U+2019 @egorand Modern Android Architecture 

U+2020 

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Stored Balance 

Stored Balance P2P Payments 

Stored Balance P2P Payments Cash Card 

Stored Balance P2P Payments Cash Card In-App Receipts 

Stored Balance P2P Payments Cash Card In-App Receipts Cash-Ins 

Stored Balance P2P Payments Cash Card In-App Receipts Instant Cash-Outs* Cash-Ins 

Stored Balance P2P Payments Cash Card In-App Receipts Instant Cash-Outs* Boosts Cash-Ins 

Stored Balance P2P Payments Cash Card In-App Receipts Instant Cash-Outs* Boosts Bitcoin Cash-Ins 

Challenges 

Release Model • Release trains • Go-live every 2 weeks • Only hot-fixes allowed in between • Daily server deploys 

Backwards Compat • ~24% of customers on older versions • Mission-critical • Frequent server deploys • Moving fast 

Workstreams • Features developed in parallel • Shared codebase • Merges into master • Android/iOS parity 

Migration to Kotlin • Better developer experience • Faster builds • Large codebase • Complex code 

Architectural debt • Multiple presenter implementations • Navigation framework • Testability • Large modules 

Architecture Overview 

app presenters backend viewmodels api db protos 

protos Protocol Buffer messages Generated code Gradle configs for pulling and compiling protos 

Protocol Buffers Schema Compilers for many platforms Binary on the wire Backwards compatible 

Workflow .proto files uploaded to Artifactory .zip with protos gets pulled down Protos compiled into .java/.kt Generated code checked into git 

Wire .proto compiler built at Square Android-aware Java + Kotlin CLI + Gradle 

message Person { // The customer's full name. required string name = 1; // The customer's ID number. required int32 id = 2; // Email address for the customer. optional string email = 3; enum PhoneType { MOBILE = 0; HOME = 1; /** Could be phone or fax. */ WORK = 2; } message PhoneNumber { // The customer's phone number. required string number = 1; // The type of phone stored here. optional PhoneType type = 2 [default = HOME]; } // A list of the customer's phone numbers. repeated PhoneNumber phone = 4; } .proto public final class Person extends Message { public final String name; public final Integer id; public final String email; public final List phone; // constructors, equals, hashCode, toString public static final class Builder {} public enum PhoneType implements WireEnum { MOBILE(0), HOME(1), WORK(2); } public static final class PhoneNumber extends Message { public final String number; public final PhoneType type; // constructors, equals, hashCode, toString public static final class Builder {} } .java (458 lines in original) Wire 2 

message Person { // The customer's full name. required string name = 1; // The customer's ID number. required int32 id = 2; // Email address for the customer. optional string email = 3; enum PhoneType { MOBILE = 0; HOME = 1; /** Could be phone or fax. */ WORK = 2; } message PhoneNumber { // The customer's phone number. required string number = 1; // The type of phone stored here. optional PhoneType type = 2 [default = HOME]; } // A list of the customer's phone numbers. repeated PhoneNumber phone = 4; } .proto data class Person( val name: String, val id: Int, val email: String? = null, val phone: List = emptyList(), val unknownFields: ByteString = ByteString.EMPTY ) : Message { enum class PhoneType(private val value: Int) : WireEnum { MOBILE(0), HOME(1), WORK(2); } data class PhoneNumber( val number: String, val type: PhoneType? = PhoneType.HOME, val unknownFields: ByteString = ByteString.EMPTY ) : Message } .kt (206 lines in original) Wire 3 

Wire 2 task generateProtos(type: JavaExec) { classpath = configurations.wire main = 'com.squareup.wire.WireCompiler' args = [ '--proto_path=protos/src/test/proto', '--java_out=protos/src/test/proto-java', '--android', 'squareup.cash.Person' ] } apply plugin: 'com.squareup.wire' wire { sourcePath 'com.squareup.cash:protos:+' roots 'squareup.cash.Person' kotlin { out "${buildDir}/generated/custom" } } Wire 3 

b viewmodels api p Retrofit interface OkHttp interceptors Networking helpers 

interface AppService { @POST("/2.0/cash/initiate-payment") fun initiatePayment( @Body request: InitiatePaymentRequest ): Observable @POST("/2.0/cash/get-profile") fun getProfile( @Body request: GetProfileRequest ): Observable // other methods } AppService RxJava Observables Request/response are protos 

Blockers InitiatePaymentResponse responseContext: ResponseContext ResponseContext blockers: List SetNameBlocker PasscodeBlocker SetAddressBlocker 

Client 1. InitiatePaymentRequest Server 1. Passcode verification required 2. Send a passcode verification blocker 

Client 3. Display PasscodeVerification screen 4. Prompt for passcode 5. Send passcode hash to server 

Client Server 6. IDV required 7. Send IDV blocker etc. 

ackend protos db DB schema in .sq files Generated SQLDelight code Migrations in .sqm files 

ORM Room SQLDelight Annotate entities Use custom DSL Annotate entities Write SQL Bring your own DSL Write SQL Use generated code 

Why? 

Everyone understands SQL SQL is standardized SQL has tooling 

import com.example.sqldelight.hockey.data.PlayerVals; import com.example.sqldelight.hockey.data.Date; CREATE TABLE player ( id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, first_name TEXT NOT NULL, last_name TEXT NOT NULL, number INTEGER AS Integer NOT NULL, team INTEGER, age INTEGER AS Integer NOT NULL, birth_date INTEGER AS Date NOT NULL, weight REAL AS Float NOT NULL, shoots TEXT AS PlayerVals.Shoots NOT NULL, position TEXT AS PlayerVals.Position NOT NULL, FOREIGN KEY (team) REFERENCES team(id) ); insertPlayer: INSERT INTO player (first_name, last_name, number, team) VALUES (?, ?, ?, (SELECT id FROM team WHERE name = ?)); selectAll: SELECT * FROM player JOIN team ON player.team = team.id; forTeam: SELECT first_name, last_name, number, team.name AS teamName FROM player JOIN team ON player.team = team.id WHERE team.id = :team_id OR :team_id = -1; SQLDelight IDE plugin Gradle plugin Migrations verifier Multiplatform! 

Workflow Add SQL to .sq files Run ./gradlew :db:assemble Use generated code 

Workflow: Migrations Add SQL to a new .sqm file Run ./gradlew :db:verifyMigration 

app enters backend Facades for :db and :api Models for upper layers based on protos Mostly interfaces 

class AppMessagePresenter( cashDatabase: CashDatabase ) { private val appMessageQueries = cashDatabase.appMessageQueries } 

class AppMessagePresenter( cashDatabase: CashDatabase ) { private val appMessageQueries = cashDatabase.appMessageQueries override fun subscribe() { val activityInlineAppMessage = appMessageQueries .foregroundVideo(ACTIVITY_INLINE) .asObservable(ioScheduler) .mapToKOptional() } } 

class AppMessagePresenter( cashDatabase: CashDatabase, profileManager: ProfileManager ) { private val appMessageQueries = cashDatabase.appMessageQueries override fun subscribe() { val activityInlineAppMessage = appMessageQueries .foregroundVideo(ACTIVITY_INLINE) .asObservable(ioScheduler) .mapToKOptional() } } 

class AppMessagePresenter( cashDatabase: CashDatabase, profileManager: ProfileManager ) { private val appMessageQueries = cashDatabase.appMessageQueries override fun subscribe() { val activityInlineAppMessage = combineLatest( appMessageQueries .foregroundVideo(ACTIVITY_INLINE) .asObservable(ioScheduler) .mapToKOptional(), profileManager.profile() ) { appMessage, profile -> // a lot of code } } } 

AppMessageManager activityInlineAppMessage(): Observable RealAppMessageManager FakeAppMessageManager 

class RealAppMessageManager( cashDatabase: CashDatabase, profileManager: ProfileManager ) { private val appMessageQueries = cashDatabase.appMessageQueries override fun activityInlineAppMessage(): Observable { return combineLatest( appMessageQueries .foregroundVideo(ACTIVITY_INLINE) .asObservable(ioScheduler) .mapToKOptional(), profileManager.profile() ) { appMessage, profile -> // a lot of code } } } 

class AppMessagePresenter( appMessageManager: AppMessageManager ) { override fun subscribe() { val activityInlineAppMessage = appMessageManager .activityInlineAppMessage .subscribe() } } 

class FakeAppMessageManager : AppMessageManager { val activityInlineAppMessages = BehaviorRelay.create() override fun activityInlineAppMessage() = activityInlineAppMessages.hide() } 

class AppMessagePresenterTest { private val appMessageManager = FakeAppMessageManager() } 

class AppMessagePresenterTest { private val appMessageManager = FakeAppMessageManager() private fun makePresenter() { return AppMessagePresenter(appMessageManager) } } 

class AppMessagePresenterTest { private val appMessageManager = FakeAppMessageManager() private fun makePresenter() { return AppMessagePresenter(appMessageManager) } @Test fun `appMessage is included in viewmodel`() { val viewModel = Observable.wrap(makePresenter()).test() val expectedAppMessage = AppMessage() appMessageManager.activityInlineAppMessages.accept(appMessage) } } 

class AppMessagePresenterTest { private val appMessageManager = FakeAppMessageManager() private fun makePresenter() { return AppMessagePresenter(appMessageManager) } @Test fun `appMessage is included in viewmodel`() { val viewModel = Observable.wrap(makePresenter()).test() val expectedAppMessage = AppMessage() appMessageManager.activityInlineAppMessages.accept(appMessage) with(viewModel.latest()) { assertThat(appMessage).isEqualTo(expectedAppMessage) } } } 

a pres viewmodels ViewModels: presenter → view ViewEvents: view → presenter Data classes 

ViewModel ViewEvent View Presenter 

ViewModel ViewEvent data class SendPaymentViewModel( val toolbarActionText: String, val toolbarAmount: String, val selectedInstrument: String? = null ) sealed class SendPaymentViewEvent { object SendPayment : SendPaymentViewEvent() class UpdateNote( val note: String = "" ) : SendPaymentViewEvent() } 

app presenters Subscribe to ViewEvents Emit ViewModels Handle navigation 

Stateless 

class FileBlockerPresenter : ObservableTransformer 

class FileBlockerPresenter : ObservableTransformer { override fun apply( viewEvents: Observable ): ObservableSource { return viewEvents.publish { events -> Observable.merge( listOf( events.filterIsInstance().compose(navigationLogic), events.filterIsInstance().compose(goToSupport), events.filterIsInstance().compose(uploadBitmaps) ) ) } } } 

class FileBlockerPresenter : ObservableTransformer { override fun apply( viewEvents: Observable ): ObservableSource { return viewEvents.publish { events -> Observable.merge( listOf( events.filterIsInstance().compose(navigationLogic), events.filterIsInstance().compose(goToSupport), events.filterIsInstance().compose(uploadBitmaps) ) ) } } private val navigationLogic = ObservableTransformer { events -> events.doOnNext { event -> navigator.goTo( when (event) { // convert an event into a navigation target } } } } 

class FileBlockerPresenterTest 

class FileBlockerPresenterTest { private lateinit var events: PublishSubject private lateinit var viewModels: TestObserver private val navigator = FakeNavigator() } 

class FileBlockerPresenterTest { private lateinit var events: PublishSubject private lateinit var viewModels: TestObserver private val navigator = FakeNavigator() private fun setupPresenter() { events = PublishSubject.create() val presenter = FileBlockerPresenter(navigator) viewModels = events .compose(presenter) .test() } } 

class FileBlockerPresenterTest { private lateinit var events: PublishSubject private lateinit var viewModels: TestObserver private val navigator = FakeNavigator() private fun setupPresenter() { events = PublishSubject.create() val presenter = FileBlockerPresenter(navigator) viewModels = events .compose(presenter) .test() } @Test fun captureCompleted_noDialogMessage() { } } 

class FileBlockerPresenterTest { private lateinit var events: PublishSubject private lateinit var viewModels: TestObserver private val navigator = FakeNavigator() private fun setupPresenter() { events = PublishSubject.create() val presenter = FileBlockerPresenter(navigator) viewModels = events .compose(presenter) .test() } @Test fun captureCompleted_noDialogMessage() { navigator.nextScreen = Profile } } 

class FileBlockerPresenterTest { private lateinit var events: PublishSubject private lateinit var viewModels: TestObserver private val navigator = FakeNavigator() private fun setupPresenter() { events = PublishSubject.create() val presenter = FileBlockerPresenter(navigator) viewModels = events .compose(presenter) .test() } @Test fun captureCompleted_noDialogMessage() { navigator.nextScreen = Profile events.onNext(CaptureCompleted()) } } 

class FileBlockerPresenterTest { private lateinit var events: PublishSubject private lateinit var viewModels: TestObserver private val navigator = FakeNavigator() private fun setupPresenter() { events = PublishSubject.create() val presenter = FileBlockerPresenter(navigator) viewModels = events .compose(presenter) .test() } @Test fun captureCompleted_noDialogMessage() { navigator.nextScreen = Profile events.onNext(CaptureCompleted()) viewModels.assertValueAt(0, Uploading) viewModels.assertValueCount(1) } } 

class FileBlockerPresenterTest { private lateinit var events: PublishSubject private lateinit var viewModels: TestObserver private val navigator = FakeNavigator() private fun setupPresenter() { events = PublishSubject.create() val presenter = FileBlockerPresenter(navigator) viewModels = events .compose(presenter) .test() } @Test fun captureCompleted_noDialogMessage() { navigator.nextScreen = Profile events.onNext(CaptureCompleted()) viewModels.assertValueAt(0, Uploading) viewModels.assertValueCount(1) assertThat(navigator.takeNextScreen()).isEqualTo(Profile) } } 

app Emit ViewEvents Render ViewModels Only talk to presenters 

class FileBlockerView( context: Context, attrs: AttributeSet? ) : RelativeLayout(context, attrs) 

class FileBlockerView( context: Context, attrs: AttributeSet? ) : RelativeLayout(context, attrs) { private lateinit var presenter: FileBlockerPresenter } 

class FileBlockerView( context: Context, attrs: AttributeSet? ) : RelativeLayout(context, attrs) { private lateinit var presenter: FileBlockerPresenter private val viewEvents = PublishSubject.create() } 

class FileBlockerView( context: Context, attrs: AttributeSet? ) : RelativeLayout(context, attrs) { private lateinit var presenter: FileBlockerPresenter private val viewEvents = PublishSubject.create() private lateinit var disposables: CompositeDisposable } 

class FileBlockerView( context: Context, attrs: AttributeSet? ) : RelativeLayout(context, attrs) { private lateinit var presenter: FileBlockerPresenter private val viewEvents = PublishSubject.create() private lateinit var disposables: CompositeDisposable override fun onAttachedToWindow() { super.onAttachedToWindow() disposables = CompositeDisposable() } } 

class FileBlockerView( context: Context, attrs: AttributeSet? ) : RelativeLayout(context, attrs) { private lateinit var presenter: FileBlockerPresenter private val viewEvents = PublishSubject.create() private lateinit var disposables: CompositeDisposable override fun onAttachedToWindow() { super.onAttachedToWindow() disposables = CompositeDisposable() disposables += viewEvents .compose(presenter) .observeOn(AndroidSchedulers.mainThread()) .errorHandlingSubscribe(::renderViewModel) } } 

class FileBlockerView( context: Context, attrs: AttributeSet? ) : RelativeLayout(context, attrs) { private lateinit var presenter: FileBlockerPresenter private val viewEvents = PublishSubject.create() private lateinit var disposables: CompositeDisposable override fun onAttachedToWindow() { super.onAttachedToWindow() disposables = CompositeDisposable() disposables += viewEvents .compose(presenter) .observeOn(AndroidSchedulers.mainThread()) .errorHandlingSubscribe(::renderViewModel) } override fun onDetachedFromWindow() { super.onDetachedFromWindow() disposables.dispose() } } 

What’s next? 

Better modularization • Key to faster builds • Shard by feature • Rely on common modules • Move Android dependencies up 

presenters backend viewmodels api db protos app 

app P2P Support Card 

More Kotlin • Finish migration to Kotlin • Remove javac build step • More libraries converted to Kotlin • Multiplatform! 

Sharing code with iOS • Kotlin, yes, kudos • SQLDelight • Retrofit + OkHttp + Wire + Okio • Entire stack except for UI 

Thanks! @egorand