implement dirty checking for @observable objects

pkg/mdv/lib/src/select_element.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;
 
 /** Extensions to the [SelectElement] API. */
 class _SelectElementExtension extends _ElementExtension {
   _SelectElementExtension(SelectElement node) : super(node);
 
@@ skipping 33 matching lines @@
   SelectElement get element => super.element;
 
   void valueChanged(value) {
     var newValue = _toInt(value);
     if (newValue <= element.length) {
       element.selectedIndex = newValue;
       return;
     }
 
     // The binding may wish to bind to an <option> which has not yet been
-    // produced by a child <template>. Delay a maximum of twice -- once for
-    // iterating <optgroup> and once for <option>.
-    var maxRetries = 2;
+    // produced by a child <template>. Furthermore, we may need to wait for
+    // <optgroup> iterating and then for <option>.
+    //
+    // Unlike the JavaScript MDV, we don't have a special "Object.observe" event
+    // loop to schedule on. (See the the "ensureScheduled" function:
+    // https://github.com/Polymer/mdv/commit/9a51ad7ed74a292bf71662cea28acbd151ff65c8)
+    //
+    // Instead we use runAsync. Each <template repeat> needs a delay of 3:
+    //   * once to happen after the child _TemplateIterator is created
+    //   * once to be after _TemplateIterator.inputs CompoundBinding resolve
+    //   * once to be after _TemplateIterator._valueBinding PathObserver fires
+    // And then we need to do this delay sequence twice:
+    //   * once for OPTGROUP
+    //   * once for OPTION.
+    // The resulting 2 * 3 is our maxRetries.
+    var maxRetries = 6;
     delaySetSelectedIndex() {
       if (newValue > element.length && --maxRetries >= 0) {
         runAsync(delaySetSelectedIndex);
       } else {
         element.selectedIndex = newValue;
       }
     }
 
-    // TODO(jmesserly): we aren't matching MDV here. They schedule "model=" to
-    // happen async and then delay using the same scheduler. By using "runAsync"
-    // we're going to happen quite a bit later.
-    // We need to port the "ensureScheduled" function:
-    // https://github.com/Polymer/mdv/commit/9a51ad7ed74a292bf71662cea28acbd151ff65c8
     runAsync(delaySetSelectedIndex);
   }
 
   void updateBinding(e) {
     binding.value = element.selectedIndex;
   }
 
   // TODO(jmesserly,sigmund): I wonder how many bindings typically convert from
   // one type to another (e.g. value-as-number) and whether it is useful to
   // have something like a int/num binding converter (either as a base class or
   // a wrapper).
   static int _toInt(value) {
     if (value is String) return int.parse(value, onError: (_) => null);
     return value is int ? value : null;
   }
 }