title: Flutter, checkbox state doesn't change, how to?
body: <p>I have a <code>ListView</code> builder that creates a few <code>ListTitle</code>'s with a checkbox inside them.</p>
  
  <p>when I <code>setState</code> on the <code>onChanged</code> on a checkbox, the value doesn't seem to change.</p>
  
  <pre><code>class ProjectPage extends StatefulWidget {
    final project;
  
    ProjectPage({Key key, this.project}) : super(key: key);
  
    @override
    _ProjectPageState createState() =&gt; new _ProjectPageState();
  }
  
  class _ProjectPageState extends State&lt;ProjectPage&gt; {
    @override
    Widget build(BuildContext context) {
      return new Scaffold(
        body: new Container(
          child: Column(
            children: &lt;Widget&gt;[
              new Expanded(
                child: new ListView.builder(
                  itemBuilder: (BuildContext context, int index) =&gt; new ItemsItem(item: widget.project.items[index]),
                  itemCount: widget.project.items.length,
                ),
              ),
            ],
          ),
        ),
      );
    }
  }
  
  class ItemsItem extends StatefulWidget {
    final item;
  
    ItemsItem({Key key, this.item}) : super(key: key);
  
    @override
    _ItemsItemState createState() =&gt; new _ItemsItemState();
  }
  
  class _ItemsItemState extends State&lt;ItemsItem&gt; {
    final GlobalKey&lt;ScaffoldState&gt; _mainState = new GlobalKey&lt;ScaffoldState&gt;();
  
    @override
    Widget build(BuildContext context) {
      bool _isCompleted = widget.item.isCompleted;
  
      return new ListTile(
        key: _mainState,
        title: new Row(
          children: &lt;Widget&gt;[
            new Expanded(child: new Text(widget.item.name)),
            new Checkbox(
              value: _isCompleted,
              onChanged: (bool newValue) {
                setState(() {
                  _isCompleted = newValue;
                });
              },
            ),
          ],
        ),
      );
    }
  }
  </code></pre>
  
  <p>this doesn't seem to change the value</p>
  
  <pre><code>setState(() {
      _isCompleted = newValue;
  });
  </code></pre>
  
  <p>any ideas?</p>
  
  <p>edit: Item class</p>
  
  <pre><code>class Item {
    final int id;
    final String name;
    final bool isCompleted;
  
    Item({
      this.id,
      this.name,
      this.isCompleted,
    });
  
    Item.fromJson(Map json)
        : id = json['id'],
          name = json['name'],
          isCompleted = json['isCompleted'],
  
    set isCompleted(bool value) {
      isCompleted = value;
    }
  }
  </code></pre>