encoding="utf-8"?><!--发现大类列表项-->
<ImageView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/ivCategory"
    android:layout_width="@dimen/x110"
    android:layout_height="@dimen/x110"
    android:src="@drawable/bt_icon_pxb"
    android:clickable="true"
    android:background="@drawable/bt_discover_category">

</ImageView>

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

    <item android:drawable="@drawable/button_d" android:state_checked="true"/>
    <item android:drawable="@drawable/button_d" android:state_pressed="true"/>
    <item android:drawable="@drawable/button_n"/>

</selector>