flutter - 类 'bool'没有实例方法 '-'。接收方:true尝试调用:-(false)

Question

我试图使用TweenAnimationBuilder<bool>，但即使有异常日志，我也真的不知道出了什么问题。在发布此问题之前，我已经厌倦了谷歌搜索，但没有帮助。
异常(exception):

Class 'bool' has no instance method '-'.
Receiver: true
Tried calling: -(false)
The relevant error-causing widget was TweenAnimationBuilder<bool> 

代码:

 @override
  Widget build(BuildContext context) {
    return TweenAnimationBuilder<bool>(
      duration: Duration(seconds: 2),
      tween: Tween<bool>(begin: false, end: true),
      builder: (BuildContext context, bool tween, Widget child) {
        print(tween.toString());
        return Visibility(visible: showNavBar, child: child);
      },
      child: BottomNavigationBar(
        selectedItemColor: Colors.green,
        unselectedItemColor: Colors.black,
        items: [
          BottomNavigationBarItem(icon: Icon(Icons.home), label: "Home"),
          BottomNavigationBarItem(icon: Icon(Icons.explore), label: "Home"),
          BottomNavigationBarItem(
              icon: Icon(Icons.library_books), label: "Home"),
          BottomNavigationBarItem(icon: Icon(Icons.settings), label: "Home")
        ],
        onTap: (index) {},
      ),
    );
  }

Answer 1

TweenAnimationBuilder<bool>尝试使用Tween<bool>，但Tween<T>不能与bool一起使用。从 Tween 文档中:

Types with special considerations

...

Types that define + and - operators to combine values (T + T → T and T - T → T) and an * operator to scale by multiplying with a double (T * double → T) can be directly used with Tween<T>.

bool不提供 +， -和 *运算符。