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类主要活动：AppCompatActivity（）{最新初始化变量绑定：ActivityMainBinding覆盖创建时的功能（保存的示例状态：创建一个新的绑定，并将其添加到新的绑定中。

override fun onCreateOptionsMenu(menu: Menu?): Boolean {
    menuInflater.inflate(R.menu.bottom_nav_bar_main , menu)
    return true
}

override fun onOptionsItemSelected(item: MenuItem): Boolean {
    val intent : Intent
    when(item.itemId) {
        R.id.itemCourses -> {
            val fragment = CoursesFragment.newInstance()
        }
    }
    return true
}

}
类课程片段：片段（）{

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    val binding = FragmentCoursesBinding.inflate(inflater , container , false)
    return binding.root
}

}