这个和kotlin没有什么关系吧，设置BottomNavigationView 的onNavigationItemSelectedListener事件就可以了

bottom_navigation.setOnNavigationItemSelectedListener {
    when (it.itemId) {
        R.id.navigation_home -> {
            view_pager.currentItem = 0
            return@setOnNavigationItemSelectedListener true
        }
        R.id.navigation_tree -> {
            view_pager.currentItem = 1
            return@setOnNavigationItemSelectedListener true
        }
        R.id.navigation_navi -> {
            view_pager.currentItem = 2
            return@setOnNavigationItemSelectedListener true
        }
        R.id.navigation_project -> {
            view_pager.currentItem = 3
            return@setOnNavigationItemSelectedListener true
        }
    }
    false
}