fix(stateDirectives): Generate the proper URL when the state changes

[frapontillo]

As of now, the generated URL changes if and only if its parameters do. So something
like ui-sref="{{myState}}({id: myId})" would simply work for the first time. When
$scope.myState changes and $scope.myId stays the same, the URL is not regenerated.
This commit fixes this by observing the attribute and regenerating the URL if it changes.
I have also added a test to make sure everything works.
This commit may also be also be relevant for #371, as it allows for empty state values ("")
to be processed and set into the href attribute. We could bind this parameter to a
directive attribute so the user can decide wether to accept this behavior or not.

[nateabele]

Other people can chime in here, but I don't think this is desired behavior (see my notes on #139). If you really want something fully dynamic, you're free to combine ng-href with $state.href().

Also, your patch alters an existing test, which, unless you're changing (breaking) existing behavior, should never, ever be done.

[ksperling]

@nateabele Not sure which comment on #139 you're referring to exactly...

I'm kind of surprised we're supporting {{}} in the state name at all. I think it should either (1) support embedded expressions, in which case they should be re-evaluated, or (2) not support them at all, in which case {{myState}} should be taken literally as a (weird) state name.

I think dynamically looking up the state name independently of the parameters is weird enough that we don't need to support it, so I'd tend towards (2) at this point.

[nateabele]

Not sure which comment on #139 you're referring to exactly...

@ksperling: "I know there was some mention of a more dynamic way of referencing states. IMO that is out-of-scope for a shorthand syntax. If people want something more advanced, they can combine ng-href with $state.href()."

I'm kind of surprised we're supporting {{}} in the state name at all.

I'm pretty sure we aren't (which is what this patch is about). If we are, it's certainly not intentional. I'm completely in agreement with option (2).

[frapontillo]

Ok, I see your points. Actually, what I'm doing is referencing a single variable (myFullState) that includes both the state name and parameters.

Regardless of this specific PR, I think you should somehow support dynamic state referencing, as it's a common expectation from AngularJS attributes and it would come very useful.

Nevertheless, I can now see it will be very straightforward to implement this behaviour by simply using 'ng-href' and the $state service.

[nateabele]

Yup, as you see, it's already fully supported using existing directives and $state services. :-)