Dependency Injection

• Software design patterns where object receives dependencies from an external source, rather than creating them itself
• essential for large, scalable systems
• good for modularity, reusability, and testability by decoupling from dependencies
  • easier to mock dependencies
  • easy to change out things because it’s created in one place

roles of dependency injection

• client
  • depends on the services provided by another source
  • receives dependencies from injector
• service
  • offers a particular functionality - independent of clients
• injector
  • creates instances of services and injects into the client
  • aware of client’s dependencies, provides services during runtime
• interface
  • defines contract that service must implement
  • clients rely on these interfaces, rather than specific implementation, enabling ease of swapping implementation beneath
    • for example - a NotificationService client receives a NotificationProvider object rather than a more specific implementation like EmailProvider , enabling loose coupling