Breaking a monolithic Android application into modules is not new, and this way of organizing code is becoming more common. We have already touched on this topic at the meeting dedicated to the best practices of working with modules among colleagues. We have collected this experience, tested it on our project and we want to share the conclusions and advice we came to. Therefore, this article can be useful both for those who are just thinking about separation, and for those who have already started it.

Developers usually think about using multi-modularity to speed up build times. But that was not the most important thing for us. In addition to the build speed, multi-modularity also provides a stricter architecture and the ability to reuse features between projects.

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1. App- — , Feature-.

   
   
   

2. Feature- — , , -. , , - (, UI- , , UI). Feature- API Feature- Core-.
   
   Feature- API , API . internal , API, «» Feature-. , . API Impl , , .
   
   .

   
   
   

3. Core- — , , Feature-. , . Core- . : module-injector.

   
   
   

4. Module-injector — , . , . .

   
   
   

— API Impl Feature-, Feature- . internal- Kotlin.

Example- , App-. ( ) , . .

:

Module-Injector

, . , . , , . , , Dagger ( DI-). , :

interface ComponentHolder<C : BaseAPI, D : BaseDependencies> { 
    fun init(dependencies: D) 
    fun get(): C
    fun reset() 
} 

interface BaseDependencies

interface BaseAPI

. Feature- . ( , ) internal, .

- , , , Kotlin ( 2020-) module-injector. . .

, , — . : , . UI, , — .

Feature- , - , Core-. , :

, API Feature-. Feature-: :feature_purchase_api :feature_purchase_impl. API- , :module-injector. API-.

, . , .

ComponentHolder-:

object PurchaseComponentHolder : ComponentHolder<PurchaseFeatureApi, PurchaseFeatureDependencies> {
    private var purchaseComponentHolder: PurchaseComponent? = null

    override fun init(dependencies: PurchaseFeatureDependencies) {
        if (purchaseComponentHolder == null) {
            synchronized(PurchaseComponentHolder::class.java) {
                if (purchaseComponentHolder == null) {
                    purchaseComponentHolder = PurchaseComponent.initAndGet(dependencies)
                }
            }
        }
    }

    override fun get(): PurchaseFeatureApi {
        checkNotNull(purchaseComponentHolder) { "PurchaseComponent was not initialized!" }
        return purchaseComponentHolder!!
    }

    override fun reset() {
        purchaseComponentHolder = null
    }
}

:

• .
• init(), .
• get(), API .
• reset(), , .

. , .

PurchaseFeatureApi PurchaseFeatureDependencies , .

ComponentHolder Dagger-:

@Component(dependencies = [PurchaseFeatureDependencies::class], modules = [PurchaseModule::class])
@PerFeature
internal abstract class PurchaseComponent : PurchaseFeatureApi {

    companion object {
        fun initAndGet(purchaseFeatureDependencies: PurchaseFeatureDependencies): PurchaseComponent {
            return DaggerPurchaseComponent.builder()
                    .purchaseFeatureDependencies(purchaseFeatureDependencies)
                    .build()
        }
    }
}

initAndGet(), ComponentHolder. , Dagger . DI- .

, app :

@Module
class AppModule {

    @Singleton
    @Provides
    fun provideScannerFeatureDependencies(featurePurchase: PurchaseFeatureApi): ScannerFeatureDependencies {
        return object : ScannerFeatureDependencies {
            override fun dbClient(): DbClient = CoreDbComponent.get().dbClient()
            override fun httpClient(): HttpClient = CoreNetworkComponent.get().httpClient()
            override fun someUtils(): SomeUtils = CoreUtilsComponent.get().someUtils()
            override fun purchaseInteractor(): PurchaseInteractor = featurePurchase.purchaseInteractor()
        }
    }

    //      -   
    @Provides
    fun provideFeatureScanner(dependencies: ScannerFeatureDependencies): ScannerFeatureApi {
        ScannerFeatureComponentHolder.init(dependencies)
        return ScannerFeatureComponentHolder.get()
    }
    ...
}

provideScannerFeatureDependencies() ScannerFeatureDependencies, provideFeatureScanner() ComponentHolder- .

, app- . , . app- , . app- .

, .

, ComponentHolder reset(), . UI, reset() Lifecycle Observer- ( Activity, ):

public override fun onPause() {
   super.onPause()
           ...
   if (isFinishing) {
       AntitheftFeatureComponentHolder.reset()
   }
}

, , . get().

// get()  Feature-:
object PurchaseComponentHolder : ComponentHolder<PurchaseFeatureApi, PurchaseFeatureDependencies> {
    private var purchaseComponentHolder: PurchaseComponent? = null
...
    override fun get(): PurchaseFeatureApi {
        checkNotNull(purchaseComponentHolder) { "PurchaseComponent was not initialized!" }
        return purchaseComponentHolder!!
    }

    override fun reset() {
        purchaseComponentHolder = null
    }
// get()  app-:
//    @Singleton       Provider,  ,   get()  Dagger-
@Provides
   fun provideFeatureScanner(dependencies: ScannerFeatureDependencies): ScannerFeatureApi {
       ScannerFeatureComponentHolder.init(dependencies)
       return ScannerFeatureComponentHolder.get()
   }
}

, DI- app- (, Singleton Dagger). init() , , reset() .

API- — Provider<T> :

class GlobalNavigator @Inject constructor(
       //  Provider    get()                
       private val featureScanner: Provider<ScannerFeatureApi>,
       private val featureAntitheft: Provider<AntitheftFeatureApi>,
       private val context: Context
) : Navigator {
   ...
   featureScanner.get().scannerStarter().start(context) //  
   ...
}

UI

, API , . API , UI-, Activity, , View.

Activity API :

interface AntitheftStarter {
    fun start(context: Context)
}

Activity , Intent:

internal class AntitheftStarterImpl @Inject constructor() : AntitheftStarter {
    override fun start(context: Context) {
        val intent = Intent(context, AntitheftActivity::class.java)
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
        context.startActivity(intent)
    }
}

FragmentManager , API , . : Cicerone, Navigation Component FragmentManager.

, , . , .

Core-ui

UI- , , UI- . UIKit. , Application.

, theme.xml, (, Example-, , ). core-ui , , , . UIKit (api implementation) Feature-, UI. .

Core-strings

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Core-native

C++-. JNI-, Java-. , SDK. , , ABI. , .

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