Polymer Elements ShadowDOM child nodes - Setting style attributes dynamically

Question

I'd like some ideas on how to set a font-size to a <paper-textarea>, dynamically (i.e set font-size from JS instead of CSS).

• There's no default attribute in the docs for font-size.
• It looks like it can only be set via a mixin (which cannot be changed via JS).

For example, <paper-textarea>'s DOM tree looks something like this:

• <paper-textarea>
  • ShadowDOM begins
  • <div>
  • <div>
  • <div>
  • <textarea> <-- This is the actual textarea

Is there any way to target the child-node in it's shadowDOM via JS and set it's style directly?

An example, which of course doesn't work.

• I bind property fontSize to the <paper-textarea>
• Since there's no attribute fontSize to set for this particular element, it doesn't work.

<base href="https://polygit.org/components/">
<script src="webcomponentsjs/webcomponents-lite.min.js"></script>
<link href="polymer/polymer.html" rel="import">
<link href="paper-input/paper-textarea.html" rel="import">
<link href="paper-button/paper-button.html" rel="import">

<dom-module id="x-example">
  <style>
    :host {
      display: block;
      max-width: 320px;
    }
  </style>
  <template>
      <paper-textarea label="Textarea" value="{{inputData}}" font-size="{{fontSize}}"></paper-textarea>
    <paper-button class="primary" on-tap="incrementFontSize">Increment Font Size</paper-button>
  </template>
<script>
  HTMLImports.whenReady(function() {
    "use strict";

    Polymer({

      is: "x-example",
      properties: {
      	inputData: {
          type: String,
          value: "Hello World"
        },
        
        fontSize: {
          type: Number,
          value: 16
        }
      },
      
      incrementFontSize: function() {
        this.set("fontSize", this.fontSize + 4);  
      }

    });
  });
</script>

</dom-module>

<x-example></x-example>

Ideally, I would set up a fontSize observer and imperatively target the <paper-textarea>'s child node to set it's style when fontSize changes.

How can I target those nodes?

Note: I'd prefer an "official" way, if it exists. The Polymer spec changes often and flimsy hacks would tend to break when updating the library.

Answer 1

Here is one way of doing it:

<dom-module id="dynamic-style">
 <template>
  <style>
   :host{
    --myfont:10px; 
   }
  paper-textarea{
    --paper-input-container-input:{
     font-size:var(--my-font);
   };
 }
 </style>
 <paper-textarea on-tap="updateStyle"></paper-textarea>
</template>
</dom-module>
<script>
 Polymer({
  is:"dynamic-style",
   properties:{
    fontSize:{
     type:Number,
     value:10
    }
   },
   updateStyle:function(){
    var style={'--my-font':this.computeFont()+'px'}
    this.updateStyles(style);
   },
   computeFont:function(){
    return this.fontSize+=4;
   }
 })
</script>

Dynamic styling is currently an issue with Polymer and as per recommendation updateStyles is the only method through which it will work.

And here's a working snippet based on your example:

<base href="https://polygit.org/components/">
<script src="webcomponentsjs/webcomponents-lite.min.js"></script>
<link href="polymer/polymer.html" rel="import">
<link href="paper-input/paper-textarea.html" rel="import">
<link href="paper-button/paper-button.html" rel="import">

<dom-module id="x-example">
  <style is="custom-style">
    :host {
      display: block;
      max-width: 320px;
    }
    :root {
      --font-size: 12px;
      --paper-input-container-input: {
        font-size: var(--font-size);
      }
    }
  </style>
  <template>
    <paper-textarea label="Textarea" value="{{inputData}}"></paper-textarea>
    <paper-button class="primary" on-tap="incrementFontSize">Increment Font Size</paper-button>
  </template>
  <script>
    HTMLImports.whenReady(function() {
      "use strict";

      Polymer({

        is: "x-example",
        properties: {
          inputData: {
            type: String,
            value: "Hello World"
          },

          fontSize: {
            type: Number,
            value: 16
          }
        },

        incrementFontSize: function() {
          var style = {
            '--font-size': this.computeFont() + 'px'
          }
          this.updateStyles(style);
        },

        computeFont: function() {
          return this.fontSize += 4;
        }

      });
    });
  </script>

</dom-module>

<x-example></x-example>