Interesting thing about $watch

I guess everybody knows how to make angular to respond on the changes made to scope variables- to add some callback to "$watch" , something like this:

scope.$watch('name', function(newValue, oldValue) {
  scope.counter = scope.counter + 1;
});

But the cool thing that $watch can listen and respond to changes of functions which return something too. For example:

 $scope.tabs=[{active:true, name:'first'},{active:false, name:'second'},{active:false, name:'third'}}];
 $scope.$watch(function(){ return $scope.tabs.filter(function(t){ return t.active; })[0]; },function(tab){
   $scope.$broadcast('tab-selected',{name:tab.name})
 },true)