build method Null safety

  1. @override
Widget build(
  1. BuildContext context
)
override

Describes the part of the user interface represented by this widget.

The framework calls this method in a number of different situations. For example:

This method can potentially be called in every frame and should not have any side effects beyond building a widget.

The framework replaces the subtree below this widget with the widget returned by this method, either by updating the existing subtree or by removing the subtree and inflating a new subtree, depending on whether the widget returned by this method can update the root of the existing subtree, as determined by calling Widget.canUpdate.

Typically implementations return a newly created constellation of widgets that are configured with information from this widget's constructor, the given BuildContext, and the internal state of this State object.

The given BuildContext contains information about the location in the tree at which this widget is being built. For example, the context provides the set of inherited widgets for this location in the tree. The BuildContext argument is always the same as the context property of this State object and will remain the same for the lifetime of this object. The BuildContext argument is provided redundantly here so that this method matches the signature for a WidgetBuilder.

Design discussion

Why is the build method on State, and not StatefulWidget?

Putting a Widget build(BuildContext context) method on State rather than putting a Widget build(BuildContext context, State state) method on StatefulWidget gives developers more flexibility when subclassing StatefulWidget.

For example, AnimatedWidget is a subclass of StatefulWidget that introduces an abstract Widget build(BuildContext context) method for its subclasses to implement. If StatefulWidget already had a build method that took a State argument, AnimatedWidget would be forced to provide its State object to subclasses even though its State object is an internal implementation detail of AnimatedWidget.

Conceptually, StatelessWidget could also be implemented as a subclass of StatefulWidget in a similar manner. If the build method were on StatefulWidget rather than State, that would not be possible anymore.

Putting the build function on State rather than StatefulWidget also helps avoid a category of bugs related to closures implicitly capturing this. If you defined a closure in a build function on a StatefulWidget, that closure would implicitly capture this, which is the current widget instance, and would have the (immutable) fields of that instance in scope:

class MyButton extends StatefulWidget {
  ...
  final Color color;

  @override
  Widget build(BuildContext context, MyButtonState state) {
    ... () { print("color: $color"); } ...
  }
}

For example, suppose the parent builds MyButton with color being blue, the $color in the print function refers to blue, as expected. Now, suppose the parent rebuilds MyButton with green. The closure created by the first build still implicitly refers to the original widget and the $color still prints blue even through the widget has been updated to green.

In contrast, with the build function on the State object, closures created during build implicitly capture the State instance instead of the widget instance:

class MyButtonState extends State<MyButton> {
  ...
  @override
  Widget build(BuildContext context) {
    ... () { print("color: ${widget.color}"); } ...
  }
}

Now when the parent rebuilds MyButton with green, the closure created by the first build still refers to State object, which is preserved across rebuilds, but the framework has updated that State object's widget property to refer to the new MyButton instance and ${widget.color} prints green, as expected.

See also:

  • StatefulWidget, which contains the discussion on performance considerations.

Implementation

@override
Widget build(BuildContext context) {
  return GestureDetector(
    /// 앱의 어떤 화면에서든 빈 공간을 탭하면 텍스트필드에 포커스를 해제하여 키보드를 닫습니다.
    onTap: () {
      if (FocusScope.of(context).hasFocus) {
        FocusScope.of(context).requestFocus(new FocusNode());
        FocusScope.of(context).unfocus();
      }
    },
    child: MaterialApp(
      navigatorKey: Helper().rootNavigatorKey,
      navigatorObservers: [NavigatorHistory()],

      /// 스크롤이 마지막에 닿았을때 스마트폰에서 나타나는 이펙트를 제거합니다.
      builder: (context, child) {
        return ScrollConfiguration(
          behavior: NoGlowScrollBehavior(),
          child: child!,
        );
      },

      /// [AppBar] 에서 요구하는 사양이므로 넣어주어야 합니다.
      localizationsDelegates: [
        GlobalMaterialLocalizations.delegate,
        GlobalCupertinoLocalizations.delegate,
      ],
      supportedLocales: [
        Locale('ko', ''),
        Locale('en', ''),
      ],
      title: AppConfig.appDisplayName,

      /// 글로벌 테마를 정의합니다.
      theme: ThemeData(
        textTheme: GoogleFonts.notoSansTextTheme(
          Theme.of(context).textTheme,
        ),
        primarySwatch: Colors.blue,
        scaffoldBackgroundColor: CustomColors.pale_grey,
        canvasColor: Colors.white,
      ),

      /// 최초 라우팅을 정의합니다.
      initialRoute: SplashScreen.routeName,

      /// 모든 라우팅을 정의합니다.
      /// 새 화면을 추가할 경우 여기에 정의해주어야 합니다.
      /// 화면전환 방식에 있어서 Named route를 사용하고 있기 때문에 모든 화면은 routeName 이라는 스태틱 상수를 가져야 합니다.
      /// onGenerateRoute에서는 이 상수 스트링으로 화면을 구분합니다.
      /// Named Route에 관해서는 다음 링크를 참고하세요. https://flutter-ko.dev/docs/cookbook/navigation/named-routes
      onGenerateRoute: (RouteSettings settings) {
        switch (settings.name) {

          /// Splash
          case SplashScreen.routeName:
            return MaterialPageRoute(builder: (context) => SplashScreen(key: splashScreenKey), settings: settings);

          /// FarmLoading
          case FarmLoadingScreen.routeName:
            return PageRouteBuilder(
              pageBuilder: (BuildContext context, _, __) {
                final curve = CurveTween(curve: Curves.easeOut);
                return FadeTransition(opacity: _.drive(curve), child: FarmLoadingScreen());
              },
              opaque: false,
              settings: settings,
            );

        /// 메인 탭 스크린
          case MainScreen.routeName:
            return PageRouteBuilder(
              pageBuilder: (BuildContext context, _, __) {
                final curve = CurveTween(curve: Curves.easeOut);
                return FadeTransition(opacity: _.drive(curve), child: MainScreen());
              },
              opaque: false,
              settings: settings,
            );
          case HomeScreen.routeName:
            return MaterialPageRoute(builder: (context) => HomeScreen(), settings: settings);
          case CowCurrentStatusScreen.routeName:
            return MaterialPageRoute(builder: (context) => CowCurrentStatusScreen(), settings: settings);
          case CowSettingsScreen.routeName:
            return MaterialPageRoute(builder: (context) => CowSettingsScreen(), settings: settings);
          case DiseaseObservationCreateScreen.routeName:
            final args = settings.arguments as DiseaseObservationCreateScreenArguments;
            return MaterialPageRoute(builder: (context) => DiseaseObservationCreateScreen(diseaseObservation: args.diseaseObservation, inputMode: args.inputMode), settings: settings);
          case DiseaseObservationListScreen.routeName:
            return MaterialPageRoute(builder: (context) => DiseaseObservationListScreen(), settings: settings);
          case SettingsScreen.routeName:
            return MaterialPageRoute(builder: (context) => SettingsScreen(), settings: settings);

        /// 화면
          case ReportScreen.routeName:
            final args = settings.arguments as ReportScreenArguments;
            return MaterialPageRoute(builder: (context) => ReportScreen(reportGroup: args.reportGroup, cow: args.cow), settings: settings);
          case ChatScreen.routeName:
            return MaterialPageRoute(builder: (context) => ChatScreen(), settings: settings);
          case SignInScreen.routeName:
            return PageRouteBuilder(
              pageBuilder: (BuildContext context, _, __) {
                final curve = CurveTween(curve: Curves.easeOut);
                return FadeTransition(opacity: _.drive(curve), child: SignInScreen());
              },
              opaque: false,
              settings: settings,
            );
          case SignupScreen.routeName:
            return CupertinoPageRoute(builder: (context) => SignupScreen(), settings: settings);
          case CowDetailScreen.routeName:
            final args = settings.arguments as CowDetailScreenArguments;
            return MaterialPageRoute(builder: (context) => CowDetailScreen(cow: args.cow, selectedDate: args.selectedDate), settings: settings);
          case SelfDiagnosisScreen.routeName:
            return PageRouteBuilder(pageBuilder: (BuildContext context, _, __) => SelfDiagnosisScreen(), opaque: false, settings: settings);
          case PhoneAuthScreen.routeName:
            final args = settings.arguments as PhoneAuthScreenArguments;
            return MaterialPageRoute(builder: (context) => PhoneAuthScreen(args.username), settings: settings);
          case PhoneAuthResendScreen.routeName:
            return MaterialPageRoute(builder: (context) => PhoneAuthResendScreen(), settings: settings);
          case FarmInputCodeScreen.routeName:
            return PageRouteBuilder(pageBuilder: (BuildContext context, _, __) => FarmInputCodeScreen(), opaque: false, settings: settings);
          case FarmSelectScreen.routeName:
            return MaterialPageRoute(builder: (context) => FarmSelectScreen(), settings: settings);
          case UserInfoSubScreen.routeName:
            return MaterialPageRoute(builder: (context) => UserInfoSubScreen(), settings: settings);
          case FindUsernameScreen.routeName:
            return MaterialPageRoute(builder: (context) => FindUsernameScreen(), settings: settings);
          case FindPasswordScreen.routeName:
            return MaterialPageRoute(builder: (context) => FindPasswordScreen(), settings: settings);
          case WebviewScreen.routeName:
            final args = settings.arguments as WebviewScreenArguments;
            return MaterialPageRoute(builder: (context) => WebviewScreen(initialUrl: args.initialUrl), settings: settings);

        /// 기타
          case PopupScreen.routeName:
            final args = settings.arguments as PopupScreenArguments;
            return PageRouteBuilder(
              barrierColor: Colors.black.withOpacity(0.233),
              pageBuilder: (BuildContext context, _, __) {
                final curve = CurveTween(curve: Curves.easeOutBack);
                return ScaleTransition(
                  scale: _.drive(curve),
                  child: PopupScreen(
                    type: args.type,
                    onTapCancel: args.onTapCancel,
                    onTapNormal: args.onTapNormal,
                    onTapNo: args.onTapNo,
                    onTapYes: args.onTapYes,
                    onTapOutside: args.onTapOutside,
                    onClose: args.onClose,
                    data: args.data,
                  ),
                );
              },
              opaque: false,
              settings: settings,
            );
          case SearchCowScreen.routeName:
            return PageRouteBuilder(
              pageBuilder: (BuildContext context, _, __) {
                const begin = Offset(0.0, -1.0);
                const end = Offset(0.0, 0.0);
                const curve = Curves.easeOutBack;

                var tween = Tween(begin: begin, end: end).chain(CurveTween(curve: curve));

                return SlideTransition(
                  position: _.drive(tween),
                  child: SearchCowScreen(),
                );
              },
              opaque: false,
              settings: settings,
            );
          case PhotoViewScreen.routeName:
            final args = settings.arguments as PhotoViewScreenArguments;
            return PageRouteBuilder(
              pageBuilder: (BuildContext context, _, __) {
                final curve = CurveTween(curve: Curves.easeOut);
                return FadeTransition(opacity: _.drive(curve), child: PhotoViewScreen(imageProvider: args.imageProvider, title: args.title, tag: args.tag));
              },
              opaque: false,
              settings: settings,
            );
          case DateTimePickerScreen.routeName:
            final args = settings.arguments as DateTimePickerScreenArguments;
            return PageRouteBuilder(
              barrierColor: Colors.black.withOpacity(0.233),
              pageBuilder: (BuildContext context, _, __) {
                final curve = CurveTween(curve: Curves.easeOutBack);
                return ScaleTransition(
                  scale: _.drive(curve),
                  child: DateTimePickerScreen(currentDateTime: args.currentDateTime),
                );
              },
              opaque: false,
              settings: settings,
            );

        /// 기본
          default:
            return MaterialPageRoute(builder: (context) => EmptyScreen(), settings: settings);
        }
      },
    ),
  );
}