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docs/quickstart/android-annotations.md

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+---
+title: Android - Annotations
+---
+
+> This tutorial lets you write an Android application and use Koin dependency injection to retrieve your components.
+> You need around __10/15 min__ to do the tutorial.
+
+## Get the code
+
+:::info
+[The source code is available at on Github](https://github.com/InsertKoinIO/koin-getting-started/tree/main/android)
+:::
+
+## Gradle Setup
+
+Let's configure the KSP Plugin like this:
+
+```groovy
+apply plugin: 'com.google.devtools.ksp'
+
+android {
+    sourceSets {
+        main.java.srcDirs += 'src/main/kotlin'
+        test.java.srcDirs += 'src/test/kotlin'
+    }
+    // For KSP
+    applicationVariants.configureEach { variant ->
+        kotlin.sourceSets {
+            getByName(name) {
+                kotlin.srcDir("build/generated/ksp/${variant.name}/kotlin")
+            }
+        }
+    }
+}
+```
+
+Add the Koin Android dependency like below:
+
+```groovy
+dependencies {
+    // Koin
+    implementation "io.insert-koin:koin-android:$koin_version"
+    implementation "io.insert-koin:koin-annotations:$koin_ksp_version"
+    ksp "io.insert-koin:koin-ksp-compiler:$koin_ksp_version"
+}
+```
+
+
+
+## Application Overview
+
+The idea of the application is to manage a list of users, and display it in our `MainActivity` class with a Presenter or a ViewModel:
+
+> Users -> UserRepository -> (Presenter or ViewModel) -> MainActivity
+
+## The "User" Data
+
+We will manage a collection of Users. Here is the data class: 
+
+```kotlin
+data class User(val name : String)
+```
+
+We create a "Repository" component to manage the list of users (add users or find one by name). Here below, the `UserRepository` interface and its implementation:
+
+```kotlin
+interface UserRepository {
+    fun findUser(name : String): User?
+    fun addUsers(users : List<User>)
+}
+
+class UserRepositoryImpl : UserRepository {
+
+    private val _users = arrayListOf<User>()
+
+    override fun findUser(name: String): User? {
+        return _users.firstOrNull { it.name == name }
+    }
+
+    override fun addUsers(users : List<User>) {
+        _users.addAll(users)
+    }
+}
+```
+
+## The Koin module
+
+Let's declare a `AppModule` module class like below. 
+
+```kotlin
+@Module
+@ComponentScan("org.koin.sample")
+class AppModule
+```
+
+* We use the `@Module` to declare our class as Koin module
+* The `@ComponentScan("org.koin.sample")` allow to scann any Koin definition in `"org.koin.sample"`package
+
+Let's simply add `@Single` on `UserRepositoryImpl` class to declare it as singleton:
+
+```kotlin
+@Single
+class UserRepositoryImpl : UserRepository {
+    // ...
+}
+```
+
+## Displaying User with Presenter
+
+Let's write a presenter component to display a user:
+
+```kotlin
+class UserPresenter(private val repository: UserRepository) {
+
+    fun sayHello(name : String) : String{
+        val foundUser = repository.findUser(name)
+        return foundUser?.let { "Hello '$it' from $this" } ?: "User '$name' not found!"
+    }
+}
+```
+
+> UserRepository is referenced in UserPresenter`s constructor
+
+We declare `UserPresenter` in our Koin module. We declare it as a `factory` definition with the `@Factory` annotation, to not keep any instance in memory (avoid any leak with Android lifecycle):
+
+```kotlin
+@Factory
+class UserPresenter(private val repository: UserRepository) {
+    // ...
+}
+```
+
+## Injecting Dependencies in Android
+
+The `UserPresenter` component will be created, resolving the `UserRepository` instance with it. To get it into our Activity, let's inject it with the `by inject()` delegate function: 
+
+```kotlin
+class MainActivity : AppCompatActivity() {
+
+    private val presenter: UserPresenter by inject()
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+
+        //...
+    }
+}
+```
+
+That's it, your app is ready.
+
+:::info
+The `by inject()` function allows us to retrieve Koin instances, in Android components runtime (Activity, fragment, Service...)
+:::
+
+## Start Koin
+
+We need to start Koin with our Android application. Just call the `startKoin()` function in the application's main entry point, our `MainApplication` class:
+
+```kotlin
+// generated
+import org.koin.ksp.generated.*
+
+class MainApplication : Application(){
+    override fun onCreate() {
+        super.onCreate()
+
+        startKoin{
+            androidLogger()
+            androidContext(this@MainApplication)
+            modules(AppModule().module)
+        }
+    }
+}
+```
+
+The Koin module is generated from `AppModule` with the `.module` extension: Just use the `AppModule().module` expression to get the Koin module from the annotations.
+
+:::info
+The `import org.koin.ksp.generated.*` import is required to allow to use generated Koin module content
+:::
+
+## Displaying User with ViewModel
+
+Let's write a ViewModel component to display a user:
+
+```kotlin
+@KoinViewModel
+class UserViewModel(private val repository: UserRepository) : ViewModel() {
+
+    fun sayHello(name : String) : String{
+        val foundUser = repository.findUser(name)
+        return foundUser?.let { "Hello '$it' from $this" } ?: "User '$name' not found!"
+    }
+}
+```
+
+> UserRepository is referenced in UserViewModel`s constructor
+
+The `UserViewModel` is tagged with `@KoinViewModel` annotation to declare the Koin ViewModel definition, to not keep any instance in memory (avoid any leak with Android lifecycle).
+
+
+## Injecting ViewModel in Android
+
+The `UserViewModel` component will be created, resolving the `UserRepository` instance with it. To get it into our Activity, let's inject it with the `by viewModel()` delegate function: 
+
+```kotlin
+class MainActivity : AppCompatActivity() {
+
+    private val viewModel: UserViewModel by viewModel()
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+
+        //...
+    }
+}
+```
+
+## Compile Time Checks
+
+Koin Annotations allows to check your Koin configuration at compile time. This is available by jusing the following Gradle option:
+
+```groovy
+ksp {
+    arg("KOIN_CONFIG_CHECK","true")
+}
+```
+
+## Verifying your App!
+
+We can ensure that our Koin configuration is good before launching our app, by verifying our Koin configuration with a simple JUnit Test.
+
+### Gradle Setup
+
+Add the Koin Android dependency like below:
+
+```groovy
+// Add Maven Central to your repositories if needed
+repositories {
+	mavenCentral()    
+}
+
+dependencies {
+    
+    // Koin for Tests
+    testImplementation "io.insert-koin:koin-test-junit4:$koin_version"
+}
+```
+
+### Checking your modules
+
+The `verify()` function allow to verify the given Koin modules:
+
+```kotlin
+class CheckModulesTest : KoinTest {
+
+    @Test
+    fun checkAllModules() {
+
+        AppModule().module.verify(
+            extraTypes = listOf(
+                SavedStateHandle::class
+            ))
+    }
+}
+```
+
+With just a JUnit test, you can ensure your definitions configuration are not missing anything!