Dependency Injection and Testing

Testing becomes easier when a component does not create all of its own dependencies. Dependency injection means providing those dependencies from outside the component.

The idea is simple: give a screen model the services it needs instead of making it construct a database, network client, and clock internally.

Start with constructor injection

An order screen model might accept these dependencies:

OrdersScreenModel(ordersRepository, clock)

The repository provides order data. The clock provides the current time for rules that depend on time.