키패드를 보여주는 extension 함수

ShowKeyboard.kt

fun View.focusAndShowKeyboard() {
   /**
    * This is to be called when the window already has focus.
    */
   fun View.showTheKeyboardNow() {
       if (isFocused) {
           post {
               // We still post the call, just in case we are being notified of the windows focus
               // but InputMethodManager didn't get properly setup yet.
               val imm = context.getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager
               imm.showSoftInput(this, InputMethodManager.SHOW_IMPLICIT)
           }
       }
   }

   requestFocus()
   if (hasWindowFocus()) {
       // No need to wait for the window to get focus.
       showTheKeyboardNow()
   } else {
       // We need to wait until the window gets focus.
       viewTreeObserver.addOnWindowFocusChangeListener(
           object : ViewTreeObserver.OnWindowFocusChangeListener {
               override fun onWindowFocusChanged(hasFocus: Boolean) {
                   // This notification will arrive just before the InputMethodManager gets set up.
                   if (hasFocus) {
                       this@focusAndShowKeyboard.showTheKeyboardNow()
                       // It’s very important to remove this listener once we are done.
                       viewTreeObserver.removeOnWindowFocusChangeListener(this)
                   }
               }
           })
   }
}