Overriding

The Scope package supports the concept of overriding a scoped key.
The ability to override a scope key solves a number of common design pattern problems with DI particularly when it comes to unit testing.
At any point in your code you can introduce a new Scope that will override existing Scopes and your GlobalScope.
When calling use or Scope.use, the scope package performs a search for the passed key.
The search order is critical to understanding the overriding mechanism.
search for the nearest Scope on the call stack
search up the hierarchy of Scopes on the call stack
search the GlobalScope
check if the withDefault argument was passed to use
check if the key was created using ScopeKey.withDefault .
throw a MissingDependencyException if the key wasn't found.
The best way to understand this is via an example:
import 'package:scope/scope.dart';
​
final userKey = ScopeKey<User>();
final counterKey = ScopeKey<int>();
​
final globalUser = User('real');
final testUser = User('test');
final innerUser = User('inner');
​
final globalDb = Db('global_db');
final liveDb = Db('live_db');
final testDb = Db('test_db');
final dbKey = ScopeKey<Db>.withDefault(liveDb);
​
/// Demonstrates how the `use` method resolves overridden
/// scope keys.
void main() {
  var counter = 0;
  GlobalScope()
    ..value<User>(userKey, globalUser)
    ..sequence<int>(counterKey, () => counter++);
​
  /// just GlobalScope in scope
  assert(use(userKey) == globalUser, 'take from global scope');
​
  /// override the GlobalScope with outerscope
  Scope('outerscope')
    ..value<User>(userKey, testUser)
    ..run(() {
      assert(use(userKey) == testUser, 'take from outerscope');
      assert(use(counterKey) == 0, 'always from the GlobalScope');
    });
  assert(use(userKey) == globalUser, 'take from GlobalScope');
  assert(use(counterKey) == 1, 'always from the GlobalScope');
​
  /// override the GlobalScope and outerscope with
  /// innerscope
  Scope('outerscope')
    ..value<User>(userKey, testUser)
    ..run(() {
      assert(use(counterKey) == 2, 'always from the GlobalScope');
      
      Scope('innerscope')
        ..value<User>(userKey, innerUser)
        ..run(() {
          assert(use(userKey) == innerUser, 'take from innerscope');
          assert(use(counterKey) == 3, 'always from the GlobalScope');
        });
​
      assert(use(userKey) == testUser, 'take from outerscope');
    });
​
  /// we are out of all Scope's so GlobalScope back in play
  assert(use(userKey) == globalUser, 'take from GlobalScope');
  assert(use(counterKey) == 4, 'always from the GlobalScope');
​
  /// No key in scope; get the keys default
  assert(use(dbKey) == liveDb, 'take from the withDefault value of the dbKey');
​
  /// No key in scope; use the default value provided to use
  assert(use(dbKey, withDefault: () => testDb) == testDb,
      'take from the withDefault provided to use');
​
  /// inject a dbKey into the global scope
  GlobalScope().value<Db>(dbKey, globalDb);
​
  /// Global key in scope; so use it.
  assert(
      use(dbKey, withDefault: () => testDb) == globalDb, 'use the global key');
​
  /// override the GlobalScope with outerscope
  Scope('outerscope')
    ..value<Db>(dbKey, globalDb)
    ..run(() {
      assert(use(dbKey) == globalDb, 'take from outerscope');
      assert(use(counterKey) == 5, 'always from the GlobalScope');
    });
}
​
class User {
  User(this.name);
  String name;
}
​
class Db {
  Db(this.databaseName);
​
  String databaseName;
}
​
Last modified 1yr ago