android 5.0 水波纹 实现

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <solid android:color="#FFFFFF" />
    <corners android:radius="4dp" />
</shape>

<?xml version="1.0" encoding="utf-8"?>
<ripple xmlns:android="http://schemas.android.com/apk/res/android"
  android:color="#FF21272B">
  <item>
    <shape android:shape="rectangle">
      <solid android:color="#FFFFFF" />
      <corners android:radius="4dp" />
    </shape>
  </item>
  <item android:drawable="@drawable/rounded_corners" />
</ripple>

  <Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@drawable/ripple_bg"
    android:text="@string/hello_world" />