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

1 files changed, 6 insertions, 2 deletions

diff --git a/src/ng/compile.js b/src/ng/compile.js
index 5508605e..f39b0935 100644
--- a/src/ng/compile.js
+++ b/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