Android 依据系统时间整点、半点倒计时

activity_main.xml中的代码

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
  xmlns:android="https://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="@android:color/white"
  >
  <TextView
      android:id="@+id/mytime"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:gravity="center"
      android:textColor="@android:color/black"
      android:textSize="36sp"/>
  
</LinearLayout><?xml version="1.0"