Forging the path from monolith to multi-module app | droidcon Italy

@marcoGomier
Marco Gomiero
👨💻 Senior Android Engineer @ TIER
 
Google Developer Expert for Kotlin
Forging the path from
monolith to

multi-module app

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@marcoGomier
We are a micro-mobility company, providing
people e-scooters, e-bikes and e-mopeds,
powered by a proprietary energy network
TIER Mobility

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@marcoGomier
🧑💻 👩💻

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@marcoGomier
🧑💻 👩💻
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@marcoGomier
🧑💻
👩💻
👨💻
👨💻
👨💻
🧑💻
👩💻
👩💻
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👩💻
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@marcoGomier
🧩 Reusable modules

🤓 Simplify development

🧑💻 Split responsibilities & clarify ownership

🚀 Improved build time
Modularization in a nutshell

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@marcoGomier
There is not a "right way"
Disclaimer:

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@marcoGomier
#1 Decide your
architecture

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@marcoGomier
Standard module layers
App
Features
Libraries

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:app
:shell
:features:feat1
:features:feat1-contract
:libraries:lib1 :libraries:lib2
:features:feat2
:features
:libraries
TIER module layers

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@marcoGomier
Shell
● Shared core common code

● Should not be a new monolith with a different name 😅

● Not depend on upper layers
:shell

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:app
:features:feat1
:features:feat2
:features
TIER module layers
:shell
:libraries

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@marcoGomier
Libraries
● Small reusable components

● Can depend on other libraries or shell
:libraries:lib1

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:app
TIER module layers
:shell
:features:feat1
:features:feat2
:features
:libraries

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@marcoGomier
Features
:features:feat1
● A specific feature, could be user-facing or not

● Every feature has an implementation and
 
a contract module

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@marcoGomier
Feature: contract
● A contract for the outside world

● What a feature can do and how it can be launched

● Contains Interfaces and Data Classes

● Can depend on libraries or shell
@marcoGomier
:features:location-contract
interface LocationUseCase {

fun getLocation(): Observable

fun getLatestLocation(): Single

fun fetchLocation(): Single

}
data class Location(

val latitude: Double,

val longitude: Double,

)

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@marcoGomier
Feature: implementation :features:feat1
● The implementation of the contract

● Has a dependency on its contract

● Can depend on other feature contracts, libraries or shell

● Invisible for other modules (except for :app)
@marcoGomier
:features:location internal class LocationUseCaseImpl(

...

) : LocationUseCase {

override fun getLocation(): Observable {

...

}
 
...

}

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@marcoGomier
Feature structure
:features:feat1
:features:feat2

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@marcoGomier
Why?
● Low coupling: avoid knowledge of inner workings.

● Risk of need to import lot of transitive dependencies, when a module
depends on many others

● Avoid increasing incremental build time

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:app
:shell
:features:feat1
:features:feat2
:features
:libraries
TIER module layers

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@marcoGomier
App
● App initialization

● Dependency Injection setup

● Link all the module together

● It should be as slim as possible
:app

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:app
:shell
:features:feat1
:features:feat2
:features
:libraries
TIER module layers

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:app
:features:feat1
:legacy
:features:feat2
:features
:libraries
:shell
TIER module layers

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@marcoGomier
Legacy
● A huge module with the non-modularized codebase

● Extract future features from here

● Try to not put any new code in it

● Can depend on feature contracts, libraries or shell
:legacy

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@marcoGomier
How to enforce this structure?
● Documentation

● “Computer says no” 🤖

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@marcoGomier
#2 Start doing
things

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@marcoGomier
Starting point
● Select a feature to start with

● Can be a feature with UI or without UI

● A non UI feature will be easier: less things to figure out (spoiler:
navigation)

● Extract Ride Handling to :features:ride

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@marcoGomier
Find shared deps → Dependecies Analysis

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@marcoGomier

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@marcoGomier
● Figure out shared dependencies

● Move them to :shell

● Extract the Ride Handling into a new module
Start modularizing

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@marcoGomier
🎉

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:app
:features:ride
:features:ride-contract
:features
:shell

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@marcoGomier
● Move the remaing code from app to the legacy module

● Keep moving shared dependencies to shell
Legacy code

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:app
:features:ride
:features
:shell

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:app
:features:ride
:features
:shell
:legacy

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@marcoGomier
Extract UI Features
● Time to extract a UI feature

● How to navigate between feature modules?

● 🚧 ⛔

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@marcoGomier
#3 Rethink
Navigation

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@marcoGomier
Current Navigation Status
● Jetpack Navigation

● Every “feature” has its own navigation graph

● Not a single Activity Architecture

● Mostly every “feature” has its own hosting Activity

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@marcoGomier
ShopActivity

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@marcoGomier
ShopActivity
ArticleList

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@marcoGomier
ShopActivity
ArticleList ArticleDetail

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@marcoGomier
ShopActivity
ArticleList ArticleDetail PurchaseSuccess

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@marcoGomier
ShopActivity
:features:shop

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@marcoGomier
:features:shop
ShopActivity
:features:map
MapActivity
Map ShopActivity
Start a feature from
another module

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@marcoGomier
:features:map
MapActivity
Map QRScannerActivity
:features:qr-scanner
QRScannerActivity
Intro Scan
Start a feature from
another module and
get a result back

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@marcoGomier
:features:customersupport
CustomerSupport
Add a Fragment in
the current flow

ShopActivity
ArticleList ArticleDetail CustomerSupport
:features:shop

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@marcoGomier
Some issues:
● Start a feature from another module
 
⛔ Deeplink with a single activity that includes all the graphs

● Start a feature and get a result back
 
⛔ Not out-of-the-box with Jetpack Navigation

● Add a Fragment in the current navigation graph
 
⛔ Not out-of-the-box with Jetpack Navigation

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@marcoGomier
Fix the issues:
● Start and end a feature module
 
Use the contract!

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@marcoGomier
Start and end a feature module
:features:shop-contract
interface ShopContract {

fun launch(fragment: Fragment)

}
:features:shop
object ShopContractImpl : ShopContract {

override fun launch(fragment: Fragment) {

fragment.startActivity(

Intent(

fragment.requireContext(),

ShopActivity::class.java,

)

)

fragment.activity?.overridePendingTransition(

R.anim.proceeding_enter,

R.anim.proceeding_exit,

)

}

}

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@marcoGomier
Fix the issues:
 
Use the contract!

● Start a feature module and get a result back
 
Use the contract!

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@marcoGomier
:features:qrscanner-contract
interface QRScannerContract {

fun launch(

fragment: Fragment,

onResultOk: () -> Unit,

): ActivityResultLauncher

}
:features:qrscanner
object QRScannerContractImpl : QRScannerContract {

override fun launch(fragment: Fragment, onResultOk: () -> Unit) {

val launcher = fragment.registerForActivityResult(

StartActivityForResult()

) { result ->

if (result.resultCode == Activity.RESULT_OK) {

onResultOk()

}

// Handle error

}

launcher.launch(

Intent(

fragment.requireContext(),

PackagesActivity::class.java,

)

)

}

}

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@marcoGomier
Fix the issues:
 
Use the contract!

 
Use the contract!

 
Use the contract!

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@marcoGomier
:features:qrscanner-contract
interface ContactSupportContract {

fun newInstance(

scopeId: String,

): Fragment

}
:features:qrscanner
object ContactSupportContractImpl : ContactSupportContract {

override fun newInstance(scopeId: String): Fragment {

return ContactSupportFragment.newInstance(scopeId)

}

}

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@marcoGomier
Fix the issues:
 
Use the contract!

 
Use the contract!

 
Use the contract!
 
But how to handle navigation? 🤔

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@marcoGomier
Introducing Router 🧭
● A module expose a Router interface for each
subcomponent

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@marcoGomier
Exposed Feature Router
interface ContactSupportRouter {

fun onContactSupportCallClick(phoneNumber: String)

fun onContactSupportBackClick()

}
:features-contactsupport-contract

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@marcoGomier
Introducing Router 🧭
● A module exposes a Router interface for each
subcomponent

● A module has an internal Router to manage navigation
that can also implement subcomponents’ Router

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@marcoGomier
Internal Module Router
internal class FeatureRouter : ContactSupportRouter, AnotherRouter {

private val navigator = SingleFiringRelay()

fun observe(): Observable = navigator.hide()

fun goToMap() {

navigator.accept(FeatureNavigationAction.OpenMap)

}

override fun onContactSupportCallClick(phoneNumber: String) {

navigator.accept(FeatureNavigationAction.OpenPhoneDialer(phoneNumber))

}

override fun onContactSupportBackClick() {

navigator.accept(FeatureNavigationAction.GoBack)

}

// Other routers’ implementation

}

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@marcoGomier
SingleFiringRelay
internal sealed class FeatureNavigationAction {

data class OpenPhoneDialer(val phoneNumber: String) : FeatureNavigationAction()

object GoBack : FeatureNavigationAction()

object OpenMap : FeatureNavigationAction()

}
@marcoGomier
private val navigator = SingleFiringRelay()
● An RX Relay that works similarly to SingleLiveEvent

● If it doesn’t have any subscribers, it emits to the first new one

● If it already has subscribers, it doesn’t re-emit to new ones

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@marcoGomier
New Navigation Approach
● Every module manages the navigation internally with the internal
Router

● A feature module subcomponent can be integrated into another
module by implementing its Router

● DI scoping is used to get the right Router implementation

● Navigation is handled “manually” and not with Jetpack Navigation

● Changes can be gradually introduced

● The navigation logic can be unit-tested

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@marcoGomier
#4 Improve
Gradle Setup

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@marcoGomier
Dependency Management
● Some version numbers were hardcoded

● Switch to Gradle Version Catalog for managing dependencies
[versions]

io-reactivex-rxjava2-rxandroid = "2.1.1"

io-reactivex-rxjava2-rxkotlin = "2.4.0"

com-jakewharton-rxrelay = "2.1.1"

[libraries]

com-jakewharton-rxrelay = { module = "com.jakewharton.rxrelay2:rxrelay", version.ref = "com-jakewharton-rxrelay" }

io-reactivex-rxjava2-rxandroid = { module = "io.reactivex.rxjava2:rxandroid", version.ref = "io-reactivex-rxjava2-rxandroid" }

io-reactivex-rxjava2-rxkotlin = { module = "io.reactivex.rxjava2:rxkotlin", version.ref = "io-reactivex-rxjava2-rxkotlin" }

[bundles]

rxjava2 = ["io-reactivex-rxjava2-rxandroid", "io-reactivex-rxjava2-rxkotlin", "com-jakewharton-rxrelay"]

@marcoGomier
gradle/libs.versions.toml

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@marcoGomier
Dependency Management
● Some version numbers were hardcoded

● Switch to Gradle Version Catalog for managing dependencies

● Bundles are awesome
[bundles]

feature-common-deps = ["androidx-appcompat", "androidx-constraintlayout", "androidx-recyclerview"]

feature-testing = [“junit”, "androidx-io-mockk"]
@marcoGomier
gradle/libs.versions.toml

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@marcoGomier
Module Configuration → buildSrc
fun configureAndroidModule(

project: Project,

isDataBindingEnabled: Boolean,

isComposeEnabled: Boolean = false,

) = project.libraryExtension.run {

val javaSourceVersion = project.rootProject.extra.get("compile_source_compatibility") as JavaVersion

val javaTargetVersion = project.rootProject.extra.get("compile_target_compatibility") as JavaVersion

compileSdkVersion(project.ANDROID_COMPILE_SDK_VERSION)

defaultConfig {

minSdk = project.ANDROID_MIN_SDK_VERSION

targetSdk = project.ANDROID_TARGET_SDK_VERSION

testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"

consumerProguardFiles("consumer-rules.pro")

}

...

}

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@marcoGomier
Module Configuration → build.gradle
plugins {

id 'com.android.library'

id 'kotlin-android'

}

ModuleConfig.configureAndroidModule(project)

dependencies {

...

}

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@marcoGomier
Module Configuration → build.gradle
plugins {

id 'com.tier.android.library'

id 'kotlin-android'

}

dependencies {

...

}

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@marcoGomier
More 🤖
● CookieCutter template to create a new feature or library module.

● Automatically add custom project configuration

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@marcoGomier
#5 Restart doing
things

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@marcoGomier
Restarting point
● Now a UI feature can be extracted

● New UI feature module with the new Navigation 🎉

● 🚧 ⛔

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@marcoGomier
Utility modules
● More utility modules are required for more complex UI
feature modules: (testing tools, feature-flags, etc)

● Time for another dependency analysis

● Resume Feature UI modularization after having (almost) all
the utilities ready

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@marcoGomier
#6 Conclusions

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@marcoGomier
July 2021
Timeline
Architecture
Decisions

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@marcoGomier
July 2021
Timeline
Nov 2021
Sept 2021
Aug 2021
Navigation
discussion
:features:ride
creation
:shell  
creation
Architecture
Decisions
Feb 2022 May 2022
Apr 2022
March 2022
Ready for bigger
UI module
More utility
modules
First UI feature
module
Gradle
improvements
● Took almost a year to have everything in place

● Every 6 weeks, 2 weeks of “cooldown” → big chunk of planning and work

● Some work happens during the 6 weeks
Modularization 
ongoing
Now

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@marcoGomier
Conclusions
● A long and difficult journey but totally worth it

● There’s no perfect recipe

● Don't be afraid to fail

● Don’t rush it

● Don’t refactor
You will have a chance to find “hidden pearls” of your project 😅

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Resources
● https://developer.android.com/guide/navigation/navigation-multi-module

● https://www.droidcon.com/2019/11/15/android-at-scale-square/

● https://medium.com/inside-aircall/why-did-we-move-away-from-navigation-component-
f2160f7c3f4b

● https://github.com/slackhq/slack-gradle-plugin

● https://github.com/cookiecutter/cookiecutter

● https://docs.gradle.org/current/userguide/platforms.html

● https://github.com/android/nowinandroid/blob/main/docs/ModularizationLearningJourney.md

● https://slack.engineering/stabilize-modularize-modernize-scaling-slacks-mobile-codebases/

● https://slack.engineering/stabilize-modularize-modernize-scaling-slacks-mobile-codebases-2/

● https://www.youtube.com/watch?v=K65M6jpyN4Y

● https://developer.android.com/topic/modularization

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@marcoGomier
Marco Gomiero
👨💻 Senior Android Engineer @ TIER
 
Google Developer Expert for Kotlin
Twitter: @marcoGomier
 
Github: prof18
 
Website: marcogomiero.com
Thank you!

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Forging the path from monolith to multi-module app | droidcon Italy

Forging the path from monolith to multi-module app | droidcon Italy

Marco Gomiero

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