$scope.$watch($rootScope.xxx,function(newVal,oldVal){
//do something
})

$rootScope.$watch("xxx",function(newVal,oldVal){
//do something
})

return function deregisterWatch() {
if (arrayRemove(array, watcher) >= 0) {
incrementWatchersCount(scope, -1);
}
lastDirtyWatch = null;
};

var watcher = $rootScope.$watch("xxx",function(){});
//手动清除 watcher 
watcher();

// Disable listeners, watchers and apply/digest methods
this.$destroy = this.$digest = this.$apply = this.$evalAsync = this.$applyAsync = noop;
this.$on = this.$watch = this.$watchGroup = function() { 
return noop; 
};
this.$$listeners = {};