title: backbone-forms with conditional fields
body: <p>First of all thanks to the guys of <a href="https://github.com/powmedia/backbone-forms" rel="noreferrer">backbone-forms</a> who made a tool which perfectly integrates in the backbone.js framework.</p>
  
  <p>I'm using backbone.js with the backbone-forms plugin, but I need to make conditional fields.</p>
  
  <p>Let's say I've the following form.
  I want to show (or not) a single line input with thext or a textarea according to the value selected in the <code>select</code>.</p>
  
  <pre><code>&lt;form method="post" action=""&gt;                  
      &lt;select &gt; 
          &lt;option value="" selected="selected"&gt;choose one&lt;/option&gt;
          &lt;option value="1" &gt;line&lt;/option&gt;
          &lt;option value="2" &gt;area&lt;/option&gt;
      &lt;/select&gt;
      &lt;input id="element_1" /&gt; 
      &lt;textarea id="element_2" &gt;&lt;/textarea&gt; 
  &lt;/form&gt; 
  </code></pre>
  
  <p>A behaviour like this one is implemented by default in backbone?</p>
  
  <p>If not, how can I implement it with javascript and backone-forms?</p>
  
  <p>thanks.</p>