/**
 * The observer that will be notified when the height of 
 * the keyboard has changed
 */
public interface KeyboardHeightObserver {

 /** 
  * Called when the keyboard height has changed, 0 means keyboard is closed,
  * >= 1 means keyboard is opened.
  * 
  * @param height  The height of the keyboard in pixels
  * @param orientation The orientation either: Configuration.ORIENTATION_PORTRAIT or 
  *      Configuration.ORIENTATION_LANDSCAPE
  */
 void onKeyboardHeightChanged(int height, int orientation);
}

/**
 * The keyboard height provider, this class uses a PopupWindow
 * to calculate the window height when the floating keyboard is opened and closed. 
 */
public class KeyboardHeightProvider extends PopupWindow {

 /** The tag for logging purposes */
 private final static String TAG = "sample_KeyboardHeightProvider";

 /** The keyboard height observer */
 private KeyboardHeightObserver observer;

 /** The cached landscape height of the keyboard */
 private int keyboardLandscapeHeight;

 /** The cached portrait height of the keyboard */
 private int keyboardPortraitHeight;

 /** The view that is used to calculate the keyboard height */
 private View popupView;

 /** The parent view */
 private View parentView;

 /** The root activity that uses this KeyboardHeightProvider */
 private Activity activity;

 /** 
  * Construct a new KeyboardHeightProvider
  * 
  * @param activity The parent activity
  */
 public KeyboardHeightProvider(Activity activity) {
  super(activity);
  this.activity = activity;

  LayoutInflater inflator = (LayoutInflater) activity.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
  this.popupView = inflator.inflate(R.layout.keyboard_popup_window, null, false);
  setContentView(popupView);

  setSoftInputMode(LayoutParams.SOFT_INPUT_ADJUST_RESIZE | LayoutParams.SOFT_INPUT_STATE_ALWAYS_VISIBLE);
  setInputMethodMode(PopupWindow.INPUT_METHOD_NEEDED);

  parentView = activity.findViewById(android.R.id.content);

  setWidth(0);
  setHeight(LayoutParams.MATCH_PARENT);

  popupView.getViewTreeObserver().addOnGlobalLayoutListener(new OnGlobalLayoutListener() {

    @Override
    public void onGlobalLayout() {
     if (popupView != null) {
      handleOnGlobalLayout();
     }
    }
   });
 }

 /**
  * Start the KeyboardHeightProvider, this must be called after the onResume of the Activity.
  * PopupWindows are not allowed to be registered before the onResume has finished
  * of the Activity.
  */
 public void start() {

  if (!isShowing() && parentView.getWindowToken() != null) {
   setBackgroundDrawable(new ColorDrawable(0));
   showAtLocation(parentView, Gravity.NO_GRAVITY, 0, 0);
  }
 }

 /**
  * Close the keyboard height provider, 
  * this provider will not be used anymore.
  */
 public void close() {
  this.observer = null;
  dismiss();
 }

 /** 
  * Set the keyboard height observer to this provider. The 
  * observer will be notified when the keyboard height has changed. 
  * For example when the keyboard is opened or closed.
  * 
  * @param observer The observer to be added to this provider.
  */
 public void setKeyboardHeightObserver(KeyboardHeightObserver observer) {
  this.observer = observer;
 }
 
 /**
  * Get the screen orientation
  *
  * @return the screen orientation
  */
 private int getScreenOrientation() {
  return activity.getResources().getConfiguration().orientation;
 }

 /**
  * Popup window itself is as big as the window of the Activity. 
  * The keyboard can then be calculated by extracting the popup view bottom 
  * from the activity window height. 
  */
 private void handleOnGlobalLayout() {

  Point screenSize = new Point();
  activity.getWindowManager().getDefaultDisplay().getSize(screenSize);

  Rect rect = new Rect();
  popupView.getWindowVisibleDisplayFrame(rect);

  // REMIND, you may like to change this using the fullscreen size of the phone
  // and also using the status bar and navigation bar heights of the phone to calculate
  // the keyboard height. But this worked fine on a Nexus.
  int orientation = getScreenOrientation();
  int keyboardHeight = screenSize.y - rect.bottom;
  
  if (keyboardHeight == 0) {
   notifyKeyboardHeightChanged(0, orientation);
  }
  else if (orientation == Configuration.ORIENTATION_PORTRAIT) {
   this.keyboardPortraitHeight = keyboardHeight; 
   notifyKeyboardHeightChanged(keyboardPortraitHeight, orientation);
  } 
  else {
   this.keyboardLandscapeHeight = keyboardHeight; 
   notifyKeyboardHeightChanged(keyboardLandscapeHeight, orientation);
  }
 }

 private void notifyKeyboardHeightChanged(int height, int orientation) {
  if (observer != null) {
   observer.onKeyboardHeightChanged(height, orientation);
  }
 }
}

@Override
public void onKeyboardHeightChanged(int height, int orientation) {
 String or = orientation == Configuration.ORIENTATION_PORTRAIT ? "portrait" : "landscape";
 Logger.d(TAG, "onKeyboardHeightChanged in pixels: " + height + " " + or);
}

private KeyboardHeightProvider mKeyboardHeightProvider;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
 ...
 mKeyboardHeightProvider = new KeyboardHeightProvider(this);
 new Handler().post(() -> mKeyboardHeightProvider.start());
}

@Override
protected void onResume() {
 super.onResume();
 mKeyboardHeightProvider.setKeyboardHeightObserver(this);
}

@Override
protected void onPause() {
 super.onPause();
 mKeyboardHeightProvider.setKeyboardHeightObserver(null);
}

@Override
protected void onDestroy() {
 super.onDestroy();
 mKeyboardHeightProvider.close();
}