无法将参数类型“Function”分配给参数类型“void Function()?” 空安全后

Question

我想实现制作一个带有不同项目的抽屉，所以我正在为 DrawerItems 创建一个单独的文件，并使用构造函数将数据传递给主文件。但是我在“onPressed”函数上收到以下错误：

“不能将参数类型‘Function’分配给参数类型‘void Function()’”

class DrawerItem extends StatelessWidget {
    
      final String text;
      final Function onPressed;
    
      const DrawerItem({Key key, this.text, this.onPressed}) : super(key: key);
    
      @override
      Widget build(BuildContext context) {
        return FlatButton(
          child: Text(
            text,
            style: TextStyle(
              fontWeight: FontWeight.w600,
              fontSize: 18.0,
            ),
          ),
          onPressed: onPressed,
        );
      }
    }

有谁知道为什么？

Answer 1

更改您的代码以接受 VoidCallback 而不是 onPressed 的 Function。顺便说一句 VoidCallback 只是简写，void Function()因此您也可以将其定义为final void Function() onPressed;

更新代码：

class DrawerItem extends StatelessWidget {
    
      final String text;
      final VoidCallback onPressed;
    
      const DrawerItem({Key key, this.text, this.onPressed}) : super(key: key);
    
      @override
      Widget build(BuildContext context) {
        return FlatButton(
          child: Text(
            text,
            style: TextStyle(
              fontWeight: FontWeight.w600,
              fontSize: 18.0,
            ),
          ),
          onPressed: onPressed,
        );
      }
    }

Answer 2

具有空安全性：

代替

final Function? onPressed;

用

final Function? onPressed;

或者

final VoidCallback? onPressed;

Answer 3

那是因为onPressedinsideFlatButton不是一个普通的函数，它的VoidCallBackFunction 。你可以尝试这样的事情：

final VoidCallBack onPressed;

同时，您正在将法线传递function给VoidCallBack

按照此处的官方文档进行操作

更新的代码：

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomeScreen(),
    );
  }
}

class HomeScreen extends StatelessWidget {
  _myFunction() => print("Being pressed!");

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            DrawerItem(
              text: "Hello Jee",
              onPressed: _myFunction,
            ),
          ],
        ),
      ),
    );
  }
}

class DrawerItem extends StatelessWidget {
  final String text;
  final Function onPressed;

  const DrawerItem({Key key, this.text, this.onPressed}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return FlatButton(
      child: Text(
        text,
        style: TextStyle(
          fontWeight: FontWeight.w600,
          fontSize: 18.0,
        ),
      ),
      onPressed: onPressed,
    );
  }
}

Answer 4

代替

final Function? onPressed; 

使用

final Function()? onPressed; // add parenthesis in Function

Answer 5

定义函数时将括号放在关键字()后面。Function

import 'package:flutter/material.dart';

class Answer extends StatelessWidget {
  final Function() onPressed; //parenthesis () on function
  final String name;
  const Answer(this.name, this.functionMessage, {Key? key}) : super(key: 
key);

  @override
  Widget build(BuildContext context) {
    return Container(
        margin: const EdgeInsets.only(top: 10),
        width: double.infinity,
        child: ElevatedButton(
           style: ElevatedButton.styleFrom(
           primary: Colors.black, // background
           onPrimary: Colors.white, // foreground
          ),
          onPressed: onPressed,
          child: Text(name),
        ));
  }
}

Answer 6

如果您了解 dart 中的数据类型和 null 安全性，则有两种可能的解决方案

第一：

添加类型 VoidCallback?

class DrawerItem extends StatelessWidget {
      final String text;
      final VoidCallback? onPressed;
    
      const DrawerItem({Key key, this.text, this.onPressed}) : super(key: key); }

第二：

如果您是初学者并且对数据类型了解不多，您可以使用动态并让 dart 编译器为您处理类型。

class DrawerItem extends StatelessWidget {
          final String text;
          final dynamic onPressed;
    
      const DrawerItem({Key key, this.text, this.onPressed}) : super(key: key); }