flutter常见问题

1. 新版 flutter，ancestorWidgetOfExactType 方法过时
   老方法：

static T of<T extends BlocBase>(BuildContext context) {
    final type = _typeOf<BlocProvider<T>>();
    BlocProvider<T> provider = context.ancestorWidgetOfExactType(type);
    return provider.bloc;
}

新方法：

static T of<T extends BlocBase>(BuildContext context) {
    BlocProvider<T> provider = context.findAncestorWidgetOfExactType<BlocProvider<T>>();
    return provider.bloc;
}