title: Get Value of Selected DropdownMenuItem in Flutter
body: <p>I am trying to build Multiple <code>DropdownButton</code> using <code>ListView.builder</code> as many time as user clicks on the float action button</p>
  
  <pre><code>new FloatingActionButton(
            onPressed: () {
              setState(() {
                counter++;
              });
            },
            child: new Icon(Icons.add),
        )
  
  new ListView.builder(
                itemBuilder: (BuildContext context, int index) {
                  return buildfields(index); },
                itemCount: counter,
                scrollDirection: Axis.vertical,
              )
  
  new DropdownButton&lt;String&gt;(
              onChanged: (String value) { setState((){
                setUn();
                _unit = value;
              });
              },
              hint: new Text('Course Unit'),
              value: _unit,
              items: &lt;String&gt;["1", "2", "3", "4", "5"].map((String value) {
                return new DropdownMenuItem&lt;String&gt;(
                  value: value,
                  child: new Text(value),
                );
              }).toList(),
            )
  </code></pre>
  
  <p>The problem am having is this, when user generates multiple <code>DropdownButton</code> and select the value for One, EVERY OTHER GENERATED <code>DropdownButton</code> CHANGES its Value to the newly selected value. How do i set a unique id for each generated <code>DropdownButton</code>?</p>