title: No Animation with AnimatedSwitcher or AnimatedOpacity
body: <p>I'm learning flutter and I'm having behavior with the animation system.</p>
  
  <p>I created a Radio Button which is a circle that should get filled when it gets clicked.</p>
  
  <p>I created a stateful widget class and a state class.</p>
  
  <p>In the state class, I build a :</p>
  
  <pre><code>   GestureDetector -&gt; Container -&gt; AnimatedSwitcher -&gt; _animatedWidget
  </code></pre>
  
  <p>_animatedWidget is a Widget that changes when I click (in the GestureDetector onTap I do _changeSelect)</p>
  
  <pre><code>  void _changeSelect(){
      _isSelected = !_isSelected;
        if(_isSelected){
          setState(() {
            _animatedWidget = Container(width: double.infinity,height: double.infinity,decoration: BoxDecoration(shape: BoxShape.circle, color: Colors.black));
          });
        }
        else{
          setState(() {
            _animatedWidget = Container();          
          });
        }
  
  
    }
  </code></pre>
  
  <p>And this code is not working properly, it should fade in the full container and fade out the empty container but instead, it just pops in and pops out (like a classic change would do)</p>
  
  <p>Here is the full code of my state class :</p>
  
  <pre><code>
  class _RadioButtonState extends State&lt;RadioButton&gt; {
  
    Widget _animatedWidget = Container();
  
  
    bool _isSelected = false;
  
    bool isSelected(){
      return _isSelected;
    }
  
    void _changeSelect(){
      _isSelected = !_isSelected;
        if(_isSelected){
          setState(() {
            _animatedWidget = Container(width: double.infinity,height: double.infinity,decoration: BoxDecoration(shape: BoxShape.circle, color: Colors.black));
          });
        }
        else{
          setState(() {
            _animatedWidget = Container();          
          });
        }
  
  
    }
  
    @override
    Widget build(BuildContext context){
      return GestureDetector(
        onTap: _changeSelect,
        child:
          Container(
            width: 16.0,
            height: 16.0,
            padding: EdgeInsets.all(2.0),
            decoration: BoxDecoration(
                shape: BoxShape.circle,
                border: Border.all(width: 2.0, color: Colors.black)
            ),
            child: AnimatedSwitcher(
              duration: Duration(seconds: 1),
              child: _animatedWidget,
            )
  
          ),
      );
    }
  }
  
  
  </code></pre>
  
  <p>Note: I also tried with AnimatedOpacity instead of AnimatedSwitcher (with the full Container with a starting opacity of 0 increased to 1 when clicked) but it doesn't even change the view, however, the javascript looks to be working during the duration time</p>