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

pkg/mdv/test/binding_syntax_test.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.
 
 library binding_syntax_test;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:html';
 import 'package:mdv/mdv.dart' as mdv;
@@ skipping 79 matching lines @@
     expect(div.nodes[2].text, 'b');
     expect(div.nodes[3].text, 'c');
 
     expect(testSyntax.log, [
       [template, model[0]],
       [template, model[1]],
       [template, model[2]],
     ]);
   });
 
+  observeTest('getInstanceModel - reorder instances', () {
+    var model = toObservable([0, 1, 2]);
+
+    var div = createTestHtml('<template repeat syntax="Test">{{}}</template>');
+    var template = div.firstChild;
+    var delegate = new TestInstanceModelSyntax();
+
+    recursivelySetTemplateModel(div, model, delegate);
+    performMicrotaskCheckpoint();
+    expect(delegate.count, 3);
+
+    // Note: intentionally mutate in place.
+    model.replaceRange(0, model.length, model.reversed.toList());
+    performMicrotaskCheckpoint();
+    expect(delegate.count, 3);
+  });
+
   observeTest('Basic', () {
     var model = fooModel(2, 4);
     var div = createTestHtml(
         '<template bind syntax="2x">'
         '{{ foo }} + {{ 2x: bar }} + {{ 4x: bar }}</template>');
     recursivelySetTemplateModel(div, model, new TimesTwoSyntax());
     performMicrotaskCheckpoint();
     expect(div.nodes.length, 2);
     expect(div.nodes.last.text, '2 + 8 + ');
 
@@ skipping 17 matching lines @@
 class TestModelSyntax extends BindingDelegate {
   var log = [];
   var altModels = new ListQueue();
 
   getInstanceModel(template, model) {
     log.add([template, model]);
     return altModels.removeFirst();
   }
 }
 
+class TestInstanceModelSyntax extends BindingDelegate {
+  int count = 0;
+  getInstanceModel(template, model) {
+    count++;
+    return model;
+  }
+}
+
 // Note: this isn't a very smart whitespace handler. A smarter one would only
 // trim indentation, not all whitespace.
 // See "trimOrCompact" in the web_ui Pub package.
 class WhitespaceRemover extends BindingDelegate {
   int trimmed = 0;
   int removed = 0;
 
   DocumentFragment getInstanceFragment(Element template) {
     var instance = template.createInstance();
     var walker = new TreeWalker(instance, NodeFilter.SHOW_TEXT);
@@ skipping 22 matching lines @@
 class TimesTwoSyntax extends BindingDelegate {
   getBinding(model, path, name, node) {
     path = path.trim();
     if (!path.startsWith('2x:')) return null;
 
     path = path.substring(3);
     return new CompoundBinding((values) => values['value'] * 2)
         ..bind('value', model, path);
   }
 }