bg 和selector 与老师设置的是一样的，但是在真机上面跑的时候，按下了button，就一直是灰色，松开按钮后无法恢复到白色。请问是什么原因呢？

white_bg:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >
    <corners android:radius="5dp"/>
    <solid android:color="@color/white"/>

</shape>

white_selector:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:drawable="@drawable/gray_bg" android:state_pressed="true"/>
    <item android:drawable="@drawable/white_bg" android:state_pressed="false"/>

</selector>

gray_bg:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >
    <corners android:radius="5dp"/>
    <solid android:color="@color/gray"/>

</shape>

button对background的引用：

<Button
                android:id="@+id/button_point"
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_marginLeft="8dp"
                android:layout_marginTop="8dp"
                android:layout_weight="1"
                android:background="@drawable/white_selector"
                android:gravity="right|bottom"
                android:paddingBottom="10dp"
                android:paddingRight="10dp"
                android:text="."
                android:textSize="25sp" />