Flutter中已定义的onTap函数在GestureDetector引用时提示未定义

问题出在你访问onTap的方式上：你写的是Widget.onTap，这里的Widget是类的名称，而你实际需要调用的是当前LoginPage实例传递过来的onTap参数，在State类里应该用**小写的widget.onTap**来访问（State类自带的widget属性就是绑定的StatefulWidget实例）。

不管你有没有移除可空符号，只要访问方式错了，就会提示函数未定义。

修正后的关键代码片段 #

GestureDetector(
  onTap: widget.onTap, // 把大写的Widget改成小写的widget
  child: const Text(
    'Create Account',
    style: TextStyle(
      fontWeight: FontWeight.w600,
      fontSize: 15,
      color: Colors.black,
    ),
  ),
),

完整修正后代码 #

import 'package:finalproject/components/my_textfield.dart';
import 'package:flutter/material.dart';

// ignore: non_constant_identifier_names

class LoginPage extends StatefulWidget {
  final void Function()? onTap;
  const LoginPage({super.key, required this.onTap});

  @override
  State<LoginPage> createState() => _LoginPageState();
}

class _LoginPageState extends State<LoginPage> {
  final emailController = TextEditingController();
  final passwordController = TextEditingController();
  // ignore: non_constant_identifier_names
  void SignIn() {}
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.grey[300],
      body: SafeArea(
        child: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              //logo
              const Icon(
                Icons.message,
                size: 80,
                color: Colors.green,
              ),

              //welcome back message
              const Text(
                'Welcome Back, You Have Been Missed',
                style: TextStyle(
                  color: Colors.grey,
                  fontSize: 20,
                  fontStyle: FontStyle.italic,
                ),
              ),
              //create space
              const SizedBox(
                height: 20,
              ),
              //email textfield
              MyTextField(
                controller: emailController,
                hintText: 'Enter email',
                obsecuretext: false,
              ),
              //create space
              const SizedBox(
                height: 8,
              ),
              //password textfield
              MyTextField(
                controller: passwordController,
                hintText: 'Enter password',
                obsecuretext: true,
              ),
              const SizedBox(
                height: 8,
              ),
              //sign in button
              MyButton(
                onTap: () {
                  SignIn();
                },
                text: 'Login',
              ),
              const SizedBox(
                height: 15,
              ),
              //not a member? register now
              Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  const Text(
                    'Not A Member?',
                    style: TextStyle(
                      // fontWeight: FontWeight.w600,
                      fontSize: 13,
                      color: Colors.black,
                    ),
                  ),
                  const SizedBox(
                    width: 10,
                  ),
                  GestureDetector(
                    onTap: widget.onTap,
                    child: const Text(
                      'Create Account',
                      style: TextStyle(
                        fontWeight: FontWeight.w600,
                        fontSize: 15,
                        color: Colors.black,
                      ),
                    ),
                  ),
                ],
              )
            ],
          ),
        ),
      ),
    );
  }
}

内容的提问来源于stack exchange，提问作者katojkalemba