fix(ng-repeat): handle the ref changing to null and back

This was broken in commit 09871cb.

If the ng-repeat expression changed from an iterable to a null,
ng-repeat would not notice it.  This would cause further bugs as it
would not reset its state and the ref became a collection again (i.e.
once out of sync, it was gone forever.)

Closes #1015

Author: chirayuk

Diff for: lib/directive/ng_repeat.dart

@@ -132,8 +132,7 @@ class NgRepeat {
     _watch = _scope.watch(
         _listExpr,
         (CollectionChangeRecord changes, _) {
-          if (changes is! CollectionChangeRecord) return;
-          _onChange(changes);
+          _onChange((changes is CollectionChangeRecord) ? changes : null);
         },
         collection: true,
         formatters: formatters
@@ -142,7 +141,8 @@ class NgRepeat {
 
   // Computes and executes DOM changes when the item list changes
   void _onChange(CollectionChangeRecord changes) {
-    final int length = changes.length;
+    final iterable = (changes == null) ? const [] : changes.iterable;
+    final int length = (changes == null) ? 0 : changes.length;
     final rows = new List<_Row>(length);
     final changeFunctions = new List<Function>(length);
     final removedIndexes = <int>[];
@@ -170,43 +170,51 @@ class NgRepeat {
       _rows = new List<_Row>(length);
       for (var i = 0; i < length; i++) {
         changeFunctions[i] = (index, previousView) {
-          addRow(index, changes.iterable.elementAt(i), previousView);
+          addRow(index, iterable.elementAt(i), previousView);
         };
       }
     } else {
-      changes.forEachRemoval((CollectionChangeItem removal) {
-        var index = removal.previousIndex;
-        var row = _rows[index];
-        row.scope.destroy();
-        _viewPort.remove(row.view);
-        leftInDom.removeAt(domLength - 1 - index);
-      });
+      if (changes == null) {
+        _rows.forEach((row) {
+          row.scope.destroy();
+          _viewPort.remove(row.view);
+        });
+        leftInDom.clear();
+      } else {
+        changes.forEachRemoval((CollectionChangeItem removal) {
+          var index = removal.previousIndex;
+          var row = _rows[index];
+          row.scope.destroy();
+          _viewPort.remove(row.view);
+          leftInDom.removeAt(domLength - 1 - index);
+        });
 
-      changes.forEachAddition((CollectionChangeItem addition) {
-        changeFunctions[addition.currentIndex] = (index, previousView) {
-          addRow(index, addition.item, previousView);
-        };
-      });
+        changes.forEachAddition((CollectionChangeItem addition) {
+          changeFunctions[addition.currentIndex] = (index, previousView) {
+            addRow(index, addition.item, previousView);
+          };
+        });
 
-      changes.forEachMove((CollectionChangeItem move) {
-        var previousIndex = move.previousIndex;
-        var value = move.item;
-        changeFunctions[move.currentIndex] = (index, previousView) {
-          var previousRow = _rows[previousIndex];
-          var childScope = previousRow.scope;
-          var childContext = _updateContext(childScope.context, index, length);
-          if (!identical(childScope.context[_valueIdentifier], value)) {
-            childContext[_valueIdentifier] = value;
-          }
-          rows[index] = _rows[previousIndex];
-          // Only move the DOM node when required
-          if (domIndex < 0 || leftInDom[domIndex] != previousIndex) {
-            _viewPort.move(previousRow.view, moveAfter: previousView);
-            leftInDom.remove(previousIndex);
-          }
-          domIndex--;
-        };
-      });
+        changes.forEachMove((CollectionChangeItem move) {
+          var previousIndex = move.previousIndex;
+          var value = move.item;
+          changeFunctions[move.currentIndex] = (index, previousView) {
+            var previousRow = _rows[previousIndex];
+            var childScope = previousRow.scope;
+            var childContext = _updateContext(childScope.context, index, length);
+            if (!identical(childScope.context[_valueIdentifier], value)) {
+              childContext[_valueIdentifier] = value;
+            }
+            rows[index] = _rows[previousIndex];
+            // Only move the DOM node when required
+            if (domIndex < 0 || leftInDom[domIndex] != previousIndex) {
+              _viewPort.move(previousRow.view, moveAfter: previousView);
+              leftInDom.remove(previousIndex);
+            }
+            domIndex--;
+          };
+        });
+      }
     }
 
     var previousView = null;

Diff for: test/directive/ng_repeat_spec.dart

@@ -107,6 +107,33 @@ main() {
     });
 
 
+    it('should gracefully handle ref changing to null and back', () {
+      scope.context['items'] = ['odin', 'dva',];
+      element = compile(
+        '<div>'
+          '<ul>'
+            '<li ng-repeat="item in items">{{item}};</li>'
+          '</ul>'
+        '</div>');
+      scope.apply();
+      expect(element.querySelectorAll('ul').length).toEqual(1);
+      expect(element.querySelectorAll('li').length).toEqual(2);
+      expect(element.text).toEqual('odin;dva;');
+
+      scope.context['items'] = null;
+      scope.apply();
+      expect(element.querySelectorAll('ul').length).toEqual(1);
+      expect(element.querySelectorAll('li').length).toEqual(0);
+      expect(element.text).toEqual('');
+
+      scope.context['items'] = ['odin', 'dva', 'tri'];
+      scope.apply();
+      expect(element.querySelectorAll('ul').length).toEqual(1);
+      expect(element.querySelectorAll('li').length).toEqual(3);
+      expect(element.text).toEqual('odin;dva;tri;');
+    });
+
+
     it('should support formatters', () {
       element = compile(
           '<div><span ng-repeat="item in items | filter:myFilter">{{item}}</span></div>');