    /**
     * @class Ext.grid.CellSelectionModel
     * @extends Ext.grid.AbstractSelectionModel
     * This class provides the basic implementation for single cell selection in a grid.
     * The object stored as the selection contains the following properties:
     *
     * cell : see {@link #getSelectedCell}
     * record : Ext.data.record The {@link Ext.data.Record Record}
     * which provides the data for the row containing the selection
     *
     * @constructor
     * @param {Object} config The object containing the configuration of this model.
     */
    Ext.grid.CellSelectionModel = Ext.extend(Ext.grid.AbstractSelectionModel,  {

        constructor : function(config){
            Ext.apply(this, config);

    	    this.selection = null;

    	    this.addEvents(
    	        /**
    	         * @event beforecellselect
    	         * Fires before a cell is selected, return false to cancel the selection.
    	         * @param {SelectionModel} this
    	         * @param {Number} rowIndex The selected row index
    	         * @param {Number} colIndex The selected cell index
    	         */
    	        "beforecellselect",
    	        /**
    	         * @event cellselect
    	         * Fires when a cell is selected.
    	         * @param {SelectionModel} this
    	         * @param {Number} rowIndex The selected row index
    	         * @param {Number} colIndex The selected cell index
    	         */
    	        "cellselect",
    	        /**
    	         * @event selectionchange
    	         * Fires when the active selection changes.
    	         * @param {SelectionModel} this
    	         * @param {Object} selection null for no selection or an object with two properties
    	         *
    	         * cell : see {@link #getSelectedCell}
    	         * record : Ext.data.recordThe {@link Ext.data.Record Record}
    	         * which provides the data for the row containing the selection
    	         *
    	         */
    	        "selectionchange"
    	    );

    	    Ext.grid.CellSelectionModel.superclass.constructor.call(this);
        },

        /** @ignore */
        initEvents : function(){
            this.grid.on('cellmousedown', this.handleMouseDown, this);
            this.grid.on(Ext.EventManager.useKeydown ? 'keydown' : 'keypress', this.handleKeyDown, this);
            this.grid.getView().on({
                scope: this,
                refresh: this.onViewChange,
                rowupdated: this.onRowUpdated,
                beforerowremoved: this.clearSelections,
                beforerowsinserted: this.clearSelections
            });
            if(this.grid.isEditor){
                this.grid.on('beforeedit', this.beforeEdit,  this);
            }
        },

    	//private
        beforeEdit : function(e){
            this.select(e.row, e.column, false, true, e.record);
        },

    	//private
        onRowUpdated : function(v, index, r){
            if(this.selection && this.selection.record == r){
                v.onCellSelect(index, this.selection.cell[1]);
            }
        },

    	//private
        onViewChange : function(){
            this.clearSelections(true);
        },

    	/**
         * Returns an array containing the row and column indexes of the currently selected cell
         * (e.g., [0, 0]), or null if none selected. The array has elements:
         *
         * rowIndex : NumberThe index of the selected row
         * cellIndex : NumberThe index of the selected cell.
         * Due to possible column reordering, the cellIndex should not be used as an
         * index into the Record's data. Instead, use the cellIndex to determine the name
         * of the selected cell and use the field name to retrieve the data value from the record:
    // get name
    var fieldName = grid.getColumnModel().getDataIndex(cellIndex);
    // get data value based on name
    var data = record.get(fieldName);
         *
         *
         * @return {Array} An array containing the row and column indexes of the selected cell, or null if none selected.
    	 */
        getSelectedCell : function(){
            return this.selection ? this.selection.cell : null;
        },

        /**
         * If anything is selected, clears all selections and fires the selectionchange event.
         * @param {Boolean} preventNotify true to prevent the gridview from
         * being notified about the change.
         */
        clearSelections : function(preventNotify){
            var s = this.selection;
            if(s){
                if(preventNotify !== true){
                    this.grid.view.onCellDeselect(s.cell[0], s.cell[1]);
                }
                this.selection = null;
                this.fireEvent("selectionchange", this, null);
            }
        },

        /**
         * Returns true if there is a selection.
         * @return {Boolean}
         */
        hasSelection : function(){
            return this.selection ? true : false;
        },

        /** @ignore */
        handleMouseDown : function(g, row, cell, e){
            if(e.button !== 0 || this.isLocked()){
                return;
            }
            this.select(row, cell);
        },

        /**
         * Selects a cell.  Before selecting a cell, fires the
         * {@link #beforecellselect} event.  If this check is satisfied the cell
         * will be selected and followed up by  firing the {@link #cellselect} and
         * {@link #selectionchange} events.
         * @param {Number} rowIndex The index of the row to select
         * @param {Number} colIndex The index of the column to select
         * @param {Boolean} preventViewNotify (optional) Specify true to
         * prevent notifying the view (disables updating the selected appearance)
         * @param {Boolean} preventFocus (optional) Whether to prevent the cell at
         * the specified rowIndex / colIndex from being focused.
         * @param {Ext.data.Record} r (optional) The record to select
         */
        select : function(rowIndex, colIndex, preventViewNotify, preventFocus, /*internal*/ r){
            if(this.fireEvent("beforecellselect", this, rowIndex, colIndex) !== false){
                this.clearSelections();
                r = r || this.grid.store.getAt(rowIndex);
                this.selection = {
                    record : r,
                    cell : [rowIndex, colIndex]
                };
                if(!preventViewNotify){
                    var v = this.grid.getView();
                    v.onCellSelect(rowIndex, colIndex);
                    if(preventFocus !== true){
                        v.focusCell(rowIndex, colIndex);
                    }
                }
                this.fireEvent("cellselect", this, rowIndex, colIndex);
                this.fireEvent("selectionchange", this, this.selection);
            }
        },

    	//private
        isSelectable : function(rowIndex, colIndex, cm){
            return !cm.isHidden(colIndex);
        },

        // private
        onEditorKey: function(field, e){
            if(e.getKey() == e.TAB){
                this.handleKeyDown(e);
            }
        },

        /** @ignore */
        handleKeyDown : function(e){
            if(!e.isNavKeyPress()){
                return;
            }

            var k = e.getKey(),
                g = this.grid,
                s = this.selection,
                sm = this,
                walk = function(row, col, step){
                    return g.walkCells(
                        row,
                        col,
                        step,
                        g.isEditor && g.editing ? sm.acceptsNav : sm.isSelectable, // *** handle tabbing while editorgrid is in edit mode
                        sm
                    );
                },
                cell, newCell, r, c, ae;

            switch(k){
                case e.ESC:
                case e.PAGE_UP:
                case e.PAGE_DOWN:
                    // do nothing
                    break;
                default:
                    // *** call e.stopEvent() only for non ESC, PAGE UP/DOWN KEYS
                    e.stopEvent();
                    break;
            }

            if(!s){
                cell = walk(0, 0, 1); // *** use private walk() function defined above
                if(cell){
                    this.select(cell[0], cell[1]);
                }
                return;
            }

            cell = s.cell;  // currently selected cell
            r = cell[0];    // current row
            c = cell[1];    // current column

            switch(k){
                case e.TAB:
                    if(e.shiftKey){
                        newCell = walk(r, c - 1, -1);
                    }else{
                        newCell = walk(r, c + 1, 1);
                    }
                    break;
                case e.DOWN:
                    newCell = walk(r + 1, c, 1);
                    break;
                case e.UP:
                    newCell = walk(r - 1, c, -1);
                    break;
                case e.RIGHT:
                    newCell = walk(r, c + 1, 1);
                    break;
                case e.LEFT:
                    newCell = walk(r, c - 1, -1);
                    break;
                case e.ENTER:
                    if (g.isEditor && !g.editing) {
                        g.startEditing(r, c);
                        return;
                    }
                    break;
            }

            if(newCell){
                // *** reassign r & c variables to newly-selected cell's row and column
                r = newCell[0];
                c = newCell[1];

                this.select(r, c); // *** highlight newly-selected cell and update selection

                if(g.isEditor && g.editing){ // *** handle tabbing while editorgrid is in edit mode
                    ae = g.activeEditor;
                    if(ae && ae.field.triggerBlur){
                        // *** if activeEditor is a TriggerField, explicitly call its triggerBlur() method
                        ae.field.triggerBlur();
                    }
                    g.startEditing(r, c);
                }
            }
        },

        acceptsNav : function(row, col, cm){
            return !cm.isHidden(col) && cm.isCellEditable(col, row);
        }
    });