核心代码：（Activity类或Fragment类中调用replaceFragment去替换碎片）
private fun replaceFragment(fragment: Fragment) {
val fragmentManager = childFragmentManager
val transaction = fragmentManager.beginTransaction()
transaction.replace(R.id.fragment_layout, fragment) //将fragment_layout用形参fragment填充
transaction.commit()
}
<FrameLayout
android:id="@+id/fragment_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_above="@+id/Bottom_Navigation">
</FrameLayout>
