Animation Definition for our button

animated_pgbutton.dart

// Defining Animations and Animation Controller
Animation<double> _animatedSize;
Animation<Color> _animatedColor;
AnimationController _boss;

// Initialize Animations and AnimationController
@override
void initState() {
  super.initState();
  // AnimationController
  _boss = AnimationController(duration: Duration(milliseconds: 300), vsync: this);
  
  // Animations
  // size using Tween<double>
  _animatedSize = Tween<double>(begin:0, end:300).animate(_boss)
    ..addListener((){
      setState(() {});
    });
  // color using a subclass of Tween, ColorTween
  _animatedColor = ColorTween(begin: Colors.yellow, end: Colors.red).animate(_boss);
  
}

// Never forget to dispose the AnimationController
// to avoid memory leaks
@override
void dispose() {
  _boss.dispose();
  super.dispose();
}