利用<layer-list >....</layer-list>叠加<item>，控制每一层间距逐渐减小，颜色逐渐减淡，最后呈现扩散模糊效果的阴影
<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android"><item><shape android:shape="rectangle"><corners android:radius="5dp" /><solid android:color="#19FFD3DA" /></shape></item><itemandroid:bottom="2dp"android:left="2dp"android:right="2dp"android:top="2dp"><shape android:shape="rectangle"><corners android:radius="5dp" /><solid android:color="#33FFD3DA" /></shape></item><itemandroid:bottom="3dp"android:left="3dp"android:right="3dp"android:top="3dp"><shape android:shape="rectangle"><corners android:radius="5dp" /><solid android:color="#4cFFD3DA" /></shape></item><item android:bottom="3.5dp"android:left="3.5dp"android:right="3.5dp"android:top="3.5dp"><shape android:shape="rectangle"><corners android:radius="5dp" /><solid android:color="#66FFD3DA" /></shape></item><!--白色背景--><itemandroid:bottom="4dp"android:left="4dp"android:right="4dp"android:top="4dp"><shape android:shape="rectangle"><corners android:radius="5dp" /><solid android:color="@color/white" /></shape></item></layer-list>