Add two way data binding.

sky/framework/sky-element/sky-element.sky

Index: sky/framework/sky-element/sky-element.sky
diff --git a/sky/framework/sky-element/sky-element.sky b/sky/framework/sky-element/sky-element.sky
index a76874a5f1215af1bbefca5594b9edc2b2c1c2e8..7b7933ab35709505eee6b2118c3228a6a28ed7ea 100644
--- a/sky/framework/sky-element/sky-element.sky
+++ b/sky/framework/sky-element/sky-element.sky
@@ -160,6 +160,8 @@ class SkyElement extends HTMLElement {
 
   createdCallback() {
     this.isAttached = false;
+    this.propertyBindings = null;
+    this.dirtyPropertyBindings = null;
     this.created();
 
     Object.preventExtensions(this);
@@ -208,6 +210,8 @@ class SkyElement extends HTMLElement {
   }
 
   notifyPropertyChanged(name, oldValue, newValue) {
+    if (oldValue == newValue)
+      return;
     var notifier = Object.getNotifier(this);
     notifier.notify({
       type: 'update',
@@ -217,6 +221,38 @@ class SkyElement extends HTMLElement {
     var handler = this[name + 'Changed'];
     if (typeof handler == 'function')
       handler.call(this, oldValue, newValue);
+    this.schedulePropertyBindingUpdate(name);
+  }
+
+  addPropertyBinding(name, binding) {
+    if (!this.propertyBindings)
+      this.propertyBindings = new Map();
+    this.propertyBindings.set(name, binding);
+  }
+
+  getPropertyBinding(name) {
+    if (!this.propertyBindings)
+      return null;
+    return this.propertyBindings.get(name);
+  }
+
+  schedulePropertyBindingUpdate(name) {
+    if (!this.dirtyPropertyBindings) {
+      this.dirtyPropertyBindings = new Set();
+      Promise.resolve().then(this.updatePropertyBindings.bind(this));
+    }
+    this.dirtyPropertyBindings.add(name);
+  }
+
+  updatePropertyBindings() {
+    for (var name of this.dirtyPropertyBindings) {
+      var binding = this.getPropertyBinding(name);
+      if (binding) {
+        binding.setValue(this[name]);
+        binding.discardChanges();
+      }
+    }
+    this.dirtyPropertyBindings = null;
   }
 };