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2020년 Kotlin + AndroidX + Navigation Menu 환경

//뒤로버튼(연속2번) 상태플래그

    private var doubleBackToExitPressedOnce :Boolean = false

//뒤로버튼 제어

//처음화면(홈프래그먼트)일때만 어플종료처리, 다른 프래그먼트가 올라온 상태이면 단순히 닫기모드

    override fun onBackPressed() {

        if(getForegroundFragment() is HomeFragment) {

            if (doubleBackToExitPressedOnce) {

                super.onBackPressed()

                return

            }

            this.doubleBackToExitPressedOnce = true

            toast(getString(R.string.msg_back_pressed_to_exit))

            Handler().postDelayed(Runnable { doubleBackToExitPressedOnce = false }, 2000)

        }else{

            super.onBackPressed()

            return

        }

    }

    //네비게이션 호스트 프래그먼트 컨테이너(R.id.nav_host_fragment)에 로드된 각 프래그먼트 정보 가져오기

    fun getForegroundFragment(): Fragment? {

        val navHostFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment)

        return navHostFragment?.childFragmentManager?.fragments?.get(0)

    }