BuildAction.cs

  public class BuildAction : GridAction
    {
        [SerializeField] private InputModule inputModule;
        [SerializeField] private ActionValidation validation;
        [SerializeField] private GameObject GhostUI;
        
        private Dictionary<Cell, bool> canBuild = new Dictionary<Cell, bool>();

        
        public override void StartAction() 
        {
            canBuild.Clear();
            
            Entity.GameGrid.cells.ForEach(cell => canBuild.Add(cell ,validation.Validate(cell, this)));
        }


        public override void Close()
        {
            
        }

        public override State Get()
        {
            return !Active ? null : inputModule.Get(this);
        }

        public bool CanBuild(Cell cell)
        {
            return canBuild.ContainsKey(cell) && canBuild[cell];
        }
        

BuildInputModule.cs

    public class BuildInputModule : InputModule
    {
        public override State Get(GridAction action)
        {
            if (!(action is BuildAction buildAction))
                return null;
            
            return new ConfirmBuildState(buildAction, inputReader, listener, onStartInputState);        
        }
        
    }

ConfirmBuildPlacementState.cs

  public class ConfirmBuildPlacementState : State
    {
        private readonly IInputReader _inputReader;
        private readonly IFilter<ISelectable> _selectionFilter;
        private readonly IFilter<ISelectable> _selectionFilterHover;
        private readonly BuildAction _buildAction;
        private readonly IListener<ISelectable> _listener;
        private readonly GridEntity _buildEntity;
        private readonly IHighlightCells _highlighter;
        private Cell _previousCell;
        private GhostUI _ghostUI;
        
        public ConfirmBuildPlacementState(IInputReader inputReader, IListener<ISelectable> listener, IFilter<ISelectable> selectionFilter, BuildAction buildAction, GridEntity buildEntity)
        {
            _inputReader = inputReader;
            _selectionFilter = selectionFilter;
            _buildAction = buildAction;
            _listener = listener;
            _buildEntity = buildEntity;
            _selectionFilter = new SelectionFilter<ISelectable>(listener.Listen, listener.ListenAll, HandleNullSelected, HandleEntitySelected, HandleCellSelected);
            _selectionFilterHover = new SelectionFilter<ISelectable>(listener.Listen, listener.ListenAll, HandlePointerAboveNull, HandlePointerAboveEntity, HandlePointerAboveCell);
            _highlighter = new DefaultHighlighter(_buildAction.Entity.GameGrid, null);
        }
        
        public override void Enter()
        {
            _ghostUI = Object.Instantiate(_buildAction.ghostUI);
            //Set the sprite of the ghost here?
            
           _highlighter.SetAsHighlighted(_buildAction.CanBuild);
           base.Enter();
        }
        
        protected override void Update()
        {
            _selectionFilter.Filter(_inputReader.Primary());
            _selectionFilterHover.Filter(true);
        }

        
        private void HandleEntitySelected(GridEntity entity)
        {
            HandleNullSelected();
        }

        private void HandleCellSelected(Cell cell)
        {
            if (_buildAction.CanBuild(cell))
            {
                Object.Destroy(_ghostUI.gameObject);
                // SOME BUILDING CODE HERE THEN? 
                nextState = null;
                Exit();
                return;
            }
            
            HandleNullSelected();
            Exit();
        }

        private void HandleNullSelected()
        {
            _buildAction.Close();
            Object.Destroy(_ghostUI.gameObject);
            nextState = null;
            Exit();
        }
        
        private void HandlePointerAboveCell(Cell cell)
        {
            if(cell == _previousCell)
                return;
            
            if (_buildAction.CanBuild(cell))
            {
                _ghostUI.gameObject.SetActive(true);
                _ghostUI.transform.position = cell.transform.position;
                _highlighter.SetAsHighlighted(_buildAction.CanBuild);
            }
            else
            {
                _ghostUI.gameObject.SetActive(false);
            }

            _previousCell = cell;
        }
        
        
        private void HandlePointerAboveEntity(GridEntity entity)
        {
            if(entity.Cell == _previousCell)
                return;
            
            _highlighter.SetAsHighlighted(_buildAction.CanBuild);
            _ghostUI.gameObject.SetActive(false);
            _previousCell = entity.Cell;
        }

        private void HandlePointerAboveNull()
        {
            _ghostUI.gameObject.SetActive(false);
        }
    }

ConfirmBuildState.cs

 public class ConfirmBuildState : State
    {
        private readonly IInputReader _inputReader;
        private readonly IFilter<ISelectable> _selectionFilter;
        private readonly IListener<ISelectable> _listener;
        private readonly BuildAction _buildAction;
        private readonly EntityCallbacks _onStartInput;
        
        public ConfirmBuildState(BuildAction buildAction, IInputReader inputReader, IListener<ISelectable> listener, EntityCallbacks onStart)
        {
            _inputReader = inputReader;
            _listener = listener;
            _selectionFilter = _selectionFilter = new SelectionFilter<ISelectable>(listener.Listen, listener.ListenAll, HandleNullSelected, HandleEntitySelected, HandleCellSelected, HandleButtonSelected);
            _buildAction = buildAction;
            _onStartInput = onStart;
        }

        public override void Enter()
        {
            _onStartInput.InvokeAll(_buildAction.Entity, action: _buildAction);
            base.Enter();
        }

        protected override void Update()
        {
            _selectionFilter.Filter(_inputReader.Primary());
        }
        
        private void HandleCellSelected(Cell cell)
        {
            HandleNullSelected();   
        }
        
        private void HandleEntitySelected(GridEntity entity)
        {
            if (SelectedSelf())
            {
                _buildAction.Close();
                var nextAction = _buildAction.Entity.GetNextAction();
                nextState = nextAction == _buildAction ? null : nextAction.Get();
                Exit();
            }
            else
            {
                HandleNullSelected();
            }
            

            bool SelectedSelf() => entity == _buildAction.Entity;
        }
        
        private void HandleButtonSelected(GridButton gridButton)
        {
            if ((gridButton is EntityButton entityButton))
            {
                if(!entityButton.Active)
                    return;
                
                _buildAction.Close();

                nextState = new ConfirmBuildPlacementState(_inputReader, _listener, _selectionFilter, _buildAction, entityButton.entity);
                
                Exit();
                return;
            }
   
            HandleNullSelected();
        }
        
        private void HandleNullSelected()
        {
            _buildAction.Entity.Deselect();
            _buildAction.Close();
            nextState = null;
            Exit();
        }
    }