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polymerstatepolymer-2.xiron-elements

Using iron-meta to manage state in Polymer 2.x

I am trying to use iron-meta to manage state in a Polymer app. Here is my Plunker demo.

On that same Plunker, I am also using Mixins to manage state. (So we can compare the two systems.)

Desired Behavior

On the demo, at the bottom, I expect the part that follows "Meta says:" to match and track the part that follows "Send notifications option is:" when the options checkbox at the top labeled "Send Notifications" is clicked.

Actual Behavior

But instead of that desired behavior, the actual behavior I get is the "Meta says:" section never populates with any data.

Demo

Plunker Demo

Demo screen recording

iron-meta demo

my-view2.html 
<link rel="import" href="my-options.html">

<base href="https://polygit.org/polymer+v2.0.0/shadycss+webcomponents+1.0.0/components/">
<link rel="import" href="polymer/polymer-element.html">
<link rel="import" href="iron-meta/iron-meta.html">

<dom-module id="my-view2">
  <template>
    <style>
      :host {
        display: block;
        padding: 10px;
      }
    </style>

    <iron-meta key="meta" value="{{meta}}"></iron-meta>

    <my-options></my-options>

    <div class="card">
      <div class="circle">2</div>
      <h1>View Two</h1>
      <p>Ea duis bonorum nec, falli paulo aliquid ei eum.</p>
      <p>Id nam odio natum malorum, tibique copiosae expetenda mel ea.Detracto suavitate repudiandae no eum. Id adhuc minim soluta nam.Id nam odio natum malorum, tibique copiosae expetenda mel ea.</p>

      <p>Send notifications option is: <b>[[ options.subscribe ]]</b></p>
      <p>Meta says: <b>[[ meta ]]</b></p>
    </div>
  </template>

  <script>
    class MyView2 extends MyOptionsMixin(Polymer.Element) {
      static get is() {
        return 'my-view2';
      }
    }

    window.customElements.define(MyView2.is, MyView2);
  </script>
</dom-module>
my-options.html 
<base href="https://polygit.org/polymer+v2.0.0/shadycss+webcomponents+1.0.0/components/">
<link rel="import" href="polymer/polymer-element.html">
<link rel="import" href="paper-checkbox/paper-checkbox.html">
<link rel="import" href="iron-meta/iron-meta.html">

<dom-module id="my-options">
  <template>
    <style>
      :host {
        display: block;
        padding: 16px;
      }
      h3, p {
        margin: 8px 0;
      }
    </style>

    <iron-meta key="meta" value="[[options.subscribe]]"></iron-meta>

    <h3>Options</h3>
    <p>
      <paper-checkbox checked="{{ options.subscribe }}">Send Notifications</paper-checkbox>
    </p>
  </template>

  <script>
    (function() {

      let optionsInstance = null;

      class MyOptions extends Polymer.Element {
        static get is() { return 'my-options'; }

        static get properties() {
          return {
            options: {
              type: Object,
              value: () => ({
                subscribe: false
              })
            },
            subscribers: {
              type: Array,
              value: () => []
            }
          }
        }

        static get observers() {
          return [
            'optionsChanged(options.*)'
          ]
        }

        constructor() {
          super();

          if (!optionsInstance) optionsInstance = this;
        }

        register(subscriber) {
          this.subscribers.push(subscriber);
          subscriber.options = this.options;
          subscriber.notifyPath('options');
        }

        unregister(subscriber) {
          var i = this.subscribers.indexOf(subscriber);
          if (i > -1) this.subscribers.splice(i, 1)
        }

        optionsChanged(change) {
          for(var i = 0; i < this.subscribers.length; i++) {
            this.subscribers[i].notifyPath(change.path);
          }
        }
      }

      window.customElements.define(MyOptions.is, MyOptions);

      MyOptionsMixin = (superClass) => {
        return class extends superClass {
          static get properties() {
            return {
              options: {
                type: Object
              }
            }
          }

          connectedCallback() {
            super.connectedCallback();
            optionsInstance.register(this);
          }

          disconnectedCallback() {
            super.disconnectedCallback();
            optionsInstance.unregister(this);
          }
        }
      }
    }());
  </script>
</dom-module>
Solution

  • The short answer is: <iron-meta> doesn't have notification support. https://github.com/PolymerElements/iron-meta/issues/9

    Your example is relying on the fact the two-way bindings work and your meta property gets updated when the observed key changes. This is not the case. Internally, values are simply assigned to a global store, and there is not publish/subscribe mechanism in-place. https://github.com/PolymerElements/iron-meta/blob/master/iron-meta.html#L91

    If you're using Polymer 1.x, maybe this element will satisfy your needs: https://www.webcomponents.org/element/trofrigo/global-variable