    /**
     * A selection model that renders a column of checkboxes that can be toggled to
     * select or deselect rows. The default mode for this selection model is MULTI.
     *
     * The selection model will inject a header for the checkboxes in the first view
     * and according to the 'injectCheckbox' configuration.
     */
    Ext.define('Ext.selection.CheckboxModel', {
        alias: 'selection.checkboxmodel',
        extend: 'Ext.selection.RowModel',

        /**
         * @cfg {String} mode
         * Modes of selection.
         * Valid values are SINGLE, SIMPLE, and MULTI.
         */
        mode: 'MULTI',

        /**
         * @cfg {Number/String} [injectCheckbox=0]
         * The index at which to insert the checkbox column.
         * Supported values are a numeric index, and the strings 'first' and 'last'.
         */
        injectCheckbox: 0,

        /**
         * @cfg {Boolean} checkOnly
         * True if rows can only be selected by clicking on the checkbox column.
         */
        checkOnly: false,

        /**
         * @cfg {Boolean} showHeaderCheckbox
         * Configure as `false` to not display the header checkbox at the top of the column.
         */
        showHeaderCheckbox: true,

        headerWidth: 24,

        // private
        checkerOnCls: Ext.baseCSSPrefix + 'grid-hd-checker-on',

        bindComponent: function(view) {
            var me = this;

            me.sortable = false;
            me.callParent(arguments);

            // if we have a locked header, only hook up to the first
            if (!me.hasLockedHeader() || view.headerCt.lockedCt) {
                if (me.showHeaderCheckbox !== false) {
                    view.headerCt.on('headerclick', me.onHeaderClick, me);
                }
                me.addCheckbox(true);
                me.mon(view.ownerCt, 'reconfigure', me.onReconfigure, me);
            }
        },

        hasLockedHeader: function(){
            var views     = this.views,
                vLen      = views.length,
                v;

            for (v = 0; v < vLen; v++) {
                if (views[v].headerCt.lockedCt) {
                    return true;
                }
            }
            return false;
        },

        /**
         * Add the header checkbox to the header row
         * @private
         * @param {Boolean} initial True if we're binding for the first time.
         */
        addCheckbox: function(initial){
            var me = this,
                checkbox = me.injectCheckbox,
                view = me.views[0],
                headerCt = view.headerCt;

            // Preserve behaviour of false, but not clear why that would ever be done.
            if (checkbox !== false) {
                if (checkbox == 'first') {
                    checkbox = 0;
                } else if (checkbox == 'last') {
                    checkbox = headerCt.getColumnCount();
                }
                Ext.suspendLayouts();
                headerCt.add(checkbox,  me.getHeaderConfig());
                Ext.resumeLayouts();
            }

            if (initial !== true) {
                view.refresh();
            }
        },

        /**
         * Handles the grid's reconfigure event.  Adds the checkbox header if the columns have been reconfigured.
         * @private
         * @param {Ext.panel.Table} grid
         * @param {Ext.data.Store} store
         * @param {Object[]} columns
         */
        onReconfigure: function(grid, store, columns) {
            if(columns) {
                this.addCheckbox();
            }
        },

        /**
         * Toggle the ui header between checked and unchecked state.
         * @param {Boolean} isChecked
         * @private
         */
        toggleUiHeader: function(isChecked) {
            var view     = this.views[0],
                headerCt = view.headerCt,
                checkHd  = headerCt.child('gridcolumn[isCheckerHd]');

            if (checkHd) {
                if (isChecked) {
                    checkHd.el.addCls(this.checkerOnCls);
                } else {
                    checkHd.el.removeCls(this.checkerOnCls);
                }
            }
        },

        /**
         * Toggle between selecting all and deselecting all when clicking on
         * a checkbox header.
         */
        onHeaderClick: function(headerCt, header, e) {
            if (header.isCheckerHd) {
                e.stopEvent();
                var me = this,
                    isChecked = header.el.hasCls(Ext.baseCSSPrefix + 'grid-hd-checker-on');

                // Prevent focus changes on the view, since we're selecting/deselecting all records
                me.preventFocus = true;
                if (isChecked) {
                    me.deselectAll();
                } else {
                    me.selectAll();
                }
                delete me.preventFocus;
            }
        },

        /**
         * Retrieve a configuration to be used in a HeaderContainer.
         * This should be used when injectCheckbox is set to false.
         */
        getHeaderConfig: function() {
            var me = this,
                showCheck = me.showHeaderCheckbox !== false;

            return {
                isCheckerHd: showCheck,
                text : '&#160;',
                width: me.headerWidth,
                sortable: false,
                draggable: false,
                resizable: false,
                hideable: false,
                menuDisabled: true,
                dataIndex: '',
                cls: showCheck ? Ext.baseCSSPrefix + 'column-header-checkbox ' : '',
                renderer: Ext.Function.bind(me.renderer, me),
                editRenderer: me.editRenderer || me.renderEmpty,
                locked: me.hasLockedHeader()
            };
        },

        renderEmpty: function(){
            return '&#160;';
        },

        /**
         * Generates the HTML to be rendered in the injected checkbox column for each row.
         * Creates the standard checkbox markup by default; can be overridden to provide custom rendering.
         * See {@link Ext.grid.column.Column#renderer} for description of allowed parameters.
         */
        renderer: function(value, metaData, record, rowIndex, colIndex, store, view) {
            var baseCSSPrefix = Ext.baseCSSPrefix;
            metaData.tdCls = baseCSSPrefix + 'grid-cell-special ' + baseCSSPrefix + 'grid-cell-row-checker';
            return '<div class="' + baseCSSPrefix + 'grid-row-checker">&#160;</div>';
        },

        // override
        onRowMouseDown: function(view, record, item, index, e) {
            view.el.focus();
            var me = this,
                checker = e.getTarget('.' + Ext.baseCSSPrefix + 'grid-row-checker'),
                mode;

            if (!me.allowRightMouseSelection(e)) {
                return;
            }

            // checkOnly set, but we didn't click on a checker.
            if (me.checkOnly && !checker) {
                return;
            }

            if (checker) {
                mode = me.getSelectionMode();
                // dont change the mode if its single otherwise
                // we would get multiple selection
                if (mode !== 'SINGLE') {
                    me.setSelectionMode('SIMPLE');
                }
                me.selectWithEvent(record, e);
                me.setSelectionMode(mode);
            } else {
                me.selectWithEvent(record, e);
            }
        },

        /**
         * Synchronize header checker value as selection changes.
         * @private
         */
        onSelectChange: function() {
            this.callParent(arguments);

            // check to see if all records are selected
            var hdSelectStatus = this.selected.getCount() === this.store.getCount();
            this.toggleUiHeader(hdSelectStatus);
        }
    });