class.html

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<link rel="import" href="legacy-element-mixin.html">
<script>

  (function() {

    'use strict';

    let LegacyElementMixin = Polymer.LegacyElementMixin;

    let metaProps = {
      attached: true,
      detached: true,
      ready: true,
      created: true,
      beforeRegister: true,
      registered: true,
      attributeChanged: true,
      // meta objects
      behaviors: true
    }

    /**
     * Applies a "legacy" behavior or array of behaviors to the provided class.
     *
     * Note: this method will automatically also apply the `Polymer.LegacyElementMixin`
     * to ensure that any legacy behaviors can rely on legacy Polymer API on
     * the underlying element.
     *
     * @param {Object|Array} behaviors Behavior object or array of behaviors
     * @param {HTMLElement} klass Element class
     * @memberof Polymer
     */
    function mixinBehaviors(behaviors, klass) {
      if (!behaviors) {
        return klass;
      }
      // NOTE: ensure the bahevior is extending a class with
      // legacy element api. This is necessary since behaviors expect to be able
      // to access 1.x legacy api.
      klass = LegacyElementMixin(klass);
      if (!Array.isArray(behaviors)) {
        behaviors = [behaviors];
      }
      let superBehaviors = klass.prototype.behaviors;
      // get flattened, deduped list of behaviors *not* already on super class
      behaviors = flattenBehaviors(behaviors, null, superBehaviors);
      // mixin new behaviors
      klass = _mixinBehaviors(behaviors, klass);
      if (superBehaviors) {
        behaviors = superBehaviors.concat(behaviors);
      }
      // Set behaviors on prototype for BC...
      klass.prototype.behaviors = behaviors;
      return klass;
    }

    // NOTE:
    // 1.x
    // Behaviors were mixed in *in reverse order* and de-duped on the fly.
    // The rule was that behavior properties were copied onto the element
    // prototype if and only if the property did not already exist.
    // Given: Polymer{ behaviors: [A, B, C, A, B]}, property copy order was:
    // (1), B, (2), A, (3) C. This means prototype properties win over
    // B properties win over A win over C. This mirrors what would happen
    // with inheritance if element extended B extended A extended C.
    //
    // Again given, Polymer{ behaviors: [A, B, C, A, B]}, the resulting
    // `behaviors` array was [C, A, B].
    // Behavior lifecycle methods were called in behavior array order
    // followed by the element, e.g. (1) C.created, (2) A.created,
    // (3) B.created, (4) element.created. There was no support for
    // super, and "super-behavior" methods were callable only by name).
    //
    // 2.x
    // Behaviors are made into proper mixins which live in the
    // element's prototype chain. Behaviors are placed in the element prototype
    // eldest to youngest and de-duped youngest to oldest:
    // So, first [A, B, C, A, B] becomes [C, A, B] then,
    // the element prototype becomes (oldest) (1) Polymer.Element, (2) class(C),
    // (3) class(A), (4) class(B), (5) class(Polymer({...})).
    // Result:
    // This means element properties win over B properties win over A win
    // over C. (same as 1.x)
    // If lifecycle is called (super then me), order is
    // (1) C.created, (2) A.created, (3) B.created, (4) element.created
    // (again same as 1.x)
    function _mixinBehaviors(behaviors, klass) {
      for (let i=0; i<behaviors.length; i++) {
        let b = behaviors[i];
        if (b) {
          klass = Array.isArray(b) ? _mixinBehaviors(b, klass) :
            GenerateClassFromInfo(b, klass);
        }
      }
      return klass;
    }

    /**
     * @param {Array} behaviors
     * @param {Array=} list
     * @param {Array=} exclude
     */
    function flattenBehaviors(behaviors, list, exclude) {
      list = list || [];
      for (let i=behaviors.length-1; i >= 0; i--) {
        let b = behaviors[i];
        if (b) {
          if (Array.isArray(b)) {
            flattenBehaviors(b, list);
          } else {
            // dedup
            if (list.indexOf(b) < 0 && (!exclude || exclude.indexOf(b) < 0)) {
              list.unshift(b);
            }
          }
        } else {
          console.warn('behavior is null, check for missing or 404 import');
        }
      }
      return list;
    }

    function GenerateClassFromInfo(info, Base) {

      let registered = false;

      class PolymerGenerated extends Base {

        static get properties() {
          return info.properties;
        }

        static get observers() {
          return info.observers;
        }

        static get template() {
          // get template first from any imperative set in `info._template`
          return info._template ||
            // next look in dom-module associated with this element's is.
            Polymer.DomModule.import(this.is, 'template') ||
            // next look for superclass template (note: use superclass symbol
            // to ensure correct `this.is`)
            Base.template ||
            // finally fall back to `_template` in element's protoype.
            this.prototype._template;
        }

        created() {
          super.created();
          if (info.created) {
            info.created.call(this);
          }
        }

        _registered() {
          if (!registered) {
            super._registered();
            // call `registered` only if it was not called for *this* constructor
            registered = true;
            if (info.registered) {
              info.registered.call(Object.getPrototypeOf(this));
            }
          }
        }

        _applyListeners() {
          super._applyListeners();
          if (info.listeners) {
            for (let l in info.listeners) {
              this._addMethodEventListenerToNode(this, l, info.listeners[l]);
            }
          }
        }

        // note: exception to "super then me" rule;
        // do work before calling super so that super attributes
        // only apply if not already set.
        _ensureAttributes() {
          if (info.hostAttributes) {
            for (let a in info.hostAttributes) {
              this._ensureAttribute(a, info.hostAttributes[a]);
            }
          }
          super._ensureAttributes();
        }

        ready() {
          super.ready();
          if (info.ready) {
            info.ready.call(this);
          }
        }

        attached() {
          super.attached();
          if (info.attached) {
            info.attached.call(this);
          }
        }

        detached() {
          super.detached();
          if (info.detached) {
            info.detached.call(this);
          }
        }

        attributeChanged(name, old, value) {
          super.attributeChanged(name, old, value);
          if (info.attributeChanged) {
            info.attributeChanged.call(this, name, old, value);
          }
       }
      }

      PolymerGenerated.generatedFrom =  info

      for (let p in info) {
        // NOTE: cannot copy `metaProps` methods onto prototype at least because
        // `super.ready` must be called and is not included in the user fn.
        if (!(p in metaProps)) {
          let pd = Object.getOwnPropertyDescriptor(info, p);
          if (pd) {
            Object.defineProperty(PolymerGenerated.prototype, p, pd);
          }
        }
      }

      return PolymerGenerated;
    }

    /**
     * Generates a class that extends `Polymer.LegacyElement` based on the
     * provided info object.  Metadata objects on the `info` object
     * (`properties`, `observers`, `listeners`, `behaviors`, `is`) are used
     * for Polymer's meta-programming systems, and any functions are copied
     * to the generated class.
     *
     * Valid "metadata" values are as follows:
     *
     * `is`: String providing the tag name to register the element under. In
     * addition, if a `dom-module` with the same id exists, the first template
     * in that `dom-module` will be stamped into the shadow root of this element,
     * with support for declarative event listeners (`on-...`), Polymer data
     * bindings (`[[...]]` and `{{...}}`), and id-based node finding into
     * `this.$`.
     *
     * `properties`: Object describing property-related metadata used by Polymer
     * features (key: property names, value: object containing property metadata).
     * Valid keys in per-property metadata include:
     * - `type` (String|Number|Object|Array|...): Used by
     *   `attributeChangedCallback` to determine how string-based attributes
     *   are deserialized to JavaScript property values.
     * - `notify` (boolean): Causes a change in the property to fire a
     *   non-bubbling event called `<property>-changed`. Elements that have
     *   enabled two-way binding to the property use this event to observe changes.
     * - `readOnly` (boolean): Creates a getter for the property, but no setter.
     *   To set a read-only property, use the private setter method
     *   `_setProperty(property, value)`.
     * - `observer` (string): Observer method name that will be called when
     *   the property changes. The arguments of the method are
     *   `(value, previousValue)`.
     * - `computed` (string): String describing method and dependent properties
     *   for computing the value of this property (e.g. `'computeFoo(bar, zot)'`).
     *   Computed properties are read-only by default and can only be changed
     *   via the return value of the computing method.
     *
     * `observers`: Array of strings describing multi-property observer methods
     *  and their dependent properties (e.g. `'observeABC(a, b, c)'`).
     *
     * `listeners`: Object describing event listeners to be added to each
     *  instance of this element (key: event name, value: method name).
     *
     * `behaviors`: Array of additional `info` objects containing metadata
     * and callbacks in the same format as the `info` object here which are
     * merged into this element.
     *
     * `hostAttributes`: Object listing attributes to be applied to the host
     *  once created (key: attribute name, value: attribute value).  Values
     *  are serialized based on the type of the value.  Host attributes should
     *  generally be limited to attributes such as `tabIndex` and `aria-...`.
     *  Attributes in `hostAttributes` are only applied if a user-supplied
     *  attribute is not already present (attributes in markup override
     *  `hostAttributes`).
     *
     * In addition, the following Polymer-specific callbacks may be provided:
     * - `registered`: called after first instance of this element,
     * - `created`: called during `constructor`
     * - `attached`: called during `connectedCallback`
     * - `detached`: called during `disconnectedCallback`
     * - `ready`: called before first `attached`, after all properties of
     *   this element have been propagated to its template and all observers
     *   have run
     *
     * @param {Object} info Object containing Polymer metadata and functions
     *   to become class methods.
     * @return {Polymer.LegacyElement} Generated class
     * @memberof Polymer
     */
    Polymer.Class = function(info) {
      if (!info) {
        console.warn('Polymer.Class requires `info` argument');
      }
      let klass = GenerateClassFromInfo(info, info.behaviors ?
        // note: mixinBehaviors ensures `LegacyElementMixin`.
        mixinBehaviors(info.behaviors, HTMLElement) :
        LegacyElementMixin(HTMLElement));
      // decorate klass with registration info
      klass.is = info.is;
      // NOTE: while we could call `beforeRegister` here to maintain
      // some BC, the state of the element at this point is not as it was in 1.0
      // In 1.0, the method was called *after* mixing prototypes together
      // but before processing of meta-objects. Since this is now done
      // in 1 step via `GenerateClassFromInfo`, this is no longer possible.
      // However, *most* work (not setting `is`) that was previously done in
      // `beforeRegister` should be possible in `registered`.
      return klass;
    }

    Polymer.mixinBehaviors = mixinBehaviors;

  })();

</script>