When is it safe to use $scope.$apply()?

Question

I guess the title is pretty much clear what I am asking. I have created this fiddle : http://jsfiddle.net/Sourabh_/HB7LU/13142/

In the fiddle I have tried to replica

Answer 1

A better way is to use $scope.$applyAsync(); instead of $scope.$apply();

The reason to avoid use of $scope.$apply() is given here:

Error: [$rootScope:inprog] digest in progress. Fix