Registering watch seems to trigger it one too many times

[mhevery]

http://plnkr.co/edit/brHNIEoTQxNQF9eZikgJ?p=preview

It contains a template:
<p>Hello {{log('A')}} {{log('B')}}!</p>

Run the above plunker and open console. I would expect the console to print: A, B and then A, B as second digest runs. Instead I see A, A, B, B and then A, B when second digest runs.

Is this a bug or intended behavior?

[frfancha]

From what I know of angular watchers it should be A, B A, B.
As mhevery, I'm very curious to know why not.

[lgalfaso]

TL;DR
it is a bug, will post a patch in a few

Long version, things happen in the following order:

• There was an overhaul on $interpolate, this overhaul changed $parse and incorporated the concept of an interceptor
• Optimization on $parse landed so expressions that have filters without parameters (or the parameters are constants) would be evaluated in 2 phases, first to evaluate the expression sans the filter evaluation and then with the filter evaluation. This also used interceptors [the second evaluation issue was added here]
• More optimizations on $parse landed and now expressions could be evaluated in 2 phases. One to get all the possible values that could change (lets call this state), the state was checked by $watch to know if an expression changed. The second to continue the evaluation (as long as this state is provided). This, once again, used interceptors

The last change, was supposed to fix the issue, but there was an assumption in the existing code that the code would always generate the 2 phases functions, but that is not true. If the expression is simple enough (just like the one in your case) then the 2-phase evaluations functions are not generated. In this case, if a stateless interceptor was added (just like what $interpolate adds) then the state was not used and you see the function being evaluated twice. This explains why, if you change the expression from Hello {{log('A')}} {{log('B')}}! to Hello {{log('A') + ' ' + log('B')}}!, then the repetition is not there.