title: Flutter slideTransition is not animating
body: <p>So I'm trying to create a trivial slide transition element in flutter and I'm having some difficulty. What the below does is wait for the animation time, and then just display the <code>Text("hello there sailor")</code>. I don't know why this is not animating - it seems very similar to this previous post that has a trivial example (<a href="https://stackoverflow.com/questions/53278792/sliding-animation-to-bottom-in-flutter?noredirect=1&amp;lq=1">Sliding animation to bottom in flutter</a>).</p>
  
  <p>This is how I call the below code: <code>DeleteCheck(offsetBool: widget.model.deleteNotify, widthSlide: 0.50*width100)</code> where <code>double width100 = MediaQuery.of(context).size.width;</code>.</p>
  
  <p>Does anyone see what I am doing wrong?</p>
  
  <pre><code>class DeleteCheck extends StatefulWidget{
  
    final offsetBool;
    final double widthSlide;
  
    DeleteCheck({
      Key key, 
      this.offsetBool, 
      this.widthSlide
    }): super(key: key);
  
    @override 
    State&lt;StatefulWidget&gt; createState() {
      return new _MyDeleteCheck();
    }
  }
  
  class _MyDeleteCheck extends State&lt;DeleteCheck&gt; with TickerProviderStateMixin {
    AnimationController _controller;
    Animation&lt;Offset&gt; _offsetFloat; 
  
    @override
    void initState() {
      super.initState();
  
      _controller = AnimationController(
        vsync: this,
        duration: const Duration(seconds: 1),
      );
  
      _offsetFloat = Tween&lt;Offset&gt;(begin: Offset(widget.widthSlide, 0.0), end: Offset.zero)
          .animate(_controller);
  
      _offsetFloat.addListener((){
        setState((){});
      });
  
      _controller.forward();
  
    }
  
    @override
    void dispose() {
      _controller.dispose();
      super.dispose();
    }
  
    @override
    Widget build(BuildContext context) {
      double height100 = MediaQuery.of(context).size.height;
      double width100 = MediaQuery.of(context).size.width;
      return new SlideTransition(
        position: _offsetFloat,
        child: Container(
          color: Colors.cyan,
          width: 0.525*width100-3.0,
          child: Text("hello there sailor")
        )
      );
    }
  }
  </code></pre>