Разница между Animation Tween и Animation Controlller

В некоторых руководствах по анимации Flutter используются объекты Tween и Animation. Некоторые используют только AnimationController. Оба приведенных ниже кода, похоже, выводят один и тот же результат. Итак, когда мы используем Tween с анимацией, а когда — только AnimationController?

С Tween и анимацией

import 'dart:core';
import 'package:flutter/material.dart';

class Test extends StatefulWidget {
  @override
  _State createState() {
    return _State();
  }
}

class _State extends State<Test> with TickerProviderStateMixin {
  AnimationController _animationController;
  Animation _animation;
   bool faded = true;
  @override
  void initState() {
    super.initState();
    _animationController = new AnimationController(
        value:0.0,
        vsync: this,
        upperBound: 1.0,
        lowerBound: 0.0,
      duration: new Duration(seconds:1),
    );
    _animation = Tween(begin: 0.0, end: 1.0).animate(_animationController);

  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: new AppBar(
        elevation: 0.5,
        title: new Text(
          "Testing views",
          style: Theme.of(context).textTheme.title,
        ),
      ),
      body: _buildBodyAnimationTest(),
//      body:  _buildTuto(),
    );
  }
  Widget _buildBodyAnimationTest(){
    return FadeTransition(
      opacity: _animation, //here is the difference
      child: InkWell(
        onTap: (){
          if (faded){
            faded = false;
            _animationController.reverse();
          }else{
            faded = true;
            _animationController.forward();
          }
        },
        child: new Container(
          color: Colors.red,
        ),
      ),
    );
  }
}

Без твина и анимации

import 'dart:core';
import 'package:flutter/material.dart';

class Test extends StatefulWidget {
  @override
  _State createState() {
    return _State();
  }
}

class _State extends State<Test> with TickerProviderStateMixin {
  AnimationController _animationController;
   bool faded = true;
  @override
  void initState() {
    super.initState();
    _animationController = new AnimationController(
        value:0.0,
        vsync: this,
        upperBound: 1.0,
        lowerBound: 0.0,
      duration: new Duration(seconds:1),
    );

  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: new AppBar(
        elevation: 0.5,
        title: new Text(
          "Testing views",
          style: Theme.of(context).textTheme.title,
        ),
      ),
      body: _buildBodyAnimationTest(),
//      body:  _buildTuto(),
    );
  }
  Widget _buildBodyAnimationTest(){
    return FadeTransition(
      opacity: _animationController, //here is the difference
      child: InkWell(
        onTap: (){
          if (faded){
            faded = false;
            _animationController.reverse();
          }else{
            faded = true;
            _animationController.forward();
          }
        },
        child: new Container(
          color: Colors.red,
        ),
      ),
    );
  }
}