How does dependency injection make unit testing significantly easier by letting you substitute a real dependency, such as a live network client, with a fake or mock version during a test?

Since a properly designed class receives its dependencies from the outside rather than creating them directly and internally itself, a unit test can easily substitute a real dependency, such as an actual network client that would otherwise make genuine live network calls, with a fake or mock version instead that returns predetermined, controlled test data, letting you thoroughly test a specific class's own internal logic in complete isolation, without the test's outcome ever depending on an actual real, unpredictable, and potentially slow external network call.

final mockApiClient = MockApiClient();
when(mockApiClient.fetchOrders()).thenAnswer((_) async => [testOrder]);

final service = OrderService(mockApiClient);