An example Flutter application for the oauth2_client library.
Clone the repo, then register your application on the provider you are going to test on. For the sake of simplicity you can use mytestapp://oauth2redirect as the redirect uri in the application registration form. In this case you won't need to change anything.
If you want to use your own redirect uri instead, you must modify the AndroidManifest.xml file changing the android:scheme attribute to the custom scheme of your redirect url. Then you must change the main.dart file, updating the two variables redirectUri and customUriScheme with the corresponding values.
For example, if your redirect url is "my.custom.app://redirect", the android:scheme attribute and the customUriScheme variable will be "my.custom.app", while the redirectUri variable will be "my.custom.app://redirect".
Run the app on an emulator or an actual device.
Select the client type, then fill the TextFields with your application's client id, client secret and required scopes (as a comma separated list).
Finally press the "Authorize" button. You should be redirected to the server authorization page.
Once you have granted authorization, you should be redirected back to the app and see the generated Access Token in the corresponding field.