fix(WatchGroup): Handle watching elements of array that were removed.

Closes dart-archive#1046

Author: mvuksano

lib/change_detection/watch_group.dart

@@ -795,18 +795,30 @@ class _EvalWatchRecord implements WatchRecord<_Handler> {
   }
 
   bool check() {
+    // A helper function used to apply another function. If the function happens to be fetching
+    // from an array by index and there's a RangeError, the function will return _MaybeRemoved
+    // instead of throwing an error.
+    var applyFnHelper = (fn, args, namedArgs) {
+      var value;
+      try {
+        return Function.apply(fn, args, namedArgs);
+      } on RangeError catch(e) {
+        if (args.length == 2 && args[0] is List && args[1] is num) return new _MaybeRemoved();
+        else rethrow;
+      }
+    };
     var value;
     switch (mode) {
       case _MODE_MARKER_:
       case _MODE_NULL_:
         return false;
       case _MODE_PURE_FUNCTION_:
         if (!dirtyArgs) return false;
-        value = Function.apply(fn, args, namedArgs);
+        value = applyFnHelper(fn, args, namedArgs);
         dirtyArgs = false;
         break;
       case _MODE_FUNCTION_:
-        value = Function.apply(fn, args, namedArgs);
+        value = applyFnHelper(fn, args, namedArgs);
         dirtyArgs = false;
         break;
       case _MODE_PURE_FUNCTION_APPLY_:
@@ -873,3 +885,7 @@ class _EvalWatchRecord implements WatchRecord<_Handler> {
     return '${watchGrp.id}:${handler.expression}';
   }
 }
+
+class _MaybeRemoved {
+  toString() => "maybeRemoved";
+}

test/change_detection/watch_group_spec.dart

@@ -756,6 +756,36 @@ void main() {
         expect(watchGrp.detectChanges()).not.toBe(null);
         expect(logger).toEqual([-3]);
       });
+
+      it('should return object of type _MaybeRemoved if closure is used to get an item by index '
+          'and the index is out of range', () {
+        context['a'] = ['item'];
+        var ast = new ClosureAST( 'getItem', (obj, idx) => obj[idx], [parse('a'), parse('0')]);
+        var watch = watchGrp.watch(ast, (v, p) => logger(v));
+
+        expect(watchGrp.detectChanges()).not.toBe(null);
+        expect(logger).toEqual(['item']);
+        logger.clear();
+
+        context['a'] = [];
+        expect(watchGrp.detectChanges()).not.toBe(null);
+        expect(logger.map((i) => '$i')).toEqual(['maybeRemoved']);
+      });
+
+      it('should return object of type _MaybeRemoved if pure function is used to get an item by '
+          'index and the index is out of range', () {
+        context['a'] = ['item'];
+        var ast = new PureFunctionAST( 'getItem', (obj, idx) => obj[idx], [parse('a'), parse('0')]);
+        var watch = watchGrp.watch(ast, (v, p) => logger(v));
+
+        expect(watchGrp.detectChanges()).not.toBe(null);
+        expect(logger).toEqual(['item']);
+        logger.clear();
+
+        context['a'] = [];
+        expect(watchGrp.detectChanges()).not.toBe(null);
+        expect(logger.map((i) => '$i')).toEqual(['maybeRemoved']);
+      });
     });
 
     describe('evaluation', () {

test/directive/ng_repeat_spec.dart

@@ -301,7 +301,6 @@ main() {
       expect(element.text).toEqual('misko:0|shyam:1|frodo:2|');
     });
 
-
     it(r'should expose iterator position as $first, $middle and $last when iterating over arrays',
         () {
       element = compile(
@@ -375,6 +374,55 @@ main() {
       expect(element.text).toEqual('a|b|Xc|d|X');
     });
 
+    describe('nested watching', () {
+      it('should not throw RangeError when an item is being wathed and it is removed', () {
+        element = compile(
+            '<ul>'
+            '  <li ng-repeat="i in items">'
+            '    <input ng-model="items[\$index]">'
+            '  </li>'
+            '</ul>');
+        scope.context['items'] = ['misko', 'shyam', 'frodo'];
+        scope.apply();
+        expect(element.children.length).toEqual(3);
+        scope.context['items'].remove('misko');
+        scope.apply();
+        expect(element.children.length).toEqual(2);
+      });
+
+      it('should not throw RangeError when an item (somewhere in the middle of the list) is being '
+      'watched and it is removed', () {
+        element = compile(
+            '<ul>'
+            '  <li ng-repeat="i in items">'
+            '    <input ng-model="items[\$index]">'
+            '  </li>'
+            '</ul>');
+        scope.context['items'] = ['misko', 'shyam', 'frodo', 'marko', 'pavel'];
+        scope.apply();
+        expect(element.children.length).toEqual(5);
+        scope.context['items'].remove('shyam');
+        scope.apply();
+        expect(element.children.length).toEqual(4);
+      });
+
+      it('should not throw RangeError when an items are being wathed and multiple items are removed '
+      'at the same time', () {
+        element = compile(
+            '<ul>'
+            '  <li ng-repeat="i in items">'
+            '    <input ng-model="items[\$index]">'
+            '  </li>'
+            '</ul>');
+        scope.context['items'] = ['misko', 'shyam', 'frodo'];
+        scope.apply();
+        expect(element.children.length).toEqual(3);
+        scope.context['items'].remove('misko');
+        scope.context['items'].remove('frodo');
+        scope.apply();
+        expect(element.children.length).toEqual(1);
+      });
+    });
 
     describe('stability', () {
       var a, b, c, d, lis;
@@ -394,7 +442,6 @@ main() {
         lis = element.querySelectorAll('li');
       });
 
-
       it(r'should preserve the order of elements', () {
         scope.context['items'] = [a, c, d];
         scope.apply();