Flutter典型错误：There are multiple heroes that share the same tag within a subtree.

这段错误信息描述通常如下：‘Within each subtree for which heroes are to be animated (i.e. a PageRoute subtree), each Hero must have a unique non-null tag. In this case, multiple heroes had the following tag: Here is the subtree for one of the offending heroes: Hero’.

首先得知道Hero是Flutter中的一种动画形式，两个带有同样hero标识的元素在页面切换时会有一个飞动的效果，该效果由flutter自动实现。该标识对应的属性名为heroTag，在同一个页面内，每个heroTag只能拥有一个独一无二的值（和html种的id一样），如果重复就会抛出标题所示的异常‘There are multiple heroes that share the same tag within a subtree.’。解决办法自然是修改重复的heroTag值即可。

例如，FloatingActionButton就是自带的Hero动画的按钮元素，阅读源码可以发现，它的参数定义：

const FloatingActionButton({
    Key key,
    this.child,
    this.tooltip,
    this.foregroundColor,
    this.backgroundColor,
    this.focusColor,
    this.hoverColor,
    this.splashColor,
    this.heroTag = const _DefaultHeroTag(),
    this.elevation,
    this.focusElevation,
    this.hoverElevation,
    this.highlightElevation,
    this.disabledElevation,
    @required this.onPressed,
    this.mini = false,
    this.shape,
    this.clipBehavior = Clip.none,
    this.focusNode,
    this.autofocus = false,
    this.materialTapTargetSize,
    this.isExtended = false,
  })

heroTag的值默认被初始化为，一个常量const _DefaultHeroTag()，该常量定义为：

class _DefaultHeroTag {
  const _DefaultHeroTag();
  @override
  String toString() => '';
}

所以当我们在一个页面种使用两个FloatingActionButton却不覆写它们的heroTag属性，它们会默认使用同一个标识，以致于冲突报错。同时，_DefaultHeroTag的toString方法返回‘’，这就是为什么报错信息描述中会有‘’的原因。

所以我们只需要修改一个FloatingActionButton的heroTag，让它不再是默认值即可：

@override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: navigatorBar(context,title: 'xxx'),
      floatingActionButton: FloatingActionButton(
        onPressed: null,
        tooltip: 'Increment',
        child: Icon(Icons.add),
        heroTag: 'other',
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }