feat($compile): support expansion of special ngBindon attributes

In order to reduce some of the verbosity associated with conforming to the
ideas behind unidirectional data-flow, we can introduce a special
attribute syntax that will be automatically expanded in order to simulate
two-way data binding.

For example,

`<my-component ng-bindon-foo="bar">` will be expanded into
`<my-component foo="bar" foo-change="bar=$event">`

Fixes #15455

Author: dcherman

Diff for: docs/content/guide/component.ngdoc

@@ -147,6 +147,21 @@ components should follow a few simple conventions:
         });
       }
     ```
+  - A two-way binding can be simulated by using both a one-way binding as well as an output event.
+    ```js
+      bindings: {
+        hero: '<',
+        heroChange: '&'
+      }
+    ```
+    ```html
+      <hero-detail hero="$ctrl.hero" hero-change="$ctrl.hero = $event"></hero-detail>
+    ```
+  - Since that can be rather verbose, especially with repeated properties, we provide syntactic sugar for that pattern
+    via `ng-bindon-` attributes which expand to match the example above.
+    ```html
+      <hero-detail ng-bindon-hero="$ctrl.hero"></hero-detail>
+    ```
 
 - **Components have a well-defined lifecycle**
 Each component can implement "lifecycle hooks". These are methods that will be called at certain points in the life

Diff for: src/ng/compile.js

@@ -1803,7 +1803,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
             : function denormalizeTemplate(template) {
               return template.replace(/\{\{/g, startSymbol).replace(/}}/g, endSymbol);
         },
-        NG_ATTR_BINDING = /^ngAttr[A-Z]/;
+        NG_SPECIAL_ATTR = /^ng(Attr|Bindon)[A-Z]/;
     var MULTI_ELEMENT_DIR_RE = /^(.+)Start$/;
 
     compile.$$addBindingInfo = debugInfoEnabled ? function $$addBindingInfo($element, binding) {
@@ -2139,8 +2139,8 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
               directiveNormalize(nodeName), 'E', maxPriority, ignoreDirective);
 
           // iterate over the attributes
-          for (var attr, name, nName, ngAttrName, value, isNgAttr, nAttrs = node.attributes,
-                   j = 0, jj = nAttrs && nAttrs.length; j < jj; j++) {
+          for (var attr, name, nName, ngAttrName, value, isNgAttr, isSpecialAttr, isNgBindon,
+            nAttrs = node.attributes, j = 0, jj = nAttrs && nAttrs.length; j < jj; j++) {
             var attrStartName = false;
             var attrEndName = false;
 
@@ -2150,10 +2150,14 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
 
             // support ngAttr attribute binding
             ngAttrName = directiveNormalize(name);
-            isNgAttr = NG_ATTR_BINDING.test(ngAttrName);
-            if (isNgAttr) {
+
+            isSpecialAttr = NG_SPECIAL_ATTR.exec(ngAttrName);
+            isNgAttr = isSpecialAttr && isSpecialAttr[1] === 'Attr';
+            isNgBindon = isSpecialAttr && isSpecialAttr[1] === 'Bindon';
+
+            if (isSpecialAttr) {
               name = name.replace(PREFIX_REGEXP, '')
-                .substr(8).replace(/_(.)/g, function(match, letter) {
+                .substr(isNgAttr ? 8 : 10).replace(/_(.)/g, function(match, letter) {
                   return letter.toUpperCase();
                 });
             }
@@ -2173,6 +2177,12 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                   attrs[nName] = true; // presence means true
                 }
             }
+
+            if (isNgBindon) {
+              attrs[nName] = value;
+              attrs[nName + 'Change'] = value + '=$event';
+            }
+
             addAttrInterpolateDirective(node, directives, value, nName, isNgAttr);
             addDirective(directives, nName, 'A', maxPriority, ignoreDirective, attrStartName,
                           attrEndName);

Diff for: test/ng/compileSpec.js

@@ -11441,6 +11441,36 @@ describe('$compile', function() {
     });
   });
 
+  describe('ngBindon attributes', function() {
+    it('should expand `ng-bindon-foo` to both an input and an output expression', function() {
+      module(function($compileProvider) {
+        $compileProvider.component('test', {
+          bindings: {
+            foo: '<',
+            fooChange: '&'
+          }
+        });
+      });
+
+      inject(function($compile, $rootScope) {
+        $rootScope.bar = 0;
+
+        element = $compile('<test ng-bindon-foo="bar"></test>')($rootScope);
+        var testController = element.controller('test');
+        $rootScope.$digest();
+
+        expect(testController.foo).toBe(0);
+        $rootScope.$apply('bar=1');
+        expect(testController.foo).toBe(1);
+        $rootScope.$apply(function() {
+          testController.fooChange({ $event: 2 });
+        });
+
+        expect($rootScope.bar).toBe(2);
+        expect(testController.foo).toBe(2);
+      });
+    });
+  });
 
   describe('when an attribute has an underscore-separated name', function() {