The source code

    /**
     * @class Ext.grid.RowSelectionModel
     * @extends Ext.grid.AbstractSelectionModel
     * The default SelectionModel used by {@link Ext.grid.GridPanel}.
     * It supports multiple selections and keyboard selection/navigation. The objects stored
     * as selections and returned by {@link #getSelected}, and {@link #getSelections} are
     * the {@link Ext.data.Record Record}s which provide the data for the selected rows.
     * @constructor
     * @param {Object} config
     */
    Ext.grid.RowSelectionModel = function(config){
        Ext.apply(this, config);
        this.selections = new Ext.util.MixedCollection(false, function(o){
            return o.id;
        });

        this.last = false;
        this.lastActive = false;

        this.addEvents(
            /**
             * @event selectionchange
             * Fires when the selection changes
             * @param {SelectionModel} this
             */
            "selectionchange",
            /**
             * @event beforerowselect
             * Fires before a row is selected, return false to cancel the selection.
             * @param {SelectionModel} this
             * @param {Number} rowIndex The index to be selected
             * @param {Boolean} keepExisting False if other selections will be cleared
             * @param {Record} record The record to be selected
             */
            "beforerowselect",
            /**
             * @event rowselect
             * Fires when a row is selected.
             * @param {SelectionModel} this
             * @param {Number} rowIndex The selected index
             * @param {Ext.data.Record} r The selected record
             */
            "rowselect",
            /**
             * @event rowdeselect
             * Fires when a row is deselected.  To prevent deselection
             * {@link Ext.grid.AbstractSelectionModel#lock lock the selections}.
             * @param {SelectionModel} this
             * @param {Number} rowIndex
             * @param {Record} record
             */
            "rowdeselect"
        );

        Ext.grid.RowSelectionModel.superclass.constructor.call(this);
    };

    Ext.extend(Ext.grid.RowSelectionModel, Ext.grid.AbstractSelectionModel,  {
        /**
         * @cfg {Boolean} singleSelect
         * true to allow selection of only one row at a time (defaults to false
         * allowing multiple selections)
         */
        singleSelect : false,

        /**
         * @cfg {Boolean} moveEditorOnEnter
         * false to turn off moving the editor to the next row down when the enter key is pressed
         * or the next row up when shift + enter keys are pressed.
         */
        // private
        initEvents : function(){

            if(!this.grid.enableDragDrop && !this.grid.enableDrag){
                this.grid.on("rowmousedown", this.handleMouseDown, this);
            }

            this.rowNav = new Ext.KeyNav(this.grid.getGridEl(), {
                "up" : function(e){
                    if(!e.shiftKey || this.singleSelect){
                        this.selectPrevious(false);
                    }else if(this.last !== false && this.lastActive !== false){
                        var last = this.last;
                        this.selectRange(this.last,  this.lastActive-1);
                        this.grid.getView().focusRow(this.lastActive);
                        if(last !== false){
                            this.last = last;
                        }
                    }else{
                        this.selectFirstRow();
                    }
                },
                "down" : function(e){
                    if(!e.shiftKey || this.singleSelect){
                        this.selectNext(false);
                    }else if(this.last !== false && this.lastActive !== false){
                        var last = this.last;
                        this.selectRange(this.last,  this.lastActive+1);
                        this.grid.getView().focusRow(this.lastActive);
                        if(last !== false){
                            this.last = last;
                        }
                    }else{
                        this.selectFirstRow();
                    }
                },
                scope: this
            });

            var view = this.grid.view;
            view.on("refresh", this.onRefresh, this);
            view.on("rowupdated", this.onRowUpdated, this);
            view.on("rowremoved", this.onRemove, this);
        },

        // private
        onRefresh : function(){
            var ds = this.grid.store, index;
            var s = this.getSelections();
            this.clearSelections(true);
            for(var i = 0, len = s.length; i < len; i++){
                var r = s[i];
                if((index = ds.indexOfId(r.id)) != -1){
                    this.selectRow(index, true);
                }
            }
            if(s.length != this.selections.getCount()){
                this.fireEvent("selectionchange", this);
            }
        },

        // private
        onRemove : function(v, index, r){
            if(this.selections.remove(r) !== false){
                this.fireEvent('selectionchange', this);
            }
        },

        // private
        onRowUpdated : function(v, index, r){
            if(this.isSelected(r)){
                v.onRowSelect(index);
            }
        },

        /**
         * Select records.
         * @param {Array} records The records to select
         * @param {Boolean} keepExisting (optional) true to keep existing selections
         */
        selectRecords : function(records, keepExisting){
            if(!keepExisting){
                this.clearSelections();
            }
            var ds = this.grid.store;
            for(var i = 0, len = records.length; i < len; i++){
                this.selectRow(ds.indexOf(records[i]), true);
            }
        },

        /**
         * Gets the number of selected rows.
         * @return {Number}
         */
        getCount : function(){
            return this.selections.length;
        },

        /**
         * Selects the first row in the grid.
         */
        selectFirstRow : function(){
            this.selectRow(0);
        },

        /**
         * Select the last row.
         * @param {Boolean} keepExisting (optional) true to keep existing selections
         */
        selectLastRow : function(keepExisting){
            this.selectRow(this.grid.store.getCount() - 1, keepExisting);
        },

        /**
         * Selects the row immediately following the last selected row.
         * @param {Boolean} keepExisting (optional) true to keep existing selections
         * @return {Boolean} true if there is a next row, else false
         */
        selectNext : function(keepExisting){
            if(this.hasNext()){
                this.selectRow(this.last+1, keepExisting);
                this.grid.getView().focusRow(this.last);
                return true;
            }
            return false;
        },

        /**
         * Selects the row that precedes the last selected row.
         * @param {Boolean} keepExisting (optional) true to keep existing selections
         * @return {Boolean} true if there is a previous row, else false
         */
        selectPrevious : function(keepExisting){
            if(this.hasPrevious()){
                this.selectRow(this.last-1, keepExisting);
                this.grid.getView().focusRow(this.last);
                return true;
            }
            return false;
        },

        /**
         * Returns true if there is a next record to select
         * @return {Boolean}
         */
        hasNext : function(){
            return this.last !== false && (this.last+1) < this.grid.store.getCount();
        },

        /**
         * Returns true if there is a previous record to select
         * @return {Boolean}
         */
        hasPrevious : function(){
            return !!this.last;
        },

        /**
         * Returns the selected records
         * @return {Array} Array of selected records
         */
        getSelections : function(){
            return [].concat(this.selections.items);
        },

        /**
         * Returns the first selected record.
         * @return {Record}
         */
        getSelected : function(){
            return this.selections.itemAt(0);
        },

        /**
         * Calls the passed function with each selection. If the function returns
         * false, iteration is stopped and this function returns
         * false. Otherwise it returns true.
         * @param {Function} fn
         * @param {Object} scope (optional)
         * @return {Boolean} true if all selections were iterated
         */
        each : function(fn, scope){
            var s = this.getSelections();
            for(var i = 0, len = s.length; i < len; i++){
                if(fn.call(scope || this, s[i], i) === false){
                    return false;
                }
            }
            return true;
        },

        /**
         * Clears all selections if the selection model
         * {@link Ext.grid.AbstractSelectionModel#isLocked is not locked}.
         * @param {Boolean} fast (optional) true to bypass the
         * conditional checks and events described in {@link #deselectRow}.
         */
        clearSelections : function(fast){
            if(this.isLocked()){
                return;
            }
            if(fast !== true){
                var ds = this.grid.store;
                var s = this.selections;
                s.each(function(r){
                    this.deselectRow(ds.indexOfId(r.id));
                }, this);
                s.clear();
            }else{
                this.selections.clear();
            }
            this.last = false;
        },

        /**
         * Selects all rows if the selection model
         * {@link Ext.grid.AbstractSelectionModel#isLocked is not locked}.
         */
        selectAll : function(){
            if(this.isLocked()){
                return;
            }
            this.selections.clear();
            for(var i = 0, len = this.grid.store.getCount(); i < len; i++){
                this.selectRow(i, true);
            }
        },

        /**
         * Returns true if there is a selection.
         * @return {Boolean}
         */
        hasSelection : function(){
            return this.selections.length > 0;
        },

        /**
         * Returns true if the specified row is selected.
         * @param {Number/Record} index The record or index of the record to check
         * @return {Boolean}
         */
        isSelected : function(index){
            var r = Ext.isNumber(index) ? this.grid.store.getAt(index) : index;
            return (r && this.selections.key(r.id) ? true : false);
        },

        /**
         * Returns true if the specified record id is selected.
         * @param {String} id The id of record to check
         * @return {Boolean}
         */
        isIdSelected : function(id){
            return (this.selections.key(id) ? true : false);
        },

        // private
        handleMouseDown : function(g, rowIndex, e){
            if(e.button !== 0 || this.isLocked()){
                return;
            }
            var view = this.grid.getView();
            if(e.shiftKey && !this.singleSelect && this.last !== false){
                var last = this.last;
                this.selectRange(last, rowIndex, e.ctrlKey);
                this.last = last; // reset the last
                view.focusRow(rowIndex);
            }else{
                var isSelected = this.isSelected(rowIndex);
                if(e.ctrlKey && isSelected){
                    this.deselectRow(rowIndex);
                }else if(!isSelected || this.getCount() > 1){
                    this.selectRow(rowIndex, e.ctrlKey || e.shiftKey);
                    view.focusRow(rowIndex);
                }
            }
        },

        /**
         * Selects multiple rows.
         * @param {Array} rows Array of the indexes of the row to select
         * @param {Boolean} keepExisting (optional) true to keep
         * existing selections (defaults to false)
         */
        selectRows : function(rows, keepExisting){
            if(!keepExisting){
                this.clearSelections();
            }
            for(var i = 0, len = rows.length; i < len; i++){
                this.selectRow(rows[i], true);
            }
        },

        /**
         * Selects a range of rows if the selection model
         * {@link Ext.grid.AbstractSelectionModel#isLocked is not locked}.
         * All rows in between startRow and endRow are also selected.
         * @param {Number} startRow The index of the first row in the range
         * @param {Number} endRow The index of the last row in the range
         * @param {Boolean} keepExisting (optional) True to retain existing selections
         */
        selectRange : function(startRow, endRow, keepExisting){
            var i;
            if(this.isLocked()){
                return;
            }
            if(!keepExisting){
                this.clearSelections();
            }
            if(startRow <= endRow){
                for(i = startRow; i <= endRow; i++){
                    this.selectRow(i, true);
                }
            }else{
                for(i = startRow; i >= endRow; i--){
                    this.selectRow(i, true);
                }
            }
        },

        /**
         * Deselects a range of rows if the selection model
         * {@link Ext.grid.AbstractSelectionModel#isLocked is not locked}.
         * All rows in between startRow and endRow are also deselected.
         * @param {Number} startRow The index of the first row in the range
         * @param {Number} endRow The index of the last row in the range
         */
        deselectRange : function(startRow, endRow, preventViewNotify){
            if(this.isLocked()){
                return;
            }
            for(var i = startRow; i <= endRow; i++){
                this.deselectRow(i, preventViewNotify);
            }
        },

        /**
         * Selects a row.  Before selecting a row, checks if the selection model
         * {@link Ext.grid.AbstractSelectionModel#isLocked is locked} and fires the
         * {@link #beforerowselect} event.  If these checks are satisfied the row
         * will be selected and followed up by  firing the {@link #rowselect} and
         * {@link #selectionchange} events.
         * @param {Number} row The index of the row to select
         * @param {Boolean} keepExisting (optional) true to keep existing selections
         * @param {Boolean} preventViewNotify (optional) Specify true to
         * prevent notifying the view (disables updating the selected appearance)
         */
        selectRow : function(index, keepExisting, preventViewNotify){
            if(this.isLocked() || (index < 0 || index >= this.grid.store.getCount()) || (keepExisting && this.isSelected(index))){
                return;
            }
            var r = this.grid.store.getAt(index);
            if(r && this.fireEvent("beforerowselect", this, index, keepExisting, r) !== false){
                if(!keepExisting || this.singleSelect){
                    this.clearSelections();
                }
                this.selections.add(r);
                this.last = this.lastActive = index;
                if(!preventViewNotify){
                    this.grid.getView().onRowSelect(index);
                }
                this.fireEvent("rowselect", this, index, r);
                this.fireEvent("selectionchange", this);
            }
        },

        /**
         * Deselects a row.  Before deselecting a row, checks if the selection model
         * {@link Ext.grid.AbstractSelectionModel#isLocked is locked}.
         * If this check is satisfied the row will be deselected and followed up by
         * firing the {@link #rowdeselect} and {@link #selectionchange} events.
         * @param {Number} row The index of the row to deselect
         * @param {Boolean} preventViewNotify (optional) Specify true to
         * prevent notifying the view (disables updating the selected appearance)
         */
        deselectRow : function(index, preventViewNotify){
            if(this.isLocked()){
                return;
            }
            if(this.last == index){
                this.last = false;
            }
            if(this.lastActive == index){
                this.lastActive = false;
            }
            var r = this.grid.store.getAt(index);
            if(r){
                this.selections.remove(r);
                if(!preventViewNotify){
                    this.grid.getView().onRowDeselect(index);
                }
                this.fireEvent("rowdeselect", this, index, r);
                this.fireEvent("selectionchange", this);
            }
        },

        // private
        restoreLast : function(){
            if(this._last){
                this.last = this._last;
            }
        },

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

        // private
        onEditorKey : function(field, e){
            var k = e.getKey(), newCell, g = this.grid, ed = g.activeEditor;
            var shift = e.shiftKey;
            if(k == e.TAB){
                e.stopEvent();
                ed.completeEdit();
                if(shift){
                    newCell = g.walkCells(ed.row, ed.col-1, -1, this.acceptsNav, this);
                }else{
                    newCell = g.walkCells(ed.row, ed.col+1, 1, this.acceptsNav, this);
                }
            }else if(k == e.ENTER){
                e.stopEvent();
                ed.completeEdit();
                if(this.moveEditorOnEnter !== false){
                    if(shift){
                        newCell = g.walkCells(ed.row - 1, ed.col, -1, this.acceptsNav, this);
                    }else{
                        newCell = g.walkCells(ed.row + 1, ed.col, 1, this.acceptsNav, this);
                    }
                }
            }else if(k == e.ESC){
                ed.cancelEdit();
            }
            if(newCell){
                g.startEditing(newCell[0], newCell[1]);
            }
        },

        destroy: function(){
            if(this.rowNav){
                this.rowNav.disable();
                this.rowNav = null;
            }
            Ext.grid.RowSelectionModel.superclass.destroy.call(this);
        }
    });