Angular $scope.$apply vs $timeout as a safe $apply

Question

I'm trying to better understand the nuances of using the $timeout service in Angular as a sort of "safe $apply" method. Basically in scenarios where a piece of code could run in response to either an Angular event or a non-angular event such as jQuery or some standard DOM event.

As I understand things:

1. Wrapping code in $scope.$apply works fine for scenarios where you aren't already in a digest loop (aka. jQuery event) but will raise an error if a digest is in progress
2. Wrapping code in a $timeout() call with no delay parameter works whether already in a digest cycle or not

Looking at Angular source code, it looks like $timeout makes a call to $rootScope.$apply().

1. Why doesn't $timeout() also raise an error if a digest cycle is already in progress?
2. Is the best practice to use $scope.$apply() when you know for sure that a digest won't already be in progress and $timeout() when needing it to be safe either way?
3. Is $timeout() really an acceptable "safe apply", or are there gotchas?

Thanks for any insight.

Answer 1

Looking at Angular source code, it looks like $timeout makes a call to $rootScope.$apply().

• Why doesn't $timeout() also raise an error if a digest cycle is already in progress?

$timeout makes use of an undocumented Angular service $browser. Specifically it uses $browser.defer() that defers execution of your function asynchronously via window.setTimeout(fn, delay), which will always run outside of Angular life-cycle. Only once window.setTimeout has fired your function will $timeout call $rootScope.$apply().

• Is the best practice to use $scope.$apply() when you know for sure that a digest won't already be in progress and $timeout() when needing it to be safe either way?

I would say so. Another use case is that sometimes you need to access a $scope variable that you know will only be initialized after digest. Simple example would be if you want to set a form's state to dirty inside your controller constructor (for whatever reason). Without $timeout the FormController has not been initialized and published onto $scope, so wrapping $scope.yourform.setDirty() inside $timeout ensures that FormController has been initialized. Sure you can do all this with a directive without $timeout, just giving another use case example.

• Is $timeout() really an acceptable "safe apply", or are there gotchas?

It should always be safe, but your go to method should always aim for $apply() in my opinion. The current Angular app I'm working on is fairly large and we've only had to rely on $timeout once instead of $apply().