用于空值的空检查运算符的连续错误

Question

我正在制作一个带有页面视图和自定义底部导航栏 CustomBottomNavigator 的应用。更改页面时图标会更改颜色，但是每当我为单个 CustomBottomNavigatorItem 实现 Ontap 方法时，它都会提供一个用于空值的空检查运算符。错误附加在代码之后。我不明白即使在观看了几个视频并查看了文档之后也没有任何安全性

import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart'as firebase_auth;

import 'package:flutter_fashion/main.dart';


class CustomBottomNavigator extends StatefulWidget {

  final int tab;
  final Function(int) tabPressed;
  const CustomBottomNavigator({Key? key, required this.tab, required this.tabPressed}) : super(key: key);

  @override
  _CustomBottomNavigatorState createState() => _CustomBottomNavigatorState();
}

class _CustomBottomNavigatorState extends State<CustomBottomNavigator> {
  int _selectedTab=0;
  @override
  Widget build(BuildContext context) {
    _selectedTab=widget.tab;
    return Container(
      decoration: BoxDecoration(
          color: Colors.white,
          borderRadius: BorderRadius.only(
              topLeft: Radius.circular(12), topRight: Radius.circular(12)),
          boxShadow: [
            BoxShadow(
                color: Colors.black.withOpacity(0.05),
                spreadRadius: 1.0,
                blurRadius: 30.0)
          ]),
      child: Row(
        mainAxisAlignment: MainAxisAlignment.spaceAround,
        children: [
          CustomBottomNavigatorItem(
            icon: Icons.home_outlined,
            selected: _selectedTab==0?true:false,
            onPressed: () {
              widget.tabPressed(0);
            },
          ),
          CustomBottomNavigatorItem(
            icon: Icons.code_rounded,
            selected: _selectedTab==1?true:false,
            onPressed: () {
              widget.tabPressed(1);
            },
          ),
          CustomBottomNavigatorItem(
            icon: Icons.bookmark_border_rounded,
            selected: _selectedTab==2?true:false,
            onPressed: () {
              widget.tabPressed(2);
            },
          ),
          CustomBottomNavigatorItem(
            icon: Icons.logout_rounded,
            selected: _selectedTab==3?true:false,
            onPressed: () {
              firebase_auth.FirebaseAuth.instance.signOut();
              Navigator.pushAndRemoveUntil(
                  context,
                  MaterialPageRoute(
                      builder: (builder) => MyApp()),
                      (route) => false);
            },
          ),
        ],
      ),
    );
  }
}


class CustomBottomNavigatorItem extends StatelessWidget {
  final IconData icon;
  final bool selected;
  final Function onPressed;
  CustomBottomNavigatorItem(
      {required this.icon, required this.selected, required this.onPressed,});
  @override
  Widget build(BuildContext context) {
    bool _selected = selected;
    return GestureDetector(
      onTap: () => onPressed,
      child: Container(
        padding: EdgeInsets.symmetric(horizontal: 24, vertical: 28),
        decoration: BoxDecoration(
            border: Border(
                top: BorderSide(
                    color: _selected
                        ? Theme.of(context).accentColor
                        : Colors.transparent,
                    width: 2.0))),
        child: Icon(
          icon,
          size: 24,
          color: _selected ? Theme.of(context).accentColor : Colors.black,
        ),
      ),
    );
  }
}

主页

import 'package:flutter/material.dart';
import 'package:flutter_fashion/Views/Widgets/bottom_navigator.dart';
import 'package:flutter_fashion/Views/Widgets/page_view_tabs/ml_page_tab.dart';
import 'package:flutter_fashion/Views/Widgets/page_view_tabs/saved_page_tab.dart';
import 'package:flutter_fashion/Views/Widgets/page_view_tabs/home_page_tab.dart';


class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  late final PageController _tabPageController;
  late int _selectedTab=0;

  @override
  void initState() {
    _tabPageController=PageController();
    super.initState();
  }

  @override
  void dispose() {
    _tabPageController.dispose();
    super.dispose();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          children: [
            Expanded(
              child: PageView(
                controller: _tabPageController,
                onPageChanged: (num){
                  setState(() {
                    _selectedTab=num;
                  });
                },
                children: [
                  HomeTab(),
                  MachineLearningTab(),
                  SavedTab(),
                ],
              ),
            ),
            CustomBottomNavigator(tab: _selectedTab,tabPressed: (num){
              _tabPageController.animateToPage(num, duration: Duration(milliseconds: 300), curve: Curves.easeOutCubic);
            },)
          ],
        ),
      ),
    );
  }
}

Answer 1

对空值使用空检查运算符通常在您对可空标识符使用空检查运算符（！）时发生异常，我怀疑错误原因在这一行：

class CustomBottomNavigatorItem extends StatelessWidget {
...
  @override
  Widget build(BuildContext context) {
...
      onTap: onPressed(),
...
}

应该是：

class CustomBottomNavigatorItem extends StatelessWidget {
...
  @override
  Widget build(BuildContext context) {
...
      onTap: onPressed,
...
}

请注意，我省略了 onPressed 函数上的 '()'，因为添加它只是意味着调用它，因此您传递的是 onPressed 函数的结果值，而不是函数本身。

Answer 2

将onPressed() 更改为() => onPressed

  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () => onPressed,
      child: Container(
        padding: EdgeInsets.symmetric(horizontal: 24, vertical: 28),
        decoration: BoxDecoration(
          border: Border(
            top: BorderSide(width: 2.0),
          ),
        ),
        child: Icon(
          Icons.send,
          size: 24,
        ),
      ),
    );
  }

尝试将管理注销功能的CustomBottomNavigatorItem 更新为如下内容：

CustomBottomNavigatorItem(
            icon: Icons.logout_rounded,
            selected: _selectedTab==3?true:false,
            onPressed: () {
              WidgetsBinding.instance.addPostFrameCallback(
              (_) {
                Navigator.of(context)
                    .pushReplacementNamed("You Route Name");
              },
            ),
            firebase_auth.FirebaseAuth.instance.signOut();
            },
          ),