watchGroupUpvoteExample.js

var isVotedGroup = [
  'vm.answer.isVotedAgainst(vm.user)',
  'vm.answer.isVotedFor(vm.user)'
];

var unwatch = scope.$watchGroup(isVotedGroup, function(isVotedGroup) {
  var hasVoted = isVotedGroup.some(function(bool) {
    return bool;
  });
  if (hasVoted) {
    vm.answer.disableVoting(vm.user);
    unwatch();
  } 
});