参数 'builder' 的值不能为 'null'，因为它的类型，但隐含的默认值是 'null'答案

【问题标题】：The parameter 'builder' can't have a value of 'null' because of its type, but the implicit default value is 'null'参数 'builder' 的值不能为 'null'，因为它的类型，但隐含的默认值是 'null'
【发布时间】：2022-01-03 09:22:19
【问题描述】：

我正在尝试将我以前的应用程序与以下代码一起使用，但 Flutter 中似乎发生了一些更新，我收到以下错误：

import 'package:flutter/material.dart';

class CustomRoute<T> extends MaterialPageRoute<T> {
  CustomRoute({
    WidgetBuilder builder,
    RouteSettings settings,
  }) : super(
          builder: builder,
          settings: settings,
        );

  @override
  Widget buildTransitions(
    BuildContext context,
    Animation<double> animation,
    Animation<double> secondaryAnimation,
    Widget child,
  ) {
    if (settings.name == '/') {
      return child;
    }
    return FadeTransition(
      opacity: animation,
      child: child,
    );
  }
}

class CustomPageTransitionBuilder extends PageTransitionsBuilder {
  @override
  Widget buildTransitions<T>(
    PageRoute<T> route,
    BuildContext context,
    Animation<double> animation,
    Animation<double> secondaryAnimation,
    Widget child,
  ) {
    if (route.settings.name == '/') {
      return child;
    }
    return FadeTransition(
      opacity: animation,
      child: child,
    );
  }
}

错误：

参数“builder”的值不能为“null”，因为它类型，但隐含的默认值为“null”。尝试添加一个显式非“null”默认值或“必需”修饰符。

我尝试添加required 或@required，但均未成功。`

【问题讨论】：

添加required后，错误是什么？
我添加了required，错误消失了
@MiftakhulArzak：如果我在builder 之前添加required，它会给我这个新错误：The parameter 'required' can't have a value of 'null' because of its type, but the implicit default value is 'null'. Try adding either an explicit non-'null' default value or the 'required' modifier.
@MiftakhulArzak：是的，你是对的。我把required放错地方了！

标签： flutter routes required non-nullable

【解决方案1】：

你可以这样使用吗？因为它没有给出错误：

import 'package:flutter/material.dart';

class CustomRoute<T> extends MaterialPageRoute<T> {
  CustomRoute({
   required WidgetBuilder builder,
   required RouteSettings settings,
  }) : super(
          builder: builder,
          settings: settings,
        );

  @override
  Widget buildTransitions(
    BuildContext context,
    Animation<double> animation,
    Animation<double> secondaryAnimation,
    Widget child,
  ) {
    if (settings.name == '/') {
      return child;
    }
    return FadeTransition(
      opacity: animation,
      child: child,
    );
  }
}

class CustomPageTransitionBuilder extends PageTransitionsBuilder {
  @override
  Widget buildTransitions<T>(
    PageRoute<T> route,
    BuildContext context,
    Animation<double> animation,
    Animation<double> secondaryAnimation,
    Widget child,
  ) {
    if (route.settings.name == '/') {
      return child;
    }
    return FadeTransition(
      opacity: animation,
      child: child,
    );
  }
}

我尝试添加required，它成功了。

【讨论】：