Folding time shadow AppBarLayout

Recently met a demand in the project, AppBarLayout no shadow when fully expanded, showing shadows when fully retracted, this function can be easily achieved by setting the StateListAnimator.

First, create appbar_elevation.xml file under res / animator directory, as follows:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <!--折叠状态下的阴影-->
    <item app:state_collapsed="true">
        <objectAnimator
            android:propertyName="elevation"
            android:valueTo="5dp"
            android:valueType="floatType" />
    </item>
    <!- Item<->expand the shadow state
    app:state_collapsed="false">
        <objectAnimator
            android:propertyName="elevation"
            android:valueTo="0dp"
            android:valueType="floatType" />
    </item>
</selector>

Then set AppBarLayout in the layout file stateListAnimator property as follows:

<android.support.design.widget.AppBarLayout
     android:id="@+id/appbar"
     android:layout_width="match_parent"
     android:layout_height="256dp"
     android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
     android:background="@color/white"
     android:stateListAnimator="@animator/appbar_elevation"
    >