Better attribute suppport

* Moves _propertyToAttribute to PropertiesChanged
* PropertiesMixin supports `attribute` in the `properties` object to define an attribute name.

Author: Steven Orvell

lib/mixins/properties-changed.html

@@ -117,6 +117,7 @@
           this.__dataPending = null;
           this.__dataOld = null;
           this.__dataInstanceProps = null;
+          this.__serializing = false;
           this._initializeProperties();
         }
 
@@ -356,9 +357,45 @@
          * returned from `_typeForProperty`
          */
         _attributeToProperty(attribute, value, type) {
-          const property = this._propertyForAttribute(attribute);
-          this[property] = this._deserializeValue(value, type ||
-            this._typeForProperty(property));
+          if (!this.__serializing) {
+            const property = this._propertyForAttribute(attribute);
+            this[property] = this._deserializeValue(value, type ||
+              this._typeForProperty(property));
+          }
+        }
+
+        /**
+         * Serializes a property to its associated attribute.
+         *
+         * @suppress {invalidCasts} Closure can't figure out `this` is an element.
+         *
+         * @param {string} property Property name to reflect.
+         * @param {string=} attribute Attribute name to reflect to.
+         * @param {*=} value Property value to refect.
+         */
+        _propertyToAttribute(property, attribute, value) {
+          this.__serializing = true;
+          value = (arguments.length < 3) ? this[property] : value;
+          this._valueToNodeAttribute(/** @type {!HTMLElement} */(this), value,
+            attribute || this._attributeForProperty(property));
+          this.__serializing = false;
+        }
+
+        /**
+         * Sets a typed value to an HTML attribute on a node.
+         *
+         * If the value is `undefined`, the attribute will be removed.
+         *
+         * @param {Element} node Element to set attribute to.
+         * @param {*} value Value to serialize.
+         * @param {string} attribute Attribute name to serialize to.
+         */
+        _valueToNodeAttribute(node, value, attribute) {
+          if (!value && value !== '' && value !== 0) {
+            node.removeAttribute(attribute);
+          } else {
+            node.setAttribute(attribute, value);
+          }
         }
 
         /**

lib/mixins/properties-mixin.html

@@ -42,6 +42,10 @@
      */
     const base = Polymer.PropertiesChanged(superClass);
 
+    function BooleanAttribute(value) {
+      return value !== null;
+    }
+
     /**
      * @polymer
      * @mixinClass
@@ -51,21 +55,33 @@
      */
     class Properties extends base {
 
+      static get BooleanAttribute() {
+        return BooleanAttribute;
+      }
+
       /**
        * Implements standard custom elements getter to observes the attributes
        * listed in `properties`.
        */
       static get observedAttributes() {
-        const props = this.properties;
+        this._ensurePropertyInfo();
+        const props = this.prototype.__propertyInfo;
         return props ? Object.keys(props).map(p => {
           return this.prototype._attributeForProperty(p);
         }) : [];
       }
 
+      _propertyForAttribute(name) {
+        return this.__attributeInfo[name] || name;
+      }
+
+      _attributeForProperty(name) {
+        const info = this.__propertyInfo[name];
+        return info && info.attribute || name.toLowerCase();
+      }
+
       static _ensureFinalized(name) {
-        const proto = this.prototype;
-        if (!proto.hasOwnProperty('__finalized')) {
-          proto.__finalized = true;
+        if (!this.prototype.hasOwnProperty('__finalized')) {
           this.finalize(name);
         }
       }
@@ -77,13 +93,25 @@
        * @param {string} name Name of the element
        */
       static finalize(name) { // eslint-disable-line no-unused-vars
-        const props = this.properties;
+        this.prototype.__finalized = true;
+        this._ensurePropertyInfo();
+        const props = this.prototype.__propertyInfo;
         if (props) {
-          this.prototype.__propertyInfo = props;
           this.createProperties(Object.keys(props));
         }
       }
 
+      static _ensurePropertyInfo() {
+        let proto = this.prototype;
+        if (!proto.hasOwnProperty('__propertyInfo')) {
+          const props = this.prototype.__propertyInfo = this.properties;
+          const attrInfo = this.prototype.__attributeInfo = {};
+          for (let prop in props) {
+            attrInfo[proto._attributeForProperty(prop)] = prop;
+          }
+        }
+      }
+
       /**
        * Overrides implementation in PropertiesChanged to immediately process
        * any pending changes to properties and ensure that
@@ -116,8 +144,8 @@
        * @protected
        */
       _typeForProperty(name) {
-        const props = this.__propertyInfo;
-        return props && props[name];
+        const info = this.__propertyInfo[name];
+        return info.type || info;
       }
 
       /**

lib/mixins/property-accessors.html

@@ -127,8 +127,6 @@
 
       constructor() {
         super();
-        /** @type {boolean} */
-        this.__serializing;
         /** @type {number} */
         this.__dataCounter;
       }
@@ -209,44 +207,9 @@
       }
 
       /**
-       * Deserializes an attribute to its associated property.
-       *
-       * This method calls the `_deserializeValue` method to convert the string to
-       * a typed value.
-       *
-       * @param {string} attribute Name of attribute to deserialize.
-       * @param {?string} value of the attribute.
-       * @param {*=} type type to deserialize to.
-       */
-      _attributeToProperty(attribute, value, type) {
-        // Don't deserialize back to property if currently reflecting
-        if (!this.__serializing) {
-          super._attributeToProperty(attribute, value, type);
-        }
-      }
-
-      /**
-       * Serializes a property to its associated attribute.
-       *
-       * @suppress {invalidCasts} Closure can't figure out `this` is an element.
-       *
-       * @param {string} property Property name to reflect.
-       * @param {string=} attribute Attribute name to reflect.
-       * @param {*=} value Property value to refect.
-       */
-      _propertyToAttribute(property, attribute, value) {
-        this.__serializing = true;
-        value = (arguments.length < 3) ? this[property] : value;
-        this._valueToNodeAttribute(/** @type {!HTMLElement} */(this), value,
-          attribute || this._attributeForProperty(property));
-        this.__serializing = false;
-      }
-
-      /**
-       * Sets a typed value to an HTML attribute on a node.
-       *
-       * This method calls the `_serializeValue` method to convert the typed
-       * value to a string.  If the `_serializeValue` method returns `undefined`,
+       * Overrides PropertiesChanged implementation to calls the
+       * `_serializeValue` method to convert the typed value to a string.
+       * If the `_serializeValue` method returns `undefined`,
        * the attribute will be removed (this is the default for boolean
        * type `false`).
        *
@@ -255,12 +218,7 @@
        * @param {string} attribute Attribute name to serialize to.
        */
       _valueToNodeAttribute(node, value, attribute) {
-        let str = this._serializeValue(value);
-        if (str === undefined) {
-          node.removeAttribute(attribute);
-        } else {
-          node.setAttribute(attribute, str);
-        }
+        super._valueToNodeAttribute(node, this._serializeValue(value), attribute);
       }
 
       /**