feat($compile): add one-way binding to the isolate scope definition

This change allows the developer to bind an isolate scope / controller property
to an expression, using a `<` binding, in such a way that if the value of the
expression changes, the scope/controller property is updated but not the
converse.

The binding is implemented as a single simple watch, which can also provide
performance benefits over two way bindings.

Closes #13928
Closes #13854
Closes #12835
Closes #13900

Author: Narretz

src/ng/compile.js

@@ -189,6 +189,30 @@
  *   equality check is done by value (using the {@link angular.equals} function). It's also possible
  *   to watch the evaluated value shallowly with {@link ng.$rootScope.Scope#$watchCollection
  *   `$watchCollection`}: use `=*` or `=*attr` (`=*?` or `=*?attr` if the attribute is optional).
+ *
+  * * `<` or `<attr` - set up a one-way (one-directional) binding between a local scope property and an
+ *   expression passed via the attribute `attr`. The expression is evaluated in the context of the
+ *   parent scope. If no `attr` name is specified then the attribute name is assumed to be the same as the
+ *   local name. You can also make the binding optional by adding `?`: `<?` or `<?attr`.
+ *
+ *   For example, given `<my-component my-attr="parentModel">` and directive definition of
+ *   `scope: { localModel:'<myAttr' }`, then the isolated scope property `localModel` will reflect the
+ *   value of `parentModel` on the parent scope. Any changes to `parentModel` will be reflected
+ *   in `localModel`, but changes in `localModel` will not reflect in `parentModel`. There are however
+ *   two caveats:
+ *     1. one-way binding does not copy the value from the parent to the isolate scope, it simply
+ *     sets the same value. That means if your bound value is an object, changes to its properties
+ *     in the isolated scope will be reflected in the parent scope (because both reference the same object).
+ *     2. one-way binding watches changes to the **identity** of the parent value. That means the
+ *     {@link ng.$rootScope.Scope#$watch `$watch`} on the parent value only fires if the reference
+ *     to the value has changed. In most cases, this should not be of concern, but can be important
+ *     to know if you one-way bind to an object, and then replace that object in the isolated scope.
+ *     If you now change a property of the object in your parent scope, the change will not be
+ *     propagated to the isolated scope, because the identity of the object on the parent scope
+ *     has not changed. Instead you must assign a new object.
+ *
+ *   One-way binding is useful if you do not plan to propagate changes to your isolated scope bindings
+ *   back to the parent. However, it does not make this completely impossible.
  *
  * * `&` 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 local name.
@@ -829,7 +853,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
   var EVENT_HANDLER_ATTR_REGEXP = /^(on[a-z]+|formaction)$/;
 
   function parseIsolateBindings(scope, directiveName, isController) {
-    var LOCAL_REGEXP = /^\s*([@&]|=(\*?))(\??)\s*(\w*)\s*$/;
+    var LOCAL_REGEXP = /^\s*([@&<]|=(\*?))(\??)\s*(\w*)\s*$/;
 
     var bindings = {};
 
@@ -2965,7 +2989,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
         optional = definition.optional,
         mode = definition.mode, // @, =, or &
         lastValue,
-        parentGet, parentSet, compare;
+        parentGet, parentSet, compare, removeWatch;
 
         switch (mode) {
 
@@ -3026,7 +3050,6 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
               return lastValue = parentValue;
             };
             parentValueWatch.$stateful = true;
-            var removeWatch;
             if (definition.collection) {
               removeWatch = scope.$watchCollection(attrs[attrName], parentValueWatch);
             } else {
@@ -3035,6 +3058,24 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
             removeWatchCollection.push(removeWatch);
             break;
 
+          case '<':
+            if (!hasOwnProperty.call(attrs, attrName)) {
+              if (optional) break;
+              attrs[attrName] = void 0;
+            }
+            if (optional && !attrs[attrName]) break;
+
+            parentGet = $parse(attrs[attrName]);
+
+            destination[scopeName] = parentGet(scope);
+
+            removeWatch = scope.$watch(parentGet, function parentValueWatchAction(newParentValue) {
+              destination[scopeName] = newParentValue;
+            }, parentGet.literal);
+
+            removeWatchCollection.push(removeWatch);
+            break;
+
           case '&':
             // Don't assign Object.prototype method to scope
             parentGet = attrs.hasOwnProperty(attrName) ? $parse(attrs[attrName]) : noop;