feat(ngRepeat): provide support for aliasing filtered repeater results as a scope member

ngRepeat can now alias the snapshot of the list of items evaluated after all filters have
been applied as a property on the scope. Prior to this fix, when a filter is applied on a
repeater, there is no way to trigger an event when the repeater renders zero results.

Closes angular#5919

Author: matsko

src/ng/directive/ngRepeat.js

@@ -112,6 +112,13 @@
  *     For example: `item in items | filter:searchText track by item.id` is a pattern that might be used to apply a filter
  *     to items in conjunction with a tracking expression.
  *
+ *   * `variable in expression as alias_expression` – You can also provide an optional alias expression which will then store the
+ *     intermediate results of the repeater after the filters have been applied. Typically this is used to render a special message
+ *     when a filter is active on the repeater, but the filtered result set is empty.
+ *
+ *     For example: `item in items | filter:x as results` will store the fragment of the repeated items as `results`, but only after
+ *     the items have been processed through the filter.
+ *
  * @example
  * This example initializes the scope to a list of names and
  * then uses `ngRepeat` to display every person:
@@ -132,9 +139,12 @@
         I have {{friends.length}} friends. They are:
         <input type="search" ng-model="q" placeholder="filter friends..." />
         <ul class="example-animate-container">
-          <li class="animate-repeat" ng-repeat="friend in friends | filter:q">
+          <li class="animate-repeat" ng-repeat="friend in friends | filter:q as results">
             [{{$index + 1}}] {{friend.name}} who is {{friend.age}} years old.
           </li>
+          <li class="animate-repeat" ng-if="results.length == 0">
+            <strong>No results found...</strong>
+          </li>
         </ul>
       </div>
     </file>
@@ -208,8 +218,8 @@ var ngRepeatDirective = ['$parse', '$animate', function($parse, $animate) {
     $$tlb: true,
     link: function($scope, $element, $attr, ctrl, $transclude){
         var expression = $attr.ngRepeat;
-        var match = expression.match(/^\s*([\s\S]+?)\s+in\s+([\s\S]+?)(?:\s+track\s+by\s+([\s\S]+?))?\s*$/),
-          trackByExp, trackByExpGetter, trackByIdExpFn, trackByIdArrayFn, trackByIdObjFn,
+        var match = expression.match(/^\s*([\s\S]+?)\s+in\s+([\s\S]+?)(?:\s+track\s+by\s+([\s\S]+?))?(?:\s+as\s+([\s\S]+?))?\s*$/),
+          trackByExp, trackByExpGetter, aliasAs, trackByIdExpFn, trackByIdArrayFn, trackByIdObjFn,
           lhs, rhs, valueIdentifier, keyIdentifier,
           hashFnLocals = {$id: hashKey};
 
@@ -221,6 +231,7 @@ var ngRepeatDirective = ['$parse', '$animate', function($parse, $animate) {
         lhs = match[1];
         rhs = match[2];
         trackByExp = match[3];
+        aliasAs = match[4];
 
         if (trackByExp) {
           trackByExpGetter = $parse(trackByExp);
@@ -272,6 +283,10 @@ var ngRepeatDirective = ['$parse', '$animate', function($parse, $animate) {
               nextBlockOrder = [],
               elementsToRemove;
 
+          if (aliasAs) {
+            $scope[aliasAs] = collection;
+          }
+
           var updateScope = function(scope, index) {
             scope[valueIdentifier] = value;
             if (keyIdentifier) scope[keyIdentifier] = key;

test/ng/directive/ngRepeatSpec.js

@@ -374,6 +374,58 @@ describe('ngRepeat', function() {
     });
   });
 
+  describe('alias as', function() {
+    it('should assigned the filtered to the target scope property if an alias is provided', function() {
+      element = $compile(
+        '<div ng-repeat="item in items | filter:x track by $index as results">{{item.name}}/</div>')(scope);
+
+      scope.items = [
+        { name : 'red' },
+        { name : 'blue' },
+        { name : 'green' },
+        { name : 'black' },
+        { name : 'orange' },
+        { name : 'blonde' }
+      ];
+
+      expect(scope.results).toBeUndefined();
+      scope.$digest();
+
+      scope.x = 'bl';
+      scope.$digest();
+
+      expect(scope.results).toEqual([
+        { name : 'blue' },
+        { name : 'black' },
+        { name : 'blonde' }
+      ]);
+
+      scope.items = [];
+      scope.$digest();
+
+      expect(scope.results).toEqual([]);
+    });
+
+    it('should render a message when the repeat list is empty', function() {
+      element = $compile(
+        '<div>' +
+        '  <div ng-repeat="item in items | filter:x as results">{{item}}</div>' +
+        '  <div ng-if="results.length == 0">' +
+        '    No results found...' +
+        '  </div>' +
+        '</div>')(scope);
+
+      scope.items = [1,2,3,4,5,6];
+      scope.$digest();
+      expect(trim(element.text())).toEqual('123456');
+
+      scope.x = '0';
+      scope.$digest();
+
+      expect(trim(element.text())).toEqual('No results found...');
+    });
+  });
+
 
   it('should allow expressions over multiple lines', function() {
     element = $compile(
@@ -1196,6 +1248,21 @@ describe('ngRepeat and transcludes', function() {
   });
 
 
+  it('should set the state before linking', function() {
+    module(function($compileProvider) {
+      $compileProvider.directive('assertA', valueFn(function(scope) {
+        // This linking function asserts that a is set.
+        // If we only test this by asserting binding, it will work even if the value is set later.
+        expect(scope.a).toBeDefined();
+      }));
+    });
+    inject(function($compile, $rootScope) {
+      var element = $compile('<div><span ng-repeat="a in [1]"><span assert-a></span></span></div>')($rootScope);
+      $rootScope.$digest();
+      dealoc(element);
+    });
+  });
+
   it('should set the state before linking', function() {
     module(function($compileProvider) {
       $compileProvider.directive('assertA', valueFn(function(scope) {