refactor(ngRepeat): simplify the code

This re-introduce the PR dart-archive#1018: "refactor(ng-repeat): refactor fix for
dart-archive#1015" which had been reverted.

Author: vicb

Diff for: lib/directive/ng_repeat.dart

@@ -83,7 +83,7 @@ class NgRepeat {
   String _valueIdentifier;
   String _keyIdentifier;
   String _listExpr;
-  List<_Row> _rows;
+  List<View> _views;
   Function _generateId = (key, value, index) => value;
   Watch _watch;
 
@@ -131,109 +131,95 @@ class NgRepeat {
     _watch = _scope.watch(
         _listExpr,
         (changes, _) {
-          _onChange((changes is CollectionChangeRecord) ? changes : null);
+          if (changes is CollectionChangeRecord && changes != null) {
+            _onCollectionChange(changes);
+          } else if (_views != null) {
+            _views.forEach(_viewPort.remove);
+            _views = null;
+          }
         },
         collection: true,
         formatters: formatters
     );
   }
 
-  // Computes and executes DOM changes when the item list changes
-  void _onChange(CollectionChangeRecord changes) {
-    final iterable = (changes == null) ? const [] : changes.iterable;
-    final int length = (changes == null) ? 0 : changes.length;
-    final rows = new List<_Row>(length);
+  void _onCollectionChange(CollectionChangeRecord changes) {
+    final int length = changes.length;
+    final views = new List<View>(length);
     final changeFunctions = new List<Function>(length);
     final removedIndexes = <int>[];
-    final int domLength = _rows == null ? 0 : _rows.length;
+    final int domLength = _views == null ? 0 : _views.length;
     final leftInDom = new List.generate(domLength, (i) => domLength - 1 - i);
     var domIndex;
 
-    var addRow = (int index, value, View previousView) {
-      var childContext = _updateContext(new PrototypeMap(_scope.context), index,
-          length)..[_valueIdentifier] = value;
-      var childScope = _scope.createChild(childContext);
-      var view = _boundViewFactory(childScope);
-      var nodes = view.nodes;
-      rows[index] = new _Row(_generateId(index, value, index))
-          ..view = view
-          ..scope = childScope
-          ..nodes = nodes
-          ..startNode = nodes.first
-          ..endNode = nodes.last;
-      _viewPort.insert(view, insertAfter: previousView);
-    };
+    Function addFn, moveFn, removeFn;
 
-    // todo(vicb) refactor once GH-774 gets fixed
-    if (_rows == null) {
-      _rows = new List<_Row>(length);
-      for (var i = 0; i < length; i++) {
-        changeFunctions[i] = (index, previousView) {
-          addRow(index, iterable.elementAt(i), previousView);
-        };
-      }
+    if (_views == null) {
+      addFn = changes.forEachItem;
+      moveFn = (_) {};
+      removeFn = (_) {};
     } else {
-      if (changes == null) {
-        _rows.forEach((row) {
-          _viewPort.remove(row.view);
-        });
-        leftInDom.clear();
-      } else {
-        changes.forEachRemoval((CollectionChangeItem removal) {
-          var index = removal.previousIndex;
-          var row = _rows[index];
-          _viewPort.remove(row.view);
-          leftInDom.removeAt(domLength - 1 - index);
-        });
-
-        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--;
-          };
-        });
-      }
+      addFn = changes.forEachAddition;
+      moveFn = changes.forEachMove;
+      removeFn = changes.forEachRemoval;
     }
 
+    removeFn((CollectionChangeItem removal) {
+      var index = removal.previousIndex;
+      _viewPort.remove(_views[index]);
+      leftInDom.removeAt(domLength - 1 - index);
+    });
+
+    addFn((CollectionChangeItem addition) {
+      changeFunctions[addition.currentIndex] = (index, previousView) {
+        var childContext = _updateContext(new PrototypeMap(_scope.context), index,length)
+            ..[_valueIdentifier] = addition.item;
+        var childScope = _scope.createChild(childContext);
+        var view = views[index] = _boundViewFactory(childScope);
+        _viewPort.insert(view, insertAfter: previousView);
+      };
+    });
+
+    moveFn((CollectionChangeItem move) {
+      var previousIndex = move.previousIndex;
+      var value = move.item;
+      changeFunctions[move.currentIndex] = (index, moveAfter) {
+        var previousView = _views[previousIndex];
+        var childScope = previousView.scope;
+        var childContext = _updateContext(childScope.context, index, length);
+        if (!identical(childScope.context[_valueIdentifier], value)) {
+          childContext[_valueIdentifier] = value;
+        }
+        views[index] = _views[previousIndex];
+        // Only move the DOM node when required
+        if (domIndex < 0 || leftInDom[domIndex] != previousIndex) {
+          _viewPort.move(previousView, moveAfter: moveAfter);
+          leftInDom.remove(previousIndex);
+        }
+        domIndex--;
+      };
+    });
+
     var previousView = null;
     domIndex = leftInDom.length - 1;
     for(var targetIndex = 0; targetIndex < length; targetIndex++) {
       var changeFn = changeFunctions[targetIndex];
       if (changeFn == null) {
-        rows[targetIndex] = _rows[targetIndex];
+        views[targetIndex] = _views[targetIndex];
         if (domIndex < 0 || leftInDom[domIndex] != targetIndex) {
-          _viewPort.move(rows[targetIndex].view, moveAfter: previousView);
+          _viewPort.move(views[targetIndex], moveAfter: previousView);
           leftInDom.remove(targetIndex);
         }
         domIndex--;
         // The element has not moved but `$last` and `$middle` might still need to be updated
-        _updateContext(rows[targetIndex].scope.context, targetIndex, length);
+        _updateContext(views[targetIndex].scope.context, targetIndex, length);
       } else {
         changeFn(targetIndex, previousView);
       }
-      previousView = rows[targetIndex].view;
+      previousView = views[targetIndex];
     }
 
-    _rows = rows;
+    _views = views;
   }
 
   PrototypeMap _updateContext(PrototypeMap context, int index, int length) {
@@ -248,14 +234,3 @@ class NgRepeat {
         ..[r'$even'] = index.isEven;
   }
 }
-
-class _Row {
-  final id;
-  Scope scope;
-  View view;
-  dom.Element startNode;
-  dom.Element endNode;
-  List<dom.Node> nodes;
-
-  _Row(this.id);
-}