title: How to call CupertinoPicker
body: <p>I would like to show CupertinoPicker, when some button is pressed.
  However I have no idea where I should implement CupertinoPicker.</p>
  
  <p>Following code is sample of code which I don't know where I write to call.</p>
  
  <pre><code>showCupertinoModalPopup&lt;String&gt;(
    context: context,
    builder:(BuildContext context){
      return _buildBottomPicker(CupertinoPicker(
        onSelectedItemChanged: (value){
          setState((){
            selectedValue = value;
          });
        },
        itemExtent: 32.0,
        children: const[
          Text('Item01'),
          Text('Item02'),
          Text('Item03'),
        ],
      ));
    },
  );
  </code></pre>