property-accessors.html

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<link rel="import" href="../utils/boot.html">
<link rel="import" href="../utils/mixin.html">
<link rel="import" href="../utils/case-map.html">
<link rel="import" href="../utils/async.html">

<script>
(function() {

  'use strict';

  let caseMap = Polymer.CaseMap;

  let microtask = Polymer.Async.microTask;

  // Save map of native properties; this forms a blacklist or properties
  // that won't have their values "saved" by `saveAccessorValue`, since
  // reading from an HTMLElement accessor from the context of a prototype throws
  const nativeProperties = {};
  let proto = HTMLElement.prototype;
  while (proto) {
    let props = Object.getOwnPropertyNames(proto);
    for (let i=0; i<props.length; i++) {
      nativeProperties[props[i]] = true;
    }
    proto = Object.getPrototypeOf(proto);
  }

  /**
   * Used to save the value of a property that will be overridden with
   * an accessor. If the `model` is a prototype, the values will be saved
   * in `__dataProto`, and it's up to the user (or downstream mixin) to
   * decide how/when to set these values back into the accessors.
   * If `model` is already an instance (it has a `__data` property), then
   * the value will be set as a pending property, meaning the user should
   * call `_invalidateProperties` or `_flushProperties` to take effect
   *
   * @param {Object} model Prototype or instance
   * @param {string} property Name of property
   * @private
   */
  function saveAccessorValue(model, property) {
    // Don't read/store value for any native properties since they could throw
    if (!nativeProperties[property]) {
      let value = model[property];
      if (value !== undefined) {
        if (model.__data) {
          // Adding accessor to instance; update the property
          // It is the user's responsibility to call _flushProperties
          model._setPendingProperty(property, value);
        } else {
          // Adding accessor to proto; save proto's value for instance-time use
          if (!model.__dataProto) {
            model.__dataProto = {};
          } else if (!model.hasOwnProperty(JSCompiler_renameProperty('__dataProto', model))) {
            model.__dataProto = Object.create(model.__dataProto);
          }
          model.__dataProto[property] = value;
        }
      }
    }
  }

  /**
   * Element class mixin that provides basic meta-programming for creating one
   * or more property accessors (getter/setter pair) that enqueue an async
   * (batched) `_propertiesChanged` callback.
   *
   * For basic usage of this mixin, simply declare attributes to observe via
   * the standard `static get observedAttributes()`, implement `_propertiesChanged`
   * on the class, and then call `MyClass.createPropertiesForAttributes()` once
   * on the class to generate property accessors for each observed attribute
   * prior to instancing.  Last, call `this._flushProperties()` once to enable
   * the accessors.
   *
   * Any `observedAttributes` will automatically be
   * deserialized via `attributeChangedCallback` and set to the associated
   * property using `dash-case`-to-`camelCase` convention.
   *
   * @polymerMixin
   * @memberof Polymer
   * @summary Element class mixin for reacting to property changes from
   *   generated property accessors.
   */
  Polymer.PropertyAccessors = Polymer.dedupingMixin(superClass => {

    /**
     * @polymerMixinClass
     * @implements {Polymer_PropertyAccessors}
     * @unrestricted
     */
    class PropertyAccessors extends superClass {

      /**
       * Generates property accessors for all attributes in the standard
       * static `observedAttributes` array.
       *
       * Attribute names are mapped to property names using the `dash-case` to
       * `camelCase` convention
       *
       */
      static createPropertiesForAttributes() {
        let a$ = this.observedAttributes;
        for (let i=0; i < a$.length; i++) {
          this.prototype._createPropertyAccessor(caseMap.dashToCamelCase(a$[i]));
        }
      }

      constructor() {
        super();
        this._initializeProperties();
      }

      attributeChangedCallback(name, old, value) {
        if (old !== value) {
          this._attributeToProperty(name, value);
        }
      }

      /**
       * Initializes the local storage for property accessors.
       *
       * Provided as an override point for performing any setup work prior
       * to initializing the property accessor system.
       *
       * @protected
       */
      _initializeProperties() {
        this.__serializing = false;
        this.__dataCounter = 0;
        this.__dataEnabled = false;
        this.__dataReady = false;
        this.__dataInvalid = false;
        // initialize data with prototype values saved when creating accessors
        this.__data = {};
        this.__dataPending = null;
        this.__dataOld = null;
        if (this.__dataProto) {
          this._initializeProtoProperties(this.__dataProto);
          this.__dataProto = null;
        }
        // Capture instance properties; these will be set into accessors
        // during first flush. Don't set them here, since we want
        // these to overwrite defaults/constructor assignments
        for (let p in this.__dataHasAccessor) {
          if (this.hasOwnProperty(p)) {
            this.__dataInstanceProps = this.__dataInstanceProps || {};
            this.__dataInstanceProps[p] = this[p];
            delete this[p];
          }
        }
      }

      /**
       * Called at instance time with bag of properties that were overwritten
       * by accessors on the prototype when accessors were created.
       *
       * The default implementation sets these properties back into the
       * setter at instance time.  This method is provided as an override
       * point for customizing or providing more efficient initialization.
       *
       * @param {Object} props Bag of property values that were overwritten
       *   when creating property accessors.
       * @protected
       */
      _initializeProtoProperties(props) {
        for (let p in props) {
          this._setProperty(p, props[p]);
        }
      }

      /**
       * Called at ready time with bag of instance properties that overwrote
       * accessors when the element upgraded.
       *
       * The default implementation sets these properties back into the
       * setter at ready time.  This method is provided as an override
       * point for customizing or providing more efficient initialization.
       *
       * @param {Object} props Bag of property values that were overwritten
       *   when creating property accessors.
       * @protected
       */
      _initializeInstanceProperties(props) {
        Object.assign(this, props);
      }

      /**
       * Ensures the element has the given attribute. If it does not,
       * assigns the given value to the attribute.
       *
       *
       * @param {string} attribute Name of attribute to ensure is set.
       * @param {string} value of the attribute.
       */
      _ensureAttribute(attribute, value) {
        if (!this.hasAttribute(attribute)) {
          this._valueToNodeAttribute(this, value, attribute);
        }
      }

      /**
       * 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) {
          let property = caseMap.dashToCamelCase(attribute);
          this[property] = this._deserializeValue(value, type);
        }
      }

      /**
       * Serializes a property to its associated attribute.
       *
       * @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(this, value,
          attribute || caseMap.camelToDashCase(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`,
       * the attribute will be removed (this is the default for boolean
       * type `false`).
       *
       * @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) {
        let str = this._serializeValue(value);
        if (str === undefined) {
          node.removeAttribute(attribute);
        } else {
          node.setAttribute(attribute, str);
        }
      }

      /**
       * Converts a typed JavaScript value to a string.
       *
       * This method is called by Polymer when setting JS property values to
       * HTML attributes.  Users may override this method on Polymer element
       * prototypes to provide serialization for custom types.
       *
       * @param {*} value Property value to serialize.
       * @return {string | undefined} String serialized from the provided property value.
       */
      _serializeValue(value) {
        /* eslint-disable no-fallthrough */
        switch (typeof value) {
          case 'boolean':
            return value ? '' : undefined;

          case 'object':
            if (value instanceof Date) {
              return value.toString();
            } else if (value) {
              try {
                return JSON.stringify(value);
              } catch(x) {
                return '';
              }
            }

          default:
            return value != null ? value.toString() : undefined;
        }
      }

      /**
       * Converts a string to a typed JavaScript value.
       *
       * This method is called by Polymer when reading HTML attribute values to
       * JS properties.  Users may override this method on Polymer element
       * prototypes to provide deserialization for custom `type`s.  Note,
       * the `type` argument is the value of the `type` field provided in the
       * `properties` configuration object for a given property, and is
       * by convention the constructor for the type to deserialize.
       *
       * Note: The return value of `undefined` is used as a sentinel value to
       * indicate the attribute should be removed.
       *
       * @param {string} value Attribute value to deserialize.
       * @param {*} type Type to deserialize the string to.
       * @return {*} Typed value deserialized from the provided string.
       */
      _deserializeValue(value, type) {
        /**
         * @type {*}
         */
        let outValue;
        switch (type) {
          case Number:
            outValue = Number(value);
            break;

          case Boolean:
            if (value === null) {
              outValue = false;
            } else if (value.toLowerCase() === "true") {
              outValue = true;
            } else if (value.toLowerCase() === "false") {
              outValue = false;
            } else {
              outValue = false;
            }
            break;

          case Object:
            try {
              outValue = JSON.parse(value);
            } catch(x) {
              // allow non-JSON literals like Strings and Numbers
            }
            break;

          case Array:
            try {
              outValue = JSON.parse(value);
            } catch(x) {
              outValue = null;
              console.warn(`Polymer::Attributes: couldn't decode Array as JSON: ${value}`);
            }
            break;

          case Date:
            outValue = new Date(value);
            break;

          case String:
          default:
            outValue = value;
            break;
        }

        return outValue;
      }
      /* eslint-enable no-fallthrough */

      /**
       * Creates a setter/getter pair for the named property with its own
       * local storage.  The getter returns the value in the local storage,
       * and the setter calls `_setProperty`, which updates the local storage
       * for the property and enqueues a `_propertiesChanged` callback.
       *
       * This method may be called on a prototype or an instance.  Calling
       * this method may overwrite a property value that already exists on
       * the prototype/instance by creating the accessor.  When calling on
       * a prototype, any overwritten values are saved in `__dataProto`,
       * and it is up to the subclasser to decide how/when to set those
       * properties back into the accessor.  When calling on an instance,
       * the overwritten value is set via `_setPendingProperty`, and the
       * user should call `_invalidateProperties` or `_flushProperties`
       * for the values to take effect.
       *
       * @param {string} property Name of the property
       * @param {boolean=} readOnly When true, no setter is created; the
       *   protected `_setProperty` function must be used to set the property
       * @protected
       */
      _createPropertyAccessor(property, readOnly) {
        if (!this.hasOwnProperty('__dataHasAccessor')) {
          this.__dataHasAccessor = Object.assign({}, this.__dataHasAccessor);
        }
        if (!this.__dataHasAccessor[property]) {
          this.__dataHasAccessor[property] = true;
          saveAccessorValue(this, property);
          Object.defineProperty(this, property, {
            get: function() {
              return this.__data[property];
            },
            set: readOnly ? function() { } : function(value) {
              this._setProperty(property, value);
            }
          });
        }
      }

      /**
       * Returns true if this library created an accessor for the given property.
       *
       * @param {string} property Property name
       * @return {boolean} True if an accessor was created
       */
      _hasAccessor(property) {
        return this.__dataHasAccessor && this.__dataHasAccessor[property];
      }

      /**
       * Updates the local storage for a property (via `_setPendingProperty`)
       * and enqueues a `_proeprtiesChanged` callback.
       *
       * @param {string} property Name of the property
       * @param {*} value Value to set
       * @protected
       */
      _setProperty(property, value) {
        if (this._setPendingProperty(property, value)) {
          this._invalidateProperties();
        }
      }

      /**
       * Updates the local storage for a property, records the previous value,
       * and adds it to the set of "pending changes" that will be passed to the
       * `_propertiesChanged` callback.  This method does not enqueue the
       * `_propertiesChanged` callback.
       *
       * @param {string} property Name of the property
       * @param {*} value Value to set
       * @return {boolean} Returns true if the property changed
       * @protected
       */
      _setPendingProperty(property, value) {
        let old = this.__data[property];
        if (this._shouldPropertyChange(property, value, old)) {
          if (!this.__dataPending) {
            this.__dataPending = {};
            this.__dataOld = {};
          }
          // Ensure old is captured from the last turn
          if (!(property in this.__dataOld)) {
            this.__dataOld[property] = old;
          }
          this.__data[property] = value;
          this.__dataPending[property] = value;
          return true;
        }
      }

      /**
       * Returns true if the specified property has a pending change.
       *
       * @param {string} prop Property name
       * @return {boolean} True if property has a pending change
       * @protected
       */
      _isPropertyPending(prop) {
        return this.__dataPending && (prop in this.__dataPending);
      }

      /**
       * Marks the properties as invalid, and enqueues an async
       * `_propertiesChanged` callback.
       *
       * @protected
       */
      _invalidateProperties() {
        if (!this.__dataInvalid && this.__dataReady) {
          this.__dataInvalid = true;
          microtask.run(() => {
            if (this.__dataInvalid) {
              this.__dataInvalid = false;
              this._flushProperties();
            }
          });
        }
      }

      /**
       * Call to enable property accessor processing. Before this method is
       * called accessor values will be set but side effects are
       * queued. When called, any pending side effects occur immediately.
       * For elements, generally `connectedCallback` is a normal spot to do so.
       * It is safe to call this method multiple times as it only turns on
       * property accessors once.
       */
      _enableProperties() {
        if (!this.__dataEnabled) {
          this.__dataEnabled = true;
          if (this.__dataInstanceProps) {
            this._initializeInstanceProperties(this.__dataInstanceProps);
            this.__dataInstanceProps = null;
          }
          this.ready()
        }
      }

      /**
       * Calls the `_propertiesChanged` callback with the current set of
       * pending changes (and old values recorded when pending changes were
       * set), and resets the pending set of changes. Generally, this method
       * should not be called in user code.
       *
       *
       * @protected
       */
      _flushProperties() {
        if (this.__dataPending) {
          let changedProps = this.__dataPending;
          this.__dataPending = null;
          this.__dataCounter++;
          this._propertiesChanged(this.__data, changedProps, this.__dataOld);
          this.__dataCounter--;
        }
      }

      /**
       * Lifecycle callback called the first time properties are being flushed.
       * Prior to `ready`, all property sets through accessors are queued and
       * their effects are flushed after this method returns.
       *
       * Users may override this function to implement behavior that is
       * dependent on the element having its properties initialized, e.g.
       * from defaults (initialized from `constructor`, `_initializeProperties`),
       * `attributeChangedCallback`, or values propagated from host e.g. via
       * bindings.  `super.ready()` must be called to ensure the data system
       * becomes enabled.
       *
       * @public
       */
      ready() {
        this.__dataReady = true;
        // Run normal flush
        this._flushProperties();
      }

      /**
       * Callback called when any properties with accessors created via
       * `_createPropertyAccessor` have been set.
       *
       * @param {Object} currentProps Bag of all current accessor values
       * @param {Object} changedProps Bag of properties changed since the last
       *   call to `_propertiesChanged`
       * @param {Object} oldProps Bag of previous values for each property
       *   in `changedProps`
       * @protected
       */
      _propertiesChanged(currentProps, changedProps, oldProps) { // eslint-disable-line no-unused-vars
      }

      /**
       * Method called to determine whether a property value should be
       * considered as a change and cause the `_propertiesChanged` callback
       * to be enqueued.
       *
       * The default implementation returns `true` for primitive types if a
       * strict equality check fails, and returns `true` for all Object/Arrays.
       * The method always returns false for `NaN`.
       *
       * Override this method to e.g. provide stricter checking for
       * Objects/Arrays when using immutable patterns.
       *
       * @param {string} property Property name
       * @param {*} value New property value
       * @param {*} old Previous property value
       * @return {boolean} Whether the property should be considered a change
       *   and enqueue a `_proeprtiesChanged` callback
       * @protected
       */
      _shouldPropertyChange(property, value, old) {
        return (
          // Strict equality check
          (old !== value &&
           // This ensures (old==NaN, value==NaN) always returns false
           (old === old || value === value))
        );
      }

    }

    return PropertyAccessors;

  });

})();
</script>