feat($compile): add attribute binding support via ngAttr*

Sometimes is not desirable to use interpolation on attributes because
the user agent parses them before the interpolation takes place. I.e:

<svg>
  <circle cx="{{cx}}" cy="{{cy}}" r="{{r}}"></circle>
</svg>

The snippet throws three browser errors, one for each attribute.

For some attributes, AngularJS fixes that behaviour introducing special
directives like ng-href or ng-src.

This commit is a more general solution that allows prefixing any
attribute with "ng-attr-", "ng:attr:" or "ng_attr_"  so it will
be set only when the binding is done. The prefix is then removed.

Example usage:

<svg>
  <circle ng-attr-cx="{{cx}}" ng-attr-cy="{{cy}}" ng:attr-r="{{r}}"></circle>
</svg>

Closes #1050
Closes #1925

docs/content/guide/directive.ngdoc

@@ -53,7 +53,7 @@ the following example.
   </doc:scenario>
 </doc:example>
 
-# String interpolation
+# Text and attribute bindings
 
 During the compilation process the {@link api/ng.$compile compiler} matches text and
 attributes using the {@link api/ng.$interpolate $interpolate} service to see if they
@@ -66,6 +66,31 @@ here:
 <a href="img/{{username}}.jpg">Hello {{username}}!</a>
 </pre>
 
+
+# ngAttr attribute bindings
+
+If an attribute with a binding is prefixed with `ngAttr` prefix (denormalized prefix: 'ng-attr-',
+'ng:attr-') then during the compilation the prefix will be removed and the binding will be applied
+to an unprefixed attribute. This allows binding to attributes that would otherwise be eagerly
+processed by browsers in their uncompilled form (e.g. `img[src]` or svg's `circle[cx]` attributes).
+
+For example, considering template:
+
+    <svg>
+      <circle ng-attr-cx="{{cx}}"></circle>
+    </svg>
+
+and model cx set to 5, will result in rendering this dom:
+
+    <svg>
+      <circle cx="5"></circle>
+    </svg>
+
+If you were to bind `{{cx}}` directly to the `cx` attribute, you'd get the following error:
+`Error: Invalid value for attribute cx="{{cx}}"`. With `ng-attr-cx` you can work around this
+problem.
+
+
 # Compilation process, and directive matching
 
 Compilation of HTML happens in three phases:

src/ng/compile.js

@@ -349,7 +349,8 @@ function $CompileProvider($provide) {
             ? identity
             : function denormalizeTemplate(template) {
               return template.replace(/\{\{/g, startSymbol).replace(/}}/g, endSymbol);
-            };
+        },
+        NG_ATTR_BINDING = /^ngAttr[A-Z]/;
 
 
     return compile;
@@ -514,11 +515,16 @@ function $CompileProvider($provide) {
               directiveNormalize(nodeName_(node).toLowerCase()), 'E', maxPriority);
 
           // iterate over the attributes
-          for (var attr, name, nName, value, nAttrs = node.attributes,
+          for (var attr, name, nName, ngAttrName, value, nAttrs = node.attributes,
                    j = 0, jj = nAttrs && nAttrs.length; j < jj; j++) {
             attr = nAttrs[j];
             if (attr.specified) {
               name = attr.name;
+              // support ngAttr attribute binding
+              ngAttrName = directiveNormalize(name);
+              if (NG_ATTR_BINDING.test(ngAttrName)) {
+                name = ngAttrName.substr(6).toLowerCase();
+              }
               nName = directiveNormalize(name.toLowerCase());
               attrsMap[nName] = name;
               attrs[nName] = value = trim((msie && name == 'href')

test/ng/compileSpec.js

@@ -2591,4 +2591,41 @@ describe('$compile', function() {
       });
     });
   });
+
+  describe('ngAttr* attribute binding', function() {
+
+    it('should bind after digest but not before', inject(function($compile, $rootScope) {
+      $rootScope.name = "Misko";
+      element = $compile('<span ng-attr-test="{{name}}"></span>')($rootScope);
+      expect(element.attr('test')).toBeUndefined();
+      $rootScope.$digest();
+      expect(element.attr('test')).toBe('Misko');
+    }));
+
+
+    it('should work with different prefixes', inject(function($compile, $rootScope) {
+      $rootScope.name = "Misko";
+      element = $compile('<span ng:attr:test="{{name}}" ng-Attr-test2="{{name}}" ng_Attr_test3="{{name}}"></span>')($rootScope);
+      expect(element.attr('test')).toBeUndefined();
+      expect(element.attr('test2')).toBeUndefined();
+      expect(element.attr('test2')).toBeUndefined();
+      $rootScope.$digest();
+      expect(element.attr('test')).toBe('Misko');
+      expect(element.attr('test2')).toBe('Misko');
+      expect(element.attr('test3')).toBe('Misko');
+    }));
+
+
+    it('should work if they are prefixed with x- or data-', inject(function($compile, $rootScope) {
+      $rootScope.name = "Misko";
+      element = $compile('<span data-ng-attr-test2="{{name}}" x-ng-attr-test3="{{name}}" data-ng:attr-test4="{{name}}"></span>')($rootScope);
+      expect(element.attr('test2')).toBeUndefined();
+      expect(element.attr('test3')).toBeUndefined();
+      expect(element.attr('test4')).toBeUndefined();
+      $rootScope.$digest();
+      expect(element.attr('test2')).toBe('Misko');
+      expect(element.attr('test3')).toBe('Misko');
+      expect(element.attr('test4')).toBe('Misko');
+    }));
+  });
 });