The document discusses dependency injection and compares two popular libraries for implementing it - Dagger and Koin. It provides an overview of dependency injection and how it works. It then describes how Dagger uses annotations to define dependencies that are compiled into code, while Koin uses a Kotlin DSL without code generation at runtime. It highlights pros and cons of each, such as Dagger being fully type safe but more complex, while Koin is easier to use but lacks type safety.

1. Taipei
Jintin
Dagger vs koin

2. Designed for the mobile
generation
● Snap, list, sell
● Private chat
● A.I. assisted selling
● Personalised content

3. What is Dependency Injection?

4. public class Twitter {
public void getFeeds() {
TwitterApi api = new TwitterApi();
api.getFeeds();
}
}
public class TwitterApi {
public void getFeeds () {
HttpClient httpClient = new HttpClient();
httpClient.connect("http://xxx.com")
}
}

5. public class Twitter {
public void getFeeds() {
TwitterApi api = new TwitterApi();
api.getFeeds();
}
}
public class TwitterApi {
public void getFeeds () {
HttpClient httpClient = new HttpClient();
httpClient.connect("http://xxx.com")
}
}

6. What’s the problem now?
● New is evil
● High coupling
● Hard to replace
● Hard to test

7. Dependency Injection
Dependency injection is basically providing
the objects that an object needs (its
dependencies) instead of having it construct
them itself.

8. How to Inject Object?
● Constructor Injection
● Method Injection
● Field Injection

9. public class Twitter {
public void getFeeds() {
TwitterApi api = new TwitterApi();
api.getFeeds();
}
}
public class TwitterApi {
public void getFeeds () {
HttpClient httpClient = new HttpClient();
httpClient.connect("http://xxx.com")
}
}

10. public class Twitter {
public void getFeeds(TwitterApi api) {
api.getFeeds();
}
}
public class TwitterApi {
HttpClient httpClient;
public TwitterApi(HttpClient httpClient) {
this.httpClient = httpClient;
}
}

11. What’s the problem now?
● Hard to use
● Manage dependency graph is a mess
● Dagger is the rescue

12. What is Dagger?

13. Dagger 2
● Dependency Injection tool
● Simple Annotation structure
● Readable generated code
● No reflection (fast)
● No runtime error
● Proguard friendly
● Pure Java
● https://github.com/google/dagger
● https://android.jlelse.eu/dagger-2-c00f31decda5

14. Annotation Processing
● Annotate in source code
● Start compile
● Read Annotation and relevant data
● Generate code under build folder
● Compile finish
● https://medium.com/@jintin/annotation-pr
ocessing-in-java-3621cb05343a

15. Annotations in Dagger2
● @Provides
● @Module
● @Component
● @Inject (in Java)
● @Qualifier (@Named)
● @Scope (@Singleton)

16. Bee Honey
Lemon

17. // Object class
class Lemon
class Bee
class Honey(var bee: Bee)
class HoneyLemonade
@Inject
constructor(var honey: Honey, var lemon: Lemon)

18. @Provides
Lemon provideLemon() {
return new Lemon();
}
@Provides
Honey provideHoney(Bee bee) {
return new Honey(bee);
}
@Provides
Bee provideBee() {
return new Bee();
}

19. @Module
public class DrinkModule {
@Provides
Lemon provideLemon() {...}
@Provides
Honey provideHoney(Bee bee) {...}
@Provides
Bee provideBee() {...}
}

20. @Component(modules = {DrinkModule.class})
public interface DrinkComponent {
HoneyLemonade drink();
void inject(MainActivity activity);
}

21. public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DrinkComponent co = DaggerDrinkComponent.create();
HoneyLemonade drink = co.drink();
}
}

22. public class MainActivity extends AppCompatActivity {
@Inject
HoneyLemonade drink;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DrinkComponent co = DaggerDrinkComponent.create();
co.inject(this);
}
}

23. Recap
● Provides: provide dependency
● Modules: group Provides
● Component: The relation graph
● Inject: Identifier for inject

24. Other feature
● Qualifier: Named...
● Scope: Singleton...
● Subcomponent / includes
● Lazy / Provider...

25. Demo

26. What is koin?

27. koin
● Service locator by Kotlin DSL
● No code generation
● No reflection
● No compile time overhead
● Pure Kotlin
● https://github.com/InsertKoinIO/koin

28. Service locator
The service locator pattern is a design pattern
used in software development to encapsulate the
processes involved in obtaining a service with a
strong abstraction layer.

29. Object Locator
Lemon
Honey

30. Kotlin DSL
● Function literals with receiver
● Describe data structure
● Like Anko, ktor

31. // HTML DSL
val result =
html {
head {
title {+"XML encoding with Kotlin"}
}
body {
h1 {+"XML encoding with Kotlin"}
p {+"this format can be used"}
a(href = "http://kotlinlang.org") {+"Kotlin"}
}
}

32. fun html(init: HTML.() -> Unit): HTML {
val html = HTML()
html.init()
return html
}
fun body(init: Body.() -> Unit) : Body {
val body = Body()
body.init()
return body
}

33. koin DSL
● Module - create a Koin Module
● Factory - provide a factory bean definition
● Single - provide a singleton bean definition
(also aliased as bean)
● Get - resolve a component dependency

34. // gradle install
dependencies {
// Koin for Android
compile 'org.koin:koin-android:1.0.2'
// or Koin for Lifecycle scoping
compile 'org.koin:koin-android-scope:1.0.2'
// or Koin for Android Architecture ViewModel
compile 'org.koin:koin-android-viewmodel:1.0.2'
}

35. // Object class
class Bee
class Honey(var bee: Bee)
class Lemon
class HoneyLemonade(var honey: Honey,
var lemon: Lemon)

36. class MyApplication : Application() {
override fun onCreate(){
super.onCreate()
// start Koin!
startKoin(this, listOf(myModule))
}
}
val myModule = module {
single { Bee() }
single { Honey(get()) }
single { Lemon() }
single { HoneyLemonade(get(), get()) }
}

37. // How get() work? reified
inline fun <reified T : Any> get(
name: String = "",
scopeId: String? = null,
noinline parameters: ParameterDefinition = ...
): T {
val scope: Scope? = scopeId?.let {
koinContext.getScope(scopeId)
}
return koinContext.get(name, scope, parameters)
}

38. reified function
● Work with inline
● Can access actual type T
● After compile T will replace with actual type

39. class MyActivity() : AppCompatActivity() {
val drink : HoneyLemonade by inject()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val drink : HoneyLemonade = get()
}
}

40. // How inject(), get() work?
inline fun <reified T : Any> ComponentCallbacks.inject(
name: String = "",
scope: Scope? = null,
noinline parameters: ParameterDefinition = ...
) = lazy { get<T>(name, scope, parameters) }
inline fun <reified T : Any> ComponentCallbacks.get(
name: String = "",
scope: Scope? = null,
noinline parameters: ParameterDefinition = ...
): T = getKoin().get(name, scope, parameters)
// What is ComponentCallbacks btw?

41. Demo

42. Dagger vs koin

43. Dagger
● Pros:
○ + Pure Java
○ + Stable, flexible, powerful
○ + No Runtime error
○ + Fast in Runtime
● Cons:
○ - Compile overhead
○ - Hard to learn

44. koin
● Pros:
○ + No Annotation processing
○ + Easy to learn/setup
● Cons:
○ - Hard to replace/remove
○ - Test in its env
○ - Error in Runtime

45. “you and only you are responsible
for your life choices and decisions”
- Robert T. Kiyosaki

46. Jintin
Taipei
Thank you!