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:
- 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
- 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().
- Why doesn't $timeout() also raise an error if a digest cycle is already in progress?
- 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?
- Is $timeout() really an acceptable "safe apply", or are there gotchas?
Thanks for any insight.
Answer
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().
