https://github.com/flutter/flutter/issues/27235#issuecomment-508995063

https://flutteragency.com/how-to-prevent-device-orientation-in-flutter/

세로모드로 고정시키기

import 'package:flutter/services.dart';

...

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp])
      .then((_) {
    runApp(new MyApp());
  });
}

SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp])

을 이용해서 고정

이것만으로 해결이 안되거나 에러가 발생하는 경우에는 다음 조치를 취해야 합니다.

iOS

<key>UISupportedInterfaceOrientations</key>
	<array>
		<string>UIInterfaceOrientationPortrait</string>
		<!-- <string>UIInterfaceOrientationLandscapeLeft</string> -->
		<!-- <string>UIInterfaceOrientationLandscapeRight</string> -->
	</array>
	<key>UISupportedInterfaceOrientations~ipad</key>
	<array>
		<string>UIInterfaceOrientationPortrait</string>
		<!-- <string>UIInterfaceOrientationPortraitUpsideDown</string> -->
		<!-- <string>UIInterfaceOrientationLandscapeLeft</string> -->
		<!-- <string>UIInterfaceOrientationLandscapeRight</string> -->
	</array>

Info.plist 에서 Orientation 관련 속성 수정

Android

<activity>
	...
	android:screenOrientation="portrait"

	...

AndroidManifest.xml 에서 screenOrientation 수정

가로모드로 고정

void main() {
      SystemChrome.setPreferredOrientations(
          [DeviceOrientation.landscapeLeft,DeviceOrientation.landscapeRight])
          .then((_) => runApp(MyApp()),
      );

특정 화면에서만 고정시키고 싶은 경우

class SampleScreen extends StatefulWidget {
  SampleScreen() : super();

  @override
  State<StatefulWidget> createState() => _SampleScreenState();
}

class _SampleScreenState extends State<SampleScreen>
    with PortraitStatefulModeMixin<SampleScreen> {
  @override
  Widget build(BuildContext context) {
    super.build(context);
    return Text("Flutter - Block screen rotation example");
  }

  @override
  void dispose() {
     super.dispose();
  }
}

PortraitStatefulModeMixin<SampleScreen> 을 사용한다.

super.build(context); 가 있다는 것을 주의