    /**
     * @class Ext.grid.AbstractSelectionModel
     * @extends Ext.util.Observable
     * Abstract base class for grid SelectionModels.  It provides the interface that should be
     * implemented by descendant classes.  This class should not be directly instantiated.
     * @constructor
     */
    Ext.grid.AbstractSelectionModel = Ext.extend(Ext.util.Observable,  {
        /**
         * The GridPanel for which this SelectionModel is handling selection. Read-only.
         * @type Object
         * @property grid
         */

        constructor : function(){
            this.locked = false;
            Ext.grid.AbstractSelectionModel.superclass.constructor.call(this);
        },

        /** @ignore Called by the grid automatically. Do not call directly. */
        init : function(grid){
            this.grid = grid;
            if(this.lockOnInit){
                delete this.lockOnInit;
                this.locked = false;
                this.lock();
            }
            this.initEvents();
        },

        /**
         * Locks the selections.
         */
        lock : function(){
            if(!this.locked){
                this.locked = true;
                // If the grid has been set, then the view is already initialized.
                var g = this.grid;
                if(g){
                    g.getView().on({
                        scope: this,
                        beforerefresh: this.sortUnLock,
                        refresh: this.sortLock
                    });
                }else{
                    this.lockOnInit = true;
                }
            }
        },

        // set the lock states before and after a view refresh
        sortLock : function() {
            this.locked = true;
        },

        // set the lock states before and after a view refresh
        sortUnLock : function() {
            this.locked = false;
        },

        /**
         * Unlocks the selections.
         */
        unlock : function(){
            if(this.locked){
                this.locked = false;
                var g = this.grid,
                    gv;

                // If the grid has been set, then the view is already initialized.
                if(g){
                    gv = g.getView();
                    gv.un('beforerefresh', this.sortUnLock, this);
                    gv.un('refresh', this.sortLock, this);
                }else{
                    delete this.lockOnInit;
                }
            }
        },

        /**
         * Returns true if the selections are locked.
         * @return {Boolean}
         */
        isLocked : function(){
            return this.locked;
        },

        destroy: function(){
            this.unlock();
            this.purgeListeners();
        }
    });