如何访问jqGrid外的元素？

Question

我有一个按钮和一个网格（使用jqGrid），但是当用户单击网格中的复选框时，想要禁用离网格的按钮。我该怎么办？

我正在尝试在LoadComplete中执行此操作。

<table id="Grid"></table>
<div id="GridPager"></div>

<br />

<button id="btnDownload" disabled="disabled">Download Selected</button>

<iframe id="ifr" style="display: none"></iframe>

<script>

    $(document).ready(function () {

        // make jQuery UI button
        $("#btnDownload").button();

        $("#btnDownload").click(function () {
            var selectedIDs = jQuery('#Grid').jqGrid('getGridParam', 'selarrrow');
            $("#ifr").attr("src", "/Handler/ExportInvoiceHandler.ashx?selectedIDs=" + selectedIDs);
        });

        $("#Grid").jqGrid({
            url: '/Handler/InvoiceHandler.ashx',
            datatype: 'json',
            autowidth: true,
            height: "100%",
            emptyRecords: "No records found",
            colNames: ['PO', 'SKU', 'Status', 'Date', ''],
            colModel: [
                    { name: 'PO', width: 80, sortable: true, stype: 'string', resizable: false },
                    { name: 'SKU', width: 200, sortable: true, stype: 'string', resizable: false },
                    { name: 'Status', width: 100, sortable: true, stype: 'string', resizable: false },
                    {
                        name: 'Date', width: 80, sorttype: "date", sortable: true, align: 'center', resizable: false,
                        searchoptions: {
                            dataInit: function (element) {
                                $(element).datepicker({ dateFormat: 'mm/dd/yy' })
                            }
                        }
                    },
                    { name: 'Id', width: 50, sortable: false, search: false, index: 'Id', editable: true, formatter: returnHyperLink, align: 'center', resizable: false },
            ],
            loadComplete: function () {
                $("input[type=checkbox]").click(function () {
                    var chkSelected = $("#Grid input[type='checkbox']:checked").length;
                    var btn = $("#btnDownload");
                    alert(btn);
                    if (chkSelected != 0) {
                        btn.removeAttr('disabled');
                    }
                    else {
                        btn.attr('disabled', 'disabled');
                    }
                });
            },
            rowNum: 10,
            rowList: [10, 20, 30],
            pager: '#GridPager',
            multiselect: true,
            sortname: 'PO',
            viewrecords: true,
            sortorder: 'asc',
            loadonce: true
        }).navGrid('#GridPager', { add: false, edit: false, del: false, refresh: false, search: true, refresh: true }, {}, {}, {}, { multipleSearch: true }, { closeOnEscape: true });

    });

    function returnHyperLink(cellValue, options, rowdata, action) {
        return "<a href='/Handler/ExportInvoiceHandler.ashx?Id=" + options.rowId + "' target='ifr'>Download</a>";
    }

</script>