Why would '@' binding need attrs.$observe()?

[LLoydsensei]

Hi guys.

API documentation (http://docs.angularjs.org/api/ng.$compile#description_comprehensive-directive-api_attributes) says  "[...] during the linking phase the interpolation hasn't been evaluated yet and so the value is at this time set to undefined."

1) I cannot understand why someone would use attrs.$observe() instead of $scope.$watch(), since both appear to work the same way
2) I don't understand why the documentation talks about the interpolation being undefined during the linking phase when this http://jsfiddle.net/3Vh43/1/ proves the contrary

Did I happen to stumble upon a particular case in my jsfiddle where it works or is the documentation wrong there?

Regards,

LLoyd

PS: Wiki says the same (https://github.com/angular/angular.js/wiki/Understanding-Scopes#-directives)

PPS: Tested on angular 1.1.5 and 1.2.10

[Mike Alsup]

You need to use $observe to watch DOM attributes who's values contain interpolation. 

<span title="Hello {{name}}">blah</span>

attrs.$observe('title', function() {

   ...

});

$watch can not be used for string expressions that contain interpolation.

http://ng.malsup.com/#!/$watch-vs-$observe

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[Vlad GURDIGA]

Perhaps a JSFiddle would help us understand each other better. We had a short conversation with LLoyd about this, so I will try to illustrate the context and what is that we don’t understand. Here is the JSFiddle: http://jsfiddle.net/gurdiga/9KeKM/.

As you will see in your browser console, when you click “Increase magic!” button it increases magicLevel in the parent scope, and also changes the myMagic directive’s incantation attribute that interpolates it. Then, in the link function the incantation attribute is watched for changes with both $scope.$watch and attrs.$observe, which both are triggered, and this is the very phenomenon that LLoyd and I were wondering about: which observation method is appropriate to use?

[Sander Elias]

Hi Lloyd, Vlad,

For using in the link or controller there isn't much difference. However, if you need to do some logic in your compile, there is no scope in there!

You can use attr.$observe in places where the scope isn't there, or the wrong scope. (you need to do something in the link function of an transcluded replaced directive with an isolated scope!)

Regards

Sander

[Vlad GURDIGA]

Hm… that begins to make sense! Thank you Sander!

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[LLoydsensei]

For using in the link or controller there isn't much difference. However, if you need to do some logic in your compile, there is no scope in there!

You can use attr.$observe in places where the scope isn't there, or the wrong scope. (you need to do something in the link function of an transcluded replaced directive with an isolated scope!)

Nice to know I didn't even think of it! So I get why and when we should use $observe instead of $watch.

However, can this still called "the linking phase" as specified in the doc? How could we be more precise in it?

Regards,

LLoyd