polymer polymer-1.0 jsbin polymer-1.x dom-repeat

Polymer 1.x: Applying iron-media-query inside dom-repeat to determine screen width

I want to calculate the screen width and assign that result to a number between, say, 0 and 3, for example. I am trying to use iron-media-query to do this.

I expect to see (logged to the console) one of the queries return a value of true and the other three to return false. However, they all return a value of undefined.

What am I doing wrong?

FYI, After I solve this, I plan to add a conditional to the queryResult() method to store the value of index when queryMatches is true.

e.g.

queryResult: function() {
  ...
  if(this.queryMatches) {
    this.set('mediaWidth', index);
  }
  ...
}

http://jsbin.com/kamusivebi/1/edit?html,console,output

<!doctype html>
<head>
  <meta charset="utf-8">
  <base href="https://polygit.org/components/">
  <script src="webcomponentsjs/webcomponents-lite.min.js"></script>
  <link href="polymer/polymer.html" rel="import">
</head>
<body>

<dom-module id="x-element">

<template>
  <style></style>

  <template is="dom-repeat"
            id="page"
            items="[[queries]]"
            >
    <iron-media-query id="query"
                      full
                      query="[[item]]"
                      query-matches="{{queryMatches}}"
                      >
      <span hidden>{{queryResult(index)}}</span>
    </iron-media-query>
  </template>

</template>

<script>
  (function(){
    Polymer({
      is: "x-element",
      properties: {
        queryMatches: Boolean,
        queries: {
          type: Object,
          value: function() {
            return [
              '(max-width:  699px)'                         ,
              '(min-width:  700px) AND (max-width:  899px)' ,
              '(min-width:  900px) AND (max-width: 1124px)' ,
              '(min-width: 1125px)'                         ,
            ];
          },
        },
      },
      queryResult: function(index) {
        this.set('mediaWidth', index);
        console.log('mediaWidth', this.mediaWidth);
        console.log('queryMatches', this.queryMatches);
      },
    });
  })();
</script>

</dom-module>

<x-element></x-element>

</body>

Solution

Two problems:

You're incorrectly binding the results of each iron-media-query to a single queryMatches property, so each iteration would overwrite the results of the previous iteration. To correct this, you could bind to a sub-property of the item iterator (e.g., item.queryMatches), which requires changing the type of queries from a String array into an Object array. Then, you could pass item.queryMatches to queryResult().
Your demo is missing the HTML import for <iron-media-query>, so queryMatches would always be undefined.

Here's a demo with the corrections:

<head>
  <meta charset="utf-8">
  <base href="https://polygit.org/polymer+:1.7.0/components/">
  <script src="webcomponentsjs/webcomponents-lite.min.js"></script>
  <link href="polymer/polymer.html" rel="import">
  <link href="iron-media-query/iron-media-query.html" rel="import">
</head>
<body>
  <dom-module id="x-element">
    <template>
      <template is="dom-repeat"
                id="page"
                items="[[queries]]"
                >
        <iron-media-query id="query"
                          full
                          query="[[item.q]]"
                          query-matches="{{item.queryMatches}}"
                          >
        </iron-media-query>
        <span hidden>[[queryResult(index, item.queryMatches)]]</span>
        <div>[[item.q]]: [[item.queryMatches]]</div>
      </template>
    </template>

    <script>
      HTMLImports.whenReady(function() {
        Polymer({
          is: 'x-element',
          properties: {
            queries: {
              type: Object,
              value: function() {
                return [
                  { q: '(max-width:  699px)'                         },
                  { q: '(min-width:  700px) AND (max-width:  899px)' },
                  { q: '(min-width:  900px) AND (max-width: 1124px)' },
                  { q: '(min-width: 1125px)'                         },
                ];
              },
            },
          },
          queryResult: function(index, queryMatches) {
            console.log('index', index, 'queryMatches', queryMatches);
          }
        });
      });
    </script>

  </dom-module>

  <x-element></x-element>

</body>

codepen