feat($compile): '=?' makes '=' binding optional

If you bind using '=' to a non-existant parent property, the compiler
will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception, which is right
because the model doesn't exist.

This enhancement allow to specify that a binding is optional so it
won't complain if the parent property is not defined. In order to mantain
backward compability, the new behaviour must be specified using '=?' instead
of '='. The local property will be undefined is these cases.

Closes #909
Closes #1435

Author: lrlopez

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

@@ -336,7 +336,9 @@ compiler}. The attributes are:
         Given `<widget my-attr="parentModel">` and widget definition of
         `scope: { localModel:'=myAttr' }`, then widget scope property `localModel` will reflect the
         value of `parentModel` on the parent scope. Any changes to `parentModel` will be reflected
-        in `localModel` and any changes in `localModel` will reflect in `parentModel`.
+        in `localModel` and any changes in `localModel` will reflect in `parentModel`. If the parent
+        scope property doesn't exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You
+        can avoid this behavior using `=?` or `=?attr` in order to flag the property as optional.
 
       * `&` or `&attr` - provides a way to execute an expression in the context of the parent scope.
         If no `attr` name is specified then the attribute name is assumed to be the same as the

Diff for: src/ng/compile.js

@@ -768,13 +768,14 @@ function $CompileProvider($provide) {
         $element = attrs.$$element;
 
         if (newIsolateScopeDirective) {
-          var LOCAL_REGEXP = /^\s*([@=&])\s*(\w*)\s*$/;
+          var LOCAL_REGEXP = /^\s*([@=&])(\??)\s*(\w*)\s*$/;
 
           var parentScope = scope.$parent || scope;
 
           forEach(newIsolateScopeDirective.scope, function(definiton, scopeName) {
             var match = definiton.match(LOCAL_REGEXP) || [],
-                attrName = match[2]|| scopeName,
+                attrName = match[3] || scopeName,
+                optional = (match[2] == '?'),
                 mode = match[1], // @, =, or &
                 lastValue,
                 parentGet, parentSet;
@@ -796,6 +797,9 @@ function $CompileProvider($provide) {
               }
 
               case '=': {
+                if (optional && !attrs[attrName]) {
+                  return;
+                }
                 parentGet = $parse(attrs[attrName]);
                 parentSet = parentGet.assign || function() {
                   // reset the change, or we will throw this exception on every $digest

Diff for: test/ng/compileSpec.js

@@ -1857,6 +1857,9 @@ describe('$compile', function() {
             ref: '=',
             refAlias: '= ref',
             reference: '=',
+            optref: '=?',
+            optrefAlias: '=? optref',
+            optreference: '=?',
             expr: '&',
             exprAlias: '&expr'
           },
@@ -1993,6 +1996,33 @@ describe('$compile', function() {
     });
 
 
+    describe('optional object reference', function() {
+      it('should update local when origin changes', inject(function() {
+        compile('<div><span my-component optref="name">');
+        expect(componentScope.optRef).toBe(undefined);
+        expect(componentScope.optRefAlias).toBe(componentScope.optRef);
+
+        $rootScope.name = 'misko';
+        $rootScope.$apply();
+        expect(componentScope.optref).toBe($rootScope.name);
+        expect(componentScope.optrefAlias).toBe($rootScope.name);
+
+        $rootScope.name = {};
+        $rootScope.$apply();
+        expect(componentScope.optref).toBe($rootScope.name);
+        expect(componentScope.optrefAlias).toBe($rootScope.name);
+      }));
+
+      it('should not throw exception when reference does not exist', inject(function() {
+        compile('<div><span my-component>');
+
+        expect(componentScope.optref).toBe(undefined);
+        expect(componentScope.optrefAlias).toBe(undefined);
+        expect(componentScope.optreference).toBe(undefined);
+      }));
+    });
+
+
     describe('executable expression', function() {
       it('should allow expression execution with locals', inject(function() {
         compile('<div><span my-component expr="count = count + offset">');