由于其类型，参数 'builder' 不能具有 'null' 的值，但隐式默认值为 'null'

Question

我正在尝试通过以下代码使用我以前的应用程序，但似乎 Flutter 中发生了一些更新，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：

The parameter 'builder' 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.

我尝试添加 required 或 @required，但均无效。`

Answer 1

这样可以用吗？因为它不会给出错误：

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 并且成功了。

由于其类型，参数 'builder' 不能具有 'null' 的值，但隐式默认值为 'null'

The parameter 'builder' can't have a value of 'null' because of its type, but the implicit default value is 'null'

routes

non-nullable

required

flutter