$scope.$watch doesn't see DOM changes done in current $digest cycle, but sees them in next cycle

[olexme]

Specifically it doesn't see changes made for class attribute by ng-class directive.
The issue occures for AngularJS v1.3.7, for Angular 1.2.26 is ok.
Below is easy to analyse fure Angular plunker example, with $watch based on function checking class attribute changes in each digest cycle (or more times per digest).
Steps to reproduce the issue:

1. click checkbox to apply "redClass" style on Foo Paragraph (it becomes red)
2. click "No Operation" button to poke next digest cycle and observe that $watch now "sees" applied redClass
3. switch to Angular 1.2.26 to see the difference in behaviour

http://plnkr.co/edit/sTjGayZnG8ti6X6LgElZ?p=preview

[pkozlowski-opensource]

@olexme putting aside behavior change between 1.2.x and 1.3.x you should never ever watch DOM properties - accessing DOM in the detection function of the $watch is a big perf no-no.

I'm not sure what is your real-life use-case but I don't think we want to investigate / fix the issue you've exposed as this is not a use-case we want to promote in AngularJS. If you could talk about a real-life problem you are trying to solve, maybe someone will be able to provide a more AngularJS-like solution.

[olexme]

Thanks for that comment.
It's something that I also don't like and I'm aware of its ugly solution at least from performance perspective.
That approach was used to deal with 3rd party non-angular component but I know easy workaround.

I added this issue cause I didn't find explicit breaking change note in migration guide

[gkalpak]

Without looking deep into it, it must be a change to ngAnimate in v1.3.0-rc.5, for which there is a BREAKING CHANGE notice in the Changelog:

ngAnimate: defer DOM operations for changing classes to postDigest (667183a, #8234, #9263)

_{So, technically speaking, it is a change that happened in 1.3.x; e.g. v1.3.0-rc.4 does work the same as 1.2.x.}

@olexme: It is always a good idea to check the Changelog for breaking changes when upgrading to a newer version.

[gkalpak]

Hm...looking at it again, it seems that the Changelog notice is somewhat misleading:
It seems to imply that the changes apply to ngAnimate, but actually they apply to Angular's core $animate service used when ngAnimate is not present (which is clearly explained in the relevant commit's message).

[pkozlowski-opensource]

Yeh, OK, it sounds like a documented breaking change. Anyway, as stated above, this is really a corner case with the behaviour that definitively shouldn't be encouraged.