How to prevent device orientation changes and force portrait?

Option 1. code:

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

Also, the following file must be imported:

'package:flutter/services.dart'

Option 2. Open android/app/src/main/AndroidManifest.xml and add the following line in the MainActivity:

android:screenOrientation="portrait"

If you have this:

<activity
        android:name=".MainActivity"
        android:launchMode="singleTop"
        android:theme="@style/LaunchTheme"
        android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
        android:hardwareAccelerated="true"
        android:windowSoftInputMode="adjustResize">

You should end up with something like this:

<activity
        android:name=".MainActivity"
        android:launchMode="singleTop"
        android:theme="@style/LaunchTheme"
        android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
        android:hardwareAccelerated="true"
        android:screenOrientation="portrait"
        android:windowSoftInputMode="adjustResize">

This works for Android. On iOS, you will have to change this from the Xcode page: https://i.stack.imgur.com/hswoe.png