How to know if all checkbox of jstree is checked or not

I am using jstree with checkbox.

I have another checkbox with Id chkSelectAll (Select All). When user select it, All jstree checkboxes are checked and if we unselect it, all jstree checkboex are unchecked using below code:

  $('#chkSelectAll').change(function() {          
       if ($('#chkSelectAll').is(":checked"))
       { 
           $("#drpDownSource .source-list").jstree().check_all(true);
       }
       else
       {
           $("#drpDownSource .source-list").jstree().uncheck_all(true);
       }
     }); 

Now if all jstree checkboxes are selected manually then I want to check chkSelectAll checkbox and if any one jstree checkbox is unchecked then I want chkSelectAll to be unchecked. I am using below code:

So How can I know whether all jstree checkbox are checked or not?

.on("check_node.jstree uncheck_node.jstree", function (e, data) {            
        debugger;
        if (e.type == "uncheck_node") {
            $("#chkSelectAll").prop( "checked", false );                
        }
        else if (e.type == "check_node") {
            // here I get only one checkbox's status.
            // How to check all checkboxe's status
        }
    }); 

Thanks