feat($compile): allow directives to modify interpolated attributes

A directive can now set/update/remove attribute values even those containing
interpolation during the compile phase and have the new value be picked up
during the compilation.

For example in template:

<div replace-directive some-attr-or-directive="{{originalInterpolationValue}}"></div>

the replace-directive can now replace the value of some-attr-or-directive during compilation
which produces this intermitent template:

<div replace-directive some-attr-or-directive="{{replacedInterpolationValue}}"></div>

or even

<div replace-directive some-attr-or-directive="replacedStaticValue"></div>

as well as

<div replace-directive some-attr-or-directive></div>

src/ng/compile.js

@@ -1102,11 +1102,13 @@ function $CompileProvider($provide) {
         compile: valueFn(function attrInterpolateLinkFn(scope, element, attr) {
           var $$observers = (attr.$$observers || (attr.$$observers = {}));
 
-          if (name === 'class') {
-            // we need to interpolate classes again, in the case the element was replaced
-            // and therefore the two class attrs got merged - we want to interpolate the result
-            interpolateFn = $interpolate(attr[name], true);
-          }
+          // we need to interpolate again, in case the attribute value has been updated
+          // (e.g. by another directive's compile function)
+          interpolateFn = $interpolate(attr[name], true);
+
+          // if attribute was updated so that there is no interpolation going on we don't want to
+          // register any observers
+          if (!interpolateFn) return;
 
           attr[name] = interpolateFn(scope);
           ($$observers[name] || ($$observers[name] = [])).$$inter = true;
@@ -1203,7 +1205,7 @@ function directiveNormalize(name) {
  * @param {string} name Normalized element attribute name of the property to modify. The name is
  *          revers translated using the {@link ng.$compile.directive.Attributes#$attr $attr}
  *          property to the original name.
- * @param {string} value Value to set the attribute to.
+ * @param {string} value Value to set the attribute to. The value can be an interpolated string.
  */
 
 

test/ng/compileSpec.js

@@ -1503,6 +1503,12 @@ describe('$compile', function() {
           expect(attr.$observe('someAttr', observeSpy)).toBe(observeSpy);
         };
       });
+      directive('replaceSomeAttr', valueFn({
+        compile: function(element, attr) {
+          attr.$set('someAttr', 'bar-{{1+1}}');
+          expect(element).toBe(attr.$$element);
+        }
+      }));
     }));
 
 
@@ -1544,6 +1550,14 @@ describe('$compile', function() {
     }));
 
 
+    it('should allow directive to replace interpolated attributes before attr interpolation compilation', inject(
+        function($compile, $rootScope) {
+      element = $compile('<div some-attr="foo-{{1+1}}" replace-some-attr></div>')($rootScope);
+      $rootScope.$digest();
+      expect(element.attr('some-attr')).toEqual('bar-2');
+    }));
+
+
     it('should call observer of non-interpolated attr through $evalAsync',
       inject(function($rootScope, $compile) {
         $compile('<div some-attr="nonBound" observer></div>')($rootScope);