how to access global variable in dart code example

Example: global variable flutter

Just create a library file and create fields for globals you need there. Import this library everywhere you need access to these fields.

app.dart

import 'globals.dart' as globals;

main() {
  globals.isLoggedIn = true;
}
component1.dart

import 'globals.dart' as globals;

class MyComponent {
  view() {
    if(globals.isLoggedIn) {
      doSomething();
    else {
      doSomethingElse();
    }
  }
}
globals.dart

library my_prj.globals;

bool isLoggedIn = false;
You can also

create a singleton in the globals library (see How do you build a Singleton in Dart? for more details).
use observable to get notified about changes (see Implement an Observer pattern in Dart, How can i trigger a kind of onChange event in a class for more details)