fnDrawCallback doesn't word immediately

hello, everyone. I have a loading spinner, when I use "preDrawCallback" to make it "TRUE", it works fine, but when I want to make it "FALSE" in "preDrawCallback" I have to focus on some input field or change browser tab to make it false;

this is my code :

"preDrawCallback": function (settings) {
        console.log("SSSSSS =====>>>>> preDrawCallback");
        vm.loading = true;
},
"fnDrawCallback": function (oSettings) {
     // this log will show in console 
     console.log("EEEEEE =====>>>>> fnDrawCallback");
     // but this line note work immediately
     vm.loading = false;
},

how to make vm.loading "TRUE" immediately without a focus on elements !!!