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GlobalScope

GlobalScope is only available in the 2.3 beta, is experimental and likely to change.
A GlobalScope implements the Singleton pattern with improvements.
When you inject a value into a GlobalScope it is available to your entire application (including in flutter build methods).
A classic example of using a GlobalScope is to hold the currently logged in user.
final userKey = ScopeKey<User>();
final user = validateLogin(username, password);
if (user != null) {
GlobalScope().value(userKey, user);
}
We use guse rather than use to get global ScopeKeys.
Now that you have the user you can get them from anywhere in your code.
final user = use(userKey);

An improved singleton

The problem with the Singleton pattern is that it doesn't play nicely when it comes to unit testing.
GlobalScope solves this problem by being aware of any Scope keys and allowing a Scope key to override a GlobalScope key.
void main() {
GlobalScope().value(userKey, user);
}
test('a unit test', () {
final user = use(userKey); // return user injected in main
Scope().
..value(userKey, testUser) // override the userKey
..run(() =>
{
final user = use(userKey); // returns testUser
});
// we are back outside the Scope.
final user = use(userKey); // return user injected in main
}