title: Flutter DropdownButton inside TextFormField as prefix
body: <p>As per title, I want to add a <code>DropdownButton</code> inside a <code>TextFormField</code> as <code>prefix</code> (or <code>prefixIcon</code>?). The text field is for amount, and the dropdown is for its currency.</p>
  
  <p>My straightforward approach works visually, but I can't tap the dropdown to show the list. Every time I tap it, it will show (and immediately hide) the keyboard for the text field itself.</p>
  
  <p>How to do it?</p>
  
  <pre><code>TextFormField(
      initialValue: '10.00',
      decoration: InputDecoration(
          prefix: DropdownButtonHideUnderline(
              child: DropdownButton(
                  items: CURRENCY_CODES,
                  onChanged: _onCurrencyChanged,
                  value: _currency,
              ),
          ),
      ),
  ),
  </code></pre>