feat($compile): add support for $observer deregistration

In order to make the behavior compatible with $rootScope.$watch and $rootScope.$on methods, and
make it possible to deregister an attribute observer, Attributes.$observe method now returns a
deregistration function instead of the observer itself.

BREAKING CHANGE: calling attr.$observe no longer returns the observer function, but a
    deregistration function instead.

    To migrate the code follow the example below:

    Before:

```
    directive('directiveName', function() {
      return {
        link: function(scope, elm, attr) {
          var observer = attr.$observe('someAttr', function(value) {
            console.log(value);
          });
        }
      };
    });
```

    After:

```
    directive('directiveName', function() {
      return {
        link: function(scope, elm, attr) {
          var observer = function(value) {
            console.log(value);
          };

          attr.$observe('someAttr', observer);
        }
      };
    });
```

Closes #5609

1 files changed, 5 insertions, 2 deletions

diff --git a/src/ng/compile.js b/src/ng/compile.js
index c7cd08bc..8cc518d5 100644
--- a/src/ng/compile.js
+++ b/src/ng/compile.js
@@ -775,7 +775,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
        * @param {function(interpolatedValue)} fn Function that will be called whenever
                 the interpolated value of the attribute changes.
        *        See the {@link guide/directive#Attributes Directives} guide for more info.
-       * @returns {function()} the `fn` parameter.
+       * @returns {function()} Returns a deregistration function for this observer.
        */
       $observe: function(key, fn) {
         var attrs = this,
@@ -789,7 +789,10 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
             fn(attrs[key]);
           }
         });
-        return fn;
+
+        return function() {
+          arrayRemove(listeners, fn);
+        };
       }
     };