This document discusses dependency injection in Android applications. It begins by providing an example of how dependency injection improves testability by removing tight coupling between classes. It then addresses some common questions that arise regarding dependency injection principles. The document emphasizes that dependency injection is an architectural pattern that involves separating classes into functional and construction sets. When implemented properly, dependency injection provides benefits like easier code reuse, better maintainability, and improved testability. While an overkill for small projects, it is considered one of the most beneficial patterns for larger object-oriented applications. The document concludes by offering best practices for implementing dependency injection.

1. Dependency Injection in Android
Vasiliy Zukanov
Freelancer, blogger, online instructor
www.TechYourChance.com

2. Paris city planning:
Software Architecture Software Design

3. Do you use dependency injection?
Would you like to use it?

4. Dependency injection:
public class SomeClient {
private final SomeService mSomeService;
public SomeClient() {
mSomeService = new SomeService();
}
}
public class SomeClient {
private final SomeService mSomeService;
public SomeClient(SomeService someService) {
mSomeService = someService;
}
}
That’s bad because UNIT TESTS That’s good because UNIT TESTS

5. Dependency injection is just passing services
into clients from outside!
Thank you (talk is over)!?

6. Dependency injection puzzler #1:
public class DataAggregator {
private final List<Data> mData;
public SomeClient() {
mData = new LinkedList<>();
}
public void addData(Data data) {
mData.add(data);
}
public List<Data> getAllData() {
return mData;
}
}
Does this violate dependency injection principles?

7. Dependency injection puzzler #2:
public class SomeClient {
private final SomeService1 mSomeService1;
private final SomeService2 mSomeService2;
public SomeClient(ServiceLocator serviceLocator) {
mSomeService1 = serviceLocator.getService(SomeService1.class);
mSomeService2 = serviceLocator.getService(SomeService2.class);
}
}
Constructor argument, so everything is kosher, right?

8. Dependency injection puzzler #3:
Why dependency injection frameworks exist, and why are
they so complex?

9. Dependency injection puzzler #4:
If all clients get services from outside, where all these
services being instantiated?
That’s the most important question in context of dependency injection

10. Looks like dependency injection is not that
simple after all!

11. Dependency injection, this time for real:
The main characteristic of dependency injection is segregation of
application’s logic into two sets of classes:
Functional set:
contains classes that encapsulate core application’s functionality
Construction set:
contains classes that resolve dependencies and instantiate objects from
functional set
Functional and construction sets must be disjoint.

12. Dependency injection, schematically:
Application
Construction Set
class
Functional Set
classintegration
Segregation of classes into Construction and Functional sets is
manifestation of Separation of Concerns

13. Separation of concerns at application level of
abstraction is…
… architectural pattern!
Dependency Injection = Architectural Pattern

14. Benefits of dependency injection:
Easier reuse of functionality
Better maintainability
Clients aren’t coupled to services construction details
Clients aren’t coupled to services lifecycle
Better testability

15. One of the most beneficial architectural
patterns in object-oriented design!

16. But it’s an overkill on small projects, right?

17. Dependency injection ROI analysis:
Return: all the benefits discussed previously
Investment:
You know how to do DI: ~1 hour on a greenfield project
You learn from good resource: several days on a greenfield project
You learn from bad resource: several weeks (months, never?) on a greenfield
project

18. Very difficult to introduce dependency injection
in existing codebase!

19. Dependency injection frameworks:
Application
Construction Set
class
Functional Set
classintegration
Dependency injection frameworks are templates for implementation
of Construction set
DI framework

20. Warning about dependency injection frameworks:
“… I don’t want [Dependency Injection]
framework code smeared all through my
application. I want to keep frameworks
nicely decoupled and at arms-length from
the main body of my code. I don’t want to
have @Inject attributes everywhere and
bind calls hidden under rocks.”
- Robert “Uncle Bob” Martin

21. Dependency injection best practices

22. Use constructor injection by default:
The only exceptions are either classes that you don’t construct
(Activity, Service, etc.), or classes that must be constructed using
specific constructor (Fragment)

23. Don’t violate the Law of Demeter:
public class SomeClient {
private final SharedPreferences mSharedPreferences;
public SomeClient(Context context) {
mSharedPreferences = context.getSharedPreferences("prefs", Context.MODE_PRIVATE);
}
}
public class SomeClient {
private final SharedPreferences mSharedPreferences;
public SomeClient(SharedPreferences sharedPreferences) {
mSharedPreferences = sharedPreferences;
}
}

24. Discriminate between objects and data structures:
Objects expose behavior (e.g. UpdateUserDetailsUseCase)
Data structures (entities, value objects, data classes, List, String, etc.)
expose data (e.g. User)
Even though both objects and data structures extend Object class in
Java, they represent different concepts.
Dependency injection applicable only to objects!
Rule of thumb: no data structures in Construction set

25. Exclude Android Views from dependency injection:
Don’t add Android Views to objects graph
Don’t inject into Android Views from Construction set
Rule of thumb: no Android Views in Construction set

26. Dependency injection best practices:
Use constructor injection by default
Don’t violate the Law of Demeter
Discriminate between objects and data structures
Exclude Android Views from dependency injection

27. Who would like to use dependency injection?

28. My advanced Android development courses (links on my blog):
Dependency Injection in Android with Dagger 2
Android Unit Testing and Test Driven Development
Android Applications Architecture

29. www.TechYourChance.com

30. Questions

31. Thank you!