Ensure getInstanceModel is only called when a new instance will be created

pkg/mdv/lib/src/template_iterator.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of mdv;
 
 // This code is a port of Model-Driven-Views:
 // https://github.com/polymer-project/mdv
 // The code mostly comes from src/template_element.js
 
@@ skipping 228 matching lines @@
         !identical(terminator, _templateElement)) {
       var subIterator = _mdv(terminator)._templateIterator;
       if (subIterator != null) {
         return subIterator.getTerminatorAt(subIterator.terminators.length - 1);
       }
     }
 
     return terminator;
   }
 
-  void insertInstanceAt(int index, List<Node> instanceNodes) {
+  void insertInstanceAt(int index, DocumentFragment fragment,
+        List<Node> instanceNodes) {
+
     var previousTerminator = getTerminatorAt(index - 1);
-    var terminator = instanceNodes.length > 0 ? instanceNodes.last
-        : previousTerminator;
+    var terminator = null;
+    if (fragment != null) {
+      terminator = fragment.lastChild;
+    } else if (instanceNodes.length > 0) {
+      terminator = instanceNodes.last;
+    }
+    if (terminator == null) terminator = previousTerminator;
 
     terminators.insert(index, terminator);
 
     var parent = _templateElement.parentNode;
     var insertBeforeNode = previousTerminator.nextNode;
+
+    if (fragment != null) {
+      parent.insertBefore(fragment, insertBeforeNode);
+      return;
+    }
+
     for (var node in instanceNodes) {
       parent.insertBefore(node, insertBeforeNode);
     }
   }
 
   List<Node> extractInstanceAt(int index) {
     var instanceNodes = <Node>[];
     var previousTerminator = getTerminatorAt(index - 1);
     var terminator = getTerminatorAt(index);
     terminators.removeAt(index);
 
     var parent = _templateElement.parentNode;
     while (terminator != previousTerminator) {
       var node = previousTerminator.nextNode;
       if (node == terminator) terminator = previousTerminator;
       node.remove();
       instanceNodes.add(node);
     }
     return instanceNodes;
   }
 
   getInstanceModel(model, BindingDelegate delegate) {
     if (delegate != null) {
       return delegate.getInstanceModel(_templateElement, model);
     }
     return model;
   }
 
-  List<Node> getInstanceNodes(model, BindingDelegate delegate,
-      IdentityMap instanceCache) {
-
-    var instanceNodes = instanceCache.remove(model);
-    if (instanceNodes != null) return instanceNodes;
-
-    var fragment = _templateElement.createInstance(model, delegate);
-    instanceNodes = fragment.nodes.toList();
-    fragment.nodes.clear();
-    return instanceNodes;
+  DocumentFragment getInstanceFragment(model, BindingDelegate delegate) {
+    return _templateElement.createInstance(model, delegate);
   }
 
   void _handleChanges(Iterable<ChangeRecord> splices) {
     splices = splices.where((s) => s is ListChangeRecord);
 
     var template = _templateElement;
     var delegate = template.bindingDelegate;
 
     if (template.parentNode == null || template.document.window == null) {
       abandon();
@@ skipping 16 matching lines @@
       }
 
       removeDelta -= splice.addedCount;
     }
 
     for (var splice in splices) {
       for (var addIndex = splice.index;
           addIndex < splice.index + splice.addedCount;
           addIndex++) {
 
-        var model = getInstanceModel(iteratedValue[addIndex], delegate);
+        var model = iteratedValue[addIndex];
+        var fragment = null;
+        var instanceNodes = instanceCache.remove(model);
+        if (instanceNodes == null) {
+          var actualModel = getInstanceModel(model, delegate);
+          fragment = getInstanceFragment(actualModel, delegate);
+        }
 
-        var instanceNodes = getInstanceNodes(model, delegate, instanceCache);
-        insertInstanceAt(addIndex, instanceNodes);
+        insertInstanceAt(addIndex, fragment, instanceNodes);
       }
     }
 
     for (var instanceNodes in instanceCache.values) {
       instanceNodes.forEach(_unbindAllRecursively);
     }
   }
 
   void unobserve() {
     if (_sub == null) return;
@@ skipping 19 matching lines @@
         nodeExt._templateIterator = null;
       }
     }
 
     _nodeOrCustom(node).unbindAll();
     for (var c = node.firstChild; c != null; c = c.nextNode) {
       _unbindAllRecursively(c);
     }
   }
 }