SOLID principles in practice: the Clean Architecture - Devfest Emila Romagna

SOLID principles in practice:
the clean architecture
Fabio Collini

@fabioCollini
linkedin.com/in/fabiocollini
github.com/fabioCollini
medium.com/@fabioCollini
codingjam.it
Android programmazione avanzata
Android Developers Italia
Ego slide

entitiesentities
Clean
architecture
S.O.L.I.D.
principles

class WeatherUseCase(
private val locationManager: LocationManager,
private val repository: TemperatureRepository) {
suspend fun getCityData(): String = coroutineScope {
try {
val location = locationManager.getLastLocation()
val cities = async { locationManager.getCities(location) }
val temperature = repository.getTemperature(
location.lat, location.lon)
val city = cities.await().getOrElse(0) { "No city found" }
"$city n $temperature"
} catch (e: Exception) {
"Error retrieving data: ${e.message}"
}1
}2
}3

Coroutines
try {
}1
}2
}3

Dependency Injection
try {
}1
}2
}3

class WeatherUseCaseTest {
val locationManager: LocationManager = mockk()
val repository: TemperatureRepository = mockk()
val useCase = WeatherUseCase(locationManager, repository)
@Test
fun retrieveCityData() {
coEvery { locationManager.getLastLocation() } returns LOCATION
coEvery { locationManager.getCities(LOCATION) } returns
listOf(City("Firenze", "IT"))
coEvery { repository.getTemperature(LAT, LON) } returns
Temperature(10, 8, 20)
val cityData = runBlocking { useCase.getCityData() }
assert(cityData).isEqualTo("Firenze (IT) n 10º min 8º max 20º")
}1
}2

interface LocationManager {
suspend fun getLastLocation(): Location
suspend fun getCities(location: Location): List<City>
}6
try {
}1
}2
}3
interface TemperatureRepository {
suspend fun getTemperature(lat: Double, lon: Double): Temperature
}7

}6
class AndroidLocationManager(context: Context) : LocationManager {
private val fusedLocationClient: FusedLocationProviderClient =
LocationServices.getFusedLocationProviderClient(context)
private val geocoder = Geocoder(context, Locale.getDefault())
override suspend fun getLastLocation(): Location = suspendCoroutine {
//...
}E
override suspend fun getCities(location: Location): List<City> = suspendCoroutine {
//...
}E
}E
}7
class OpenWeatherTemperatureRepository(
private val api: WeatherApi
) : TemperatureRepository {
override suspend fun getTemperature(lat: Double, lon: Double): Temperature {
val forecastDeferred = api.forecast(lat, lon)
val weather = api.currentWeather(lat, lon).await()
val temperatures = forecastDeferred.await().list.map { it.main }
return Temperature(
weather.main.temp.toInt(),
temperatures.map { it.temp_min }.min()?.toInt(),
temperatures.map { it.temp_max }.max()?.toInt()
)D
}E
}F

//...
}E
//...
}E
}E

@SuppressLint("MissingPermission")
override suspend fun getLastLocation(): Location = suspendCoroutine { continuation ->
fusedLocationClient.lastLocation
.addOnSuccessListener { location ->
if (location == null)
continuation.resumeWithException(Exception("Location not available"))
else
continuation.resume(Location(location.latitude, location.longitude))
}
.addOnFailureListener {
continuation.resumeWithException(it)
}
}E
override suspend fun getCities(location: Location): List<City> = suspendCoroutine { continuation ->
val addresses = geocoder.getFromLocation(location.lat, location.lon, 10)
continuation.resume(addresses
.filter { it.locality != null }
.map {
City(it.locality, it.countryCode)
}
)
}E
}E

return Temperature(
)D
}E
}F

interface WeatherApi {
@GET("weather?appid=$OPEN_WEATHER_APP_ID&units=metric")
fun currentWeather(@Query("lat") lat: Double, @Query("lon") lon: Double):
Deferred<TemperatureWrapper>
@GET("forecast?appid=$OPEN_WEATHER_APP_ID&units=metric")
fun forecast(@Query("lat") lat: Double, @Query("lon") lon: Double):
Deferred<Forecast>
}Z
}6
//...
}E
//...
}E
}E
}7
return Temperature(
)D
}E
}F

Deferred<Forecast>
}Z

}7
}6
//...
}E
//...
}E
}E
try {
}1
}2
}3
Deferred<Forecast>
}Z
return Temperature(
)D
}E
}F

try {
}1
}2
}3
class WeatherViewModel(app: Application) : AndroidViewModel(app) {
private val api = RetrofitFactory.createService<WeatherApi>()
private val weatherRepository = OpenWeatherTemperatureRepository(api)
private val positionManager = AndroidLocationManager(app)
private val useCase = WeatherUseCase(positionManager, weatherRepository)
val state = MutableLiveData<String>()
fun load() {
viewModelScope.launch {
val result = useCase.getCityData()
state.value = result
}O
}O
}O
class MainActivity : AppCompatActivity() {
//...
}
}6
//...
}E
//...
}E
}E
}7
Deferred<Forecast>
}Z
return Temperature(
)D
}E
}F

fun load() {
}O
}O
}O

app
api
domain
weather
location
Deferred<Forecast>
}Z
try {
}1
}2
}3
//...
}
}6
//...
}E
//...
}E
}E
}7
return Temperature(
)D
}E
}F
fun load() {
}O
}O
}O

Java/Kotlin modules are more…
Testable
Reusable

Robert C. Martin Copyright (c) 2000 by Robert C. Martin. All Rights Reserved.
www.objectmentor.com 1
Design Principles and
Design Patterns
Robert C. Martin
www.objectmentor.com
What is software architecture? The answer is multitiered. At the highest level, there
are the architecture patterns that define the overall shape and structure of software
applications1
. Down a level is the architecture that is specifically related to the pur-
pose of the software application. Yet another level down resides the architecture of
the modules and their interconnections. This is the domain of design patterns2
, pack-
akges, components, and classes. It is this level that we will concern ourselves with in
this chapter.
Our scope in this chapter is quite limitted. There is much more to be said about the
principles and patterns that are exposed here. Interested readers are referred to
[Martin99].
Architecture and Dependencies
What goes wrong with software? The design of many software applications begins as
a vital image in the minds of its designers. At this stage it is clean, elegant, and com-
pelling. It has a simple beauty that makes the designers and implementers itch to see it
working. Some of these applications manage to maintain this purity of design through
the initial development and into the first release.
But then something begins to happen. The software starts to rot. At first it isn’t so
bad. An ugly wart here, a clumsy hack there, but the beauty of the design still shows
through. Yet, over time as the rotting continues, the ugly festering sores and boils
accumulate until they dominate the design of the application. The program becomes a
festering mass of code that the developers find increasingly hard to maintain. Eventu-
1. [Shaw96]
2. [GOF96]

Single responsibility
“A class should have one, and only one,
reason to change”

app
domain
api
weather
Deferred<Forecast>
}Z
}7
return Temperature(
)D
}E
}F
try {
}1
}2
}3
fun load() {
}O
}O
}O
//...
}
location
}6
//...
}E
//...
}E
}E

Open closed
“You should be able to extend a classes
behavior, without modifying it”

Liskov substitution
“Derived classes must be substitutable
for their base classes”

open class Rectangle(
open var width: Int,
open var height: Int
) {
fun area() = width * height
}
class Square(size: Int) : Rectangle(size, size) {
override var width: Int = size
set(value) {
field = value
if (height != value)
height = value
}
override var height: Int = size
set(value) {
field = value
if (width != value)
width = value
}
}

var rectangle = Rectangle(2, 3)
var initialArea = rectangle.area()
rectangle.width *= 2
println(initialArea * 2 == rectangle.area())
rectangle = Square(2)
initialArea = rectangle.area()
rectangle.width *= 2
println(initialArea * 2 == rectangle.area())

Interface segregation
“Make ﬁne grained interfaces
that are client speciﬁc”

Dependency injection?
Inversion of control?

Dependency inversion
“Depend on abstractions,
not on concretions”

Dependency inversion
“High level modules should not depend
upon low level modules. Both should
depend upon abstractions”

domain location
}6
//...
}E
//...
}E
}E
try {
}1
}2
}3

domain location
try {
}1
}2
}3
}6
//...
}E
//...
}E
}E

app
domain
api
weather
Deferred<Forecast>
}Z
}7
return Temperature(
)D
}E
}F
location
try {
}1
}2
}3
}6
//...
}E
//...
}E
}E
fun load() {
}O
}O
}O
//...
}

location
entities
app
weather
api
domain

location
entities
app
weather
api
domain
location
entities
app
weather
api
domain

location
entities
app
weather
api
repository
entities
UI
data source
domain
domain

entitiesentitiesentitiesentities
domain
repository
UI
data source
presenter
repository
entities
UI
data source
domain

domain
repository
UI
data source
presenter
Module by feature
OR
Module by layer

entitiesentities
entities
1
domain1
apiRepository
feature1
presenter1
ap1
entities
2
entities
3
featureN
presenterN
…
…
dbRepository
db
ap2
domain2

Prosentitiesentitiesentitiesentities
domain
repository
UI
data source
presenter

Framework
independence
domain
repository
UI
data source
presenter

Architecture
Vs
code conventions
domain
repository
UI
data source
presenter

TDDentitiesentitiesentitiesentities
domain
repository
UI
data source
presenter

Consentitiesentitiesentitiesentities
domain
repository
UI
data source
presenter

More codeentitiesentitiesentitiesentities
domain
repository
UI
data source
presenter

Architecture Vs
code conventions
TDD
Pros
Cons
More code
Framework
independence
domain
repository
UI
data source
presenter

Links
Demo Project
github.com/fabioCollini/CleanWeather
2000 - Robert C. Martin - Design Principles and Design Patterns
www.cvc.uab.es/shared/teach/a21291/temes/object_oriented_design/
materials_adicionals/principles_and_patterns.pdf
2005 - Robert C. Martin - The Principles of OOD
butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod
2012 - Robert C. Martin - The Clean Architecture
8thlight.com/blog/uncle-bob/2012/08/13/the-clean-architecture.html

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