title: SetState doesn't update widgets
body: <p>I want the image to get bigger and to opacity 0 with animation
  I tried <code>setState()</code> in two functions <code>_onpre()</code> and <code>Future.delayed</code> to change size and opacity of image but it doesn't change the widgets</p>
  <pre><code>import 'dart:async';
  import 'package:flutter/cupertino.dart';
  import 'package:flutter/material.dart';
  import 'package:flutter/scheduler.dart';
  
  class splatch extends StatefulWidget {
    static String id = 'splatch';
    @override
    _splatchState createState() =&gt; _splatchState();
  }
  
  class _splatchState extends State&lt;splatch&gt; {
    @override
    Widget build(BuildContext context) {
      Size size = MediaQuery.of(context).size;
      double _height =  300;
      double _width = 3;
  
  
      Future.delayed(const Duration(milliseconds: 500), () {
        setState(() {
          _height = 40;
          _width = 40;
        });
  
      });
  
      _onpre() {
          setState(() {
            _height = 40;
          });
      }
  
      return Container(
        child: Stack(
          children: [
              AnimatedContainer(duration: Duration(microseconds: 100),
                height: _height,
                child:Container(
                  child: Image.asset('images/rond1.png'),
                ),
                curve: Curves.fastOutSlowIn,
              ),
          ]
        ),
      );
    }
  }
  </code></pre>