title: AnimatedSwitcher does not animate
body: <p>I'm trying to make a news section in my app. In this page that's gonna display the news, i want to be able to click anywhere on the page and get the news that is next in my list. So far no problem with that, but i wanted it to have a nice animation so i tried implementing AnimatedSwitcher, but i can't figure out why there is no animation showing.</p>
  
  <p>I tried changing the hierarchy of my code. Putting the gesture detector inside the animated switcher and the other way around. Letting the main container outside or inside of it too. I tried an animation builder that would scale it just in case it wasnt obvious enough but nothing. Tried changing the duration too but that wasn't it.</p>
  
  <pre><code>class ShowNews extends StatefulWidget {
    @override
    _ShowNewsState createState() =&gt; _ShowNewsState();
  }
  
  class _ShowNewsState extends State&lt;ShowNews&gt; {
    List&lt;News&gt; _news = [
      News(title: 'OYÉ OYÉ', desc: 'bla bla bla bla bla'),
      News(title: 'another one', desc: 'plus de bout d\'histoire'),
      News(title: 'boum', desc: 'attention à l\'accident'),
      News(title: 'Lorem ipsum', desc: 'Lorem ipsum in doloris'),
    ];
  
    int _currentIndex = 0;
  
    @override
    Widget build(BuildContext context) {
      return GestureDetector(
        onTap: () {
          setState(() {
            if (_currentIndex &lt; _news.length - 1) {
              _currentIndex++;
            } else {
              _currentIndex = 0;
            }
          });
        },
        child: Container(
          height: 160,
          padding: EdgeInsets.all(20.0),
          decoration: BoxDecoration(
            color: Colors.white,
            borderRadius: BorderRadius.only(
              topLeft: Radius.circular(20.0),
              topRight: Radius.circular(20.0),
            ),
          ),
          child: AnimatedSwitcher(
            duration: Duration(seconds: 5),
            child: ColumnArticle(_news, _currentIndex),
          ),
        ),
      );
    }
  }
  </code></pre>
  
  <p>Everything is working fine but the animation.</p>
  
  <p>Edit: I tried adding a key to make it different but still no animation.</p>
  
  <pre><code>class ColumnArticle extends StatelessWidget {
    final List&lt;News&gt; _news;
    final int _currentIndex;
  
    ColumnArticle(this._news, this._currentIndex);
  
    @override
    Widget build(BuildContext context) {
      return Column(
        key: ValueKey&lt;int&gt;(_currentIndex),
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: &lt;Widget&gt;[
          Text(
            _news[_currentIndex].title,
            textAlign: TextAlign.left,
            style: TextStyle(
              fontSize: 20.0,
            ),
          ),
          SizedBox(
            height: 10.0,
          ),
          Text(
            _news[_currentIndex].desc,
            style: TextStyle(
              fontSize: 14.0,
            ),
          ),
        ],
      );
    }
  }
  </code></pre>