jQuery Grid Plugin - jqGrid

version 3.6.1 - Cell editing

my code for the edition:

         cellEdit:true,
         cellsubmit: 'clientArray',
         beforeEditCell: function(rowid,cellname,value,irow,icol){
             alert("ok1");
         },
         afterEditCell: function(rowid,cellname,value,irow,icol){
             alert("ok2");
         }

and the definition of the column editable:

{
            name:'foo',
            index:'foo',
            align:'center',
            editable:true,
            formatter:'checkbox',
            formatoptions: {disabled: false},
            edittype: "checkbox",
            width:80
 },

1) one click in the cell displays ok1, and after the alert-ok, ok2

2) if I try again, ok1 is not displayed ! why ?

3) whatever 1&2, if I click on the checkbox, I never have ok1 ! why ?