我正在尝试使用颤振制作 Bmi 计算器，但出现了很多错误

Question

import 'package:flutter/material.dart';

class ReusableCard extends StatelessWidget {
  ReusableCard({@required this.colour, this.cardChild, this.onPress}); // i am getting errors here
  final Color colour ;
  final Widget cardChild;
  final Function onPress;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onPress(),
      child: Container(
//if box decoration is there in the code then the color property should go in the box decoration property
        decoration: BoxDecoration(
            color:colour,
            borderRadius: BorderRadius.circular(15.0)
        ),
        margin: EdgeInsets.all(15.0),
      ),
    );
  }
}

i am getting error: The parameter 'onPress', 'cardChild', 'colour' can't have a value of 'null' because of its type, but the implicit default value is 'null'. in the dart analysis

Answer 1

The error you facing is caused by null-safety, you may want to change your types from:

final Color colour ;
final Widget cardChild;
final Function onPress;

//to

final Color? colour ;
final Widget? cardChild;
final Function? onPress;

But I recommend you to put them all in the constructor with the decorator @required

You can learn more about null-safety syntaxes on the official documentation:
https://flutter.dev/docs/null-safety

Answer 2

I faced the same error, even using the @required decorator wasn't enough to solve the issue, What solved it for me was that i had to disable the null-safety for the project and that's by downgrading the flutter version from ">=2.12.0 <3.0.0" to ">=2.11.0 <3.0.0" through modifying the pubspec.yaml file. Specifically under dependencies -> sdk.

Keep in mind that by downgrading the project's flutter version will disable the entire usage of null-safety including the "?" checking operator

Here's a good read from Flutter Docs to better understand the null-safety feature in the Dart language : https://dart.dev/null-safety#enable-null-safety

This is a good comprehensive article as well https://petercoding.com/flutter/2021/03/04/using-null-safety-in-dart/

Answer 3

I added 'required' to the constructor for onPress. However, I could not directly assign it to the onTap: part of the GestureDetector widget. So I called it from a method that did match the expected method signature for onTap. In the end this is the (inelegant) solution that worked for me:

class ReusableCard extends StatelessWidget {

ReusableCard({required this.colour, this.cardChild, required this.onPress});

final Color colour;
final Widget? cardChild;
final Function onPress;

void onPressAction() {
    onPress();
}

@override
Widget build(BuildContext context) {
    return GestureDetector(
    onTap: onPressAction,
    child: Container(
      child: cardChild,
      margin: EdgeInsets.all(15.0),
      decoration: BoxDecoration(
        color: colour,
        borderRadius: BorderRadius.circular(10.0)
      ),
    ),
  );
}