第一个列表滚动在聚合物中结束后，铁滚动阈值永远不会执行

我已经使用以下方法完成了iron-list and iron-scroll-threshold为了在聚合物中实现无限滚动。但问题是 loadMoreDatairon-scroll-threshold在聚合物中第一个列表滚动结束后永远不会执行。请帮助我的代码中缺少或错误的一项。谢谢。

<template>

    <iron-ajax id="ajax" 
      url="https://randomuser.me/api/" 
      handle-as="json" 
      params='{"results": 20}'
      loading="{{loadingPeople}}"
      on-response="categoriesLoaded">
    </iron-ajax>

    <app-toolbar>
      <div main-title>Load data using iron-scroll-threshold</div>
    </app-toolbar>

    <iron-scroll-threshold on-lower-threshold="loadMoreData" id="threshold">
      <iron-list id="list" items="[[categoriesJobs]]" as="person" scroll-target="threshold">
        <template>
          <div>
            <div class="personItem" tabindex$="[[tabIndex]]">
              <iron-image class="avatar" sizing="contain" src="[[person.picture.medium]]"></iron-image>
              <div class="pad">
                <div class="primary">[[person.name.first]] [[person.name.last]]</div>
                <address>
                  [[person.location.street]] [[person.city]] <br />
                  [[person.location.city]], [[person.location.state]] [[person.location.postcode]]
                </address>
              </div>
            </div>
          </div>
        </template>
      </iron-list>
    </iron-scroll-threshold>

  </template>

  <script>
    Polymer({
      is: 'job-category',
      attached: function () {
        this.companyDetail = [];
        this.categoriesJobs = [];
      },
      loadMoreData: function () {
        this.$.ajax.generateRequest();
      },
      categoriesLoaded: function (e) {
        var people = e.detail.response.results;
        this.categoriesJobs = people;
        this.$.threshold.clearTriggers();
      }
    });
  </script>

</dom-module>

问题在于iron-list， 来自docs https://elements.polymer-project.org/elements/iron-list:

iron-list 必须明确调整大小，或者将滚动委托给明确调整大小的父级......

在那里您可以找到不同的方法来做到这一点，但下面您可以看到一个有效的示例。在这个例子中我还修复了categoriesLoaded函数，因此它会加载更多数据，而不仅仅是替换旧数据。

JSBin 示例 http://jsbin.com/turecox/edit?html,console,output

我还在这里添加了代码：

<dom-module id="test-app">
  <template>

    <style>

      :host {

      }

      app-toolbar {
        height: 64px;
        background-color: grey;
      }

      iron-list {
        flex: 1 1 auto;
      }

      .container {
        height: calc(100vh - 64px);
        display: flex;
        flex-direction: column;
      }

    </style>

    <iron-ajax id="ajax" 
      url="https://randomuser.me/api/" 
      handle-as="json" 
      params='{"results": 20}'
      loading="{{loadingPeople}}"
      on-response="categoriesLoaded">
    </iron-ajax>

    <app-toolbar>
      <div main-title>Load data using iron-scroll-threshold</div>
    </app-toolbar>

    <div class="container">
      <iron-scroll-threshold on-lower-threshold="loadMoreData" id="threshold">
        <iron-list id="list" items="[[categoriesJobs]]" as="person" scroll-target="threshold">
          <template>
            <div>
              <div class="personItem" tabindex$="[[tabIndex]]">
                <iron-image class="avatar" sizing="contain" src="[[person.picture.medium]]"></iron-image>
                <div class="pad">
                  <div class="primary">[[person.name.first]] [[person.name.last]]</div>
                  <address>
                    [[person.location.street]] [[person.city]] <br />
                    [[person.location.city]], [[person.location.state]] [[person.location.postcode]]
                  </address>
                </div>
              </div>
            </div>
          </template>
        </iron-list>
      </iron-scroll-threshold>
    </div>

  </template>
  <script>
    Polymer({

      is: 'test-app',

      attached: function() {
        this.companyDetail = [];
        this.categoriesJobs = [];
      },

      loadMoreData: function () {
        console.log('load more data');
        this.$.ajax.generateRequest();
      },

      categoriesLoaded: function (e) {
        var self = this;
        var people = e.detail.response.results;
        people.forEach(function(element) {
          self.push('categoriesJobs', element);
        });
        this.$.threshold.clearTriggers();
      }

    });
  </script>
</dom-module>