Perfecting Dependency Injection

It’s in code bases all around the world, but are we getting all the value that the Dependency Injection pattern has to offer? Large dependency graphs, brittle test suites and hidden complexity have lead me to believe that we aren't. However, I think we can! In this talk we will cover when to use this pattern, how to use it and the pitfalls that we should be watching out for. We’ll also cover some alternatives to Dependency Injection, just in case you decide that it is not for you.

Transcript

1. Perfecting Dependency Injection Matt Baker @mattc_baker
2. Please leave me feedback! @mattc_baker
3. Motivation for talk @mattc_baker
4. What is Dependency Injection? @mattc_baker
5. “Dependency Injection is a set of software design principles and patterns that enable us to develop loosely coupled code” - Mark Seemann @mattc_baker
6. @mattc_baker Loose coupling protects our components from rippling changes
7. Our beautiful class @mattc_baker
8. Our beautiful class’s tight coupling @mattc_baker
9. Deterministic @mattc_baker 1 1 + 2
10. Nondeterministic @mattc_baker FlightSqlRepository SQL Database
11. You’re being a control freak @mattc_baker
12. We need to let go of control @mattc_baker
13. Inverting control of FlightSqlRepository @mattc_baker
14. Method Injection @mattc_baker
15. Constructor Injection should be your default @mattc_baker
16. Three ways to inject dependencies @mattc_baker Constructor Injection Method Injection Property Injection
17. Some changes have been requested Log a message every time we save a new flight successfully Email customer every time we save a new flight successfully Email customer every time we fail to save a new flight @mattc_baker Publish a message every time we save a new flight successfully
18. FlightBooker constructor after changes @mattc_baker
19. BookFlight implementation after change @mattc_baker
20. How many dependencies is too many dependencies? @mattc_baker
21. Facade Services, or presenting many things as one @mattc_baker
22. Discovering Facade Services Save flights Email customer IFlightRepository ILogger IMessagePublisher IEmailSender @mattc_baker
23. FlightSaver Facade Service @mattc_baker
24. FlightSaver Facade Service Save method @mattc_baker
25. FlightBooker constructor after Facade Service @mattc_baker
26. BookFlight method after Facade Service @mattc_baker
27. Consuming FlightBooker, passing the buck @mattc_baker
28. @mattc_baker
29. Going a little deeper... @mattc_baker
30. Wouldn’t this be nice? Impossible without tight coupling? @mattc_baker
31. All I want is a parameterless constructor @mattc_baker
32. Inversion of Control Container @mattc_baker
33. IoC Container wiring @mattc_baker
34. What dependencies does FlightBooker need? @mattc_baker
35. Service Location, maybe an anti-pattern? @mattc_baker
36. Alternatively, use a Composition Root @mattc_baker
37. Favor Composition Roots @mattc_baker
38. IoC Container and a Composition Root @mattc_baker
39. Isn’t that still Service Location? @mattc_baker
40. Handling changes with the Composition Root Add a timestamp to every log that is written @mattc_baker
41. ILogger implementation @mattc_baker
42. Change Logger directly @mattc_baker Logger Logger Timestamp
43. Intercepting calls to the logger @mattc_baker Logger Timestamp Logger
44. ILogger implementation @mattc_baker
45. Create another ILogger implementation @mattc_baker
46. Intercepting calls using the Decorator pattern @mattc_baker Logger Timestamp Logger
47. Back in the Composition Root @mattc_baker
48. Compose the new implementation in the Composition Root @mattc_baker
49. Interception! @mattc_baker
50. One more change Get rid of calls to message publisher in FlightSaver. @mattc_baker
51. Cross cutting concerns in FlightSaver facade @mattc_baker
52. Implement IFlightSaver again @mattc_baker
53. Body of Save method @mattc_baker
54. Compose new IFlightSaver implementation @mattc_baker
55. Previous FlightSaver constructor with IMessagePublisher @mattc_baker
56. Updated FlightSaver constructor @mattc_baker
57. Previous FlightSaver Save implementation @mattc_baker
58. Updated FlightSaver Save implementation @mattc_baker
59. Compose your cross-cutting concerns with Interception! @mattc_baker Logging Caching Message Publishing
60. What about some alternatives? @mattc_baker
61. Domain events @mattc_baker
62. Registering Events and their Handlers @mattc_baker
63. FlightBooker implementation with no dependencies @mattc_baker
64. LogEmitted @mattc_baker
65. Sending data in is easy @mattc_baker Getting data out is hard
66. Ditch the IoC Container for Poor Man’s Dependency Injection @mattc_baker
67. Volatile dependencies = static abstractions @mattc_baker
68. @mattc_baker Things to keep in mind - Don’t be a Control Freak - Favor Constructor Injection - Only inject volatile, or nondeterministic, dependencies - Don’t inject more than X dependencies - Solve Constructor Over-Injection with Facade Services - Avoid Service Location - Use Interception for your cross-cutting concerns - Use Interception for your cross-cutting concerns - If you choose to use Dependency Injection, use it everywhere in your application - Use a Composition Root - Use only one Composition Root - Keep your Composition Root close to your entry point
69. Questions? @mattc_baker
70. “Dependency Injection is a set of software design principles and patterns that enable us to develop loosely coupled code” - Mark Seemann @mattc_baker
71. Thank you! @mattc_baker