Flutter中ScreenUtilInit builder参数类型不匹配错误解决

执行pub get后遇到如下错误：
The argument type 'MaterialApp Function()' can't be assigned to the parameter type 'Widget Function(BuildContext, Widget?)'

受影响的核心代码：

return ScreenUtilInit(
        designSize: Size(428.0, 926.0),
        builder: () => MaterialApp(
          title: 'MMAS',
          debugShowCheckedModeBanner: false,
          theme: ThemeData(
            textTheme: TextTheme(
              headline3: TextStyle(
                fontFamily: 'OpenSans',
                fontSize: 45.0,
                color: Colors.deepOrangeAccent,
              ),
              button: TextStyle(
                fontFamily: 'OpenSans',
              ),
              subtitle1: TextStyle(fontFamily: 'NotoSans'),
              bodyText2: TextStyle(fontFamily: 'NotoSans'),
            ),
            colorScheme: ColorScheme.fromSwatch(primarySwatch: Colors.indigo)
                .copyWith(secondary: Colors.orange),
            textSelectionTheme:
                TextSelectionThemeData(cursorColor: Colors.amberAccent),
          ),
          builder: (context, widget) => MediaQuery(
            data: MediaQuery.of(context).copyWith(textScaleFactor: 1),
            child: widget!,
          ),
          home: SignIn(),
          // Home(),
          locale: _locale,
          localizationsDelegates: [
            AppLocalization.delegate,
            GlobalMaterialLocalizations.delegate,
            GlobalWidgetsLocalizations.delegate,
            GlobalCupertinoLocalizations.delegate,
          ],
          localeResolutionCallback: (locale, supportedLocales) {
            for (var supportedLocale in supportedLocales) {
              if (supportedLocale.languageCode == locale!.languageCode &&
                  supportedLocale.countryCode == locale.countryCode) {
                return supportedLocale;
              }
            }
            return supportedLocales.first;
          },
          supportedLocales: [
            Locale("en", "US"),
            Locale("de", "DE"),
            Locale("es", "ES"),
            Locale("fr", "FR"),
            Locale("hi", "IN"),
            Locale("ja", "JP"),
            Locale("ko", "KR"),
            Locale("pt", "PT"),
            Locale("ru", "RU"),
            Locale("tr", "TR"),
            Locale("vi", "VN"),
            Locale("zh", "CN"),
          ],
        ),
      );

解决方法 #

错误原因是新版本flutter_screenutil库中，ScreenUtilInit的builder参数要求函数签名为Widget Function(BuildContext, Widget?)，但你的代码里用了无参函数。只需给builder函数补全参数签名即可：

修改后的核心代码部分：

return ScreenUtilInit(
        designSize: Size(428.0, 926.0),
        // 补全context和widget参数，即使暂时不用也要保留签名
        builder: (context, widget) => MaterialApp(
          // 原MaterialApp内部代码不变
          title: 'MMAS',
          debugShowCheckedModeBanner: false,
          // ... 其余代码省略
        ),
      );

新版本的flutter_screenutil调整了builder的参数要求，目的是更好地支持上下文传递和组件嵌套，补全参数后类型匹配错误就会消失。

内容的提问来源于stack exchange，提问作者Buiswer231