Exercise: Update TemperatureService
Get back to the TemperatureService exercise, and improve its implementation so that there is only one observer for the temperature updates, that observes since this class was created for as long as it exists.
Once you are done with the exercise, you can check your solution here.
Playground
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.currentTime
import kotlinx.coroutines.test.runTest
import org.junit.Assert.assertEquals
import org.junit.Test
import java.util.concurrent.ConcurrentHashMap
class TemperatureService(
private val temperatureDataSource: TemperatureDataSource,
backgroundScope: CoroutineScope,
) {
private val lastKnownTemperature =
ConcurrentHashMap<String, Fahrenheit>()
fun observeTemperature(city: String): Flow<Fahrenheit> =
temperatureDataSource.observeTemperatureUpdates()
.filter { it.city == city }
.map { celsiusToFahrenheit(it.temperature) }
.onEach { lastKnownTemperature[city] = it }
.onStart { lastKnownTemperature[city]?.let { emit(it) } }
fun getLastKnown(city: String): Fahrenheit? =
lastKnownTemperature[city]
fun getAllLastKnown(): Map<String, Fahrenheit> =
lastKnownTemperature.toMap()
private fun celsiusToFahrenheit(celsius: Double) =
Fahrenheit(celsius * 9 / 5 + 32)
}
interface TemperatureDataSource {
fun observeTemperatureUpdates(): Flow<TemperatureData>
}
data class TemperatureData(
val city: String,
val temperature: Double,
)
data class Fahrenheit(
val temperature: Double,
)
@ExperimentalCoroutinesApi
class TemperatureServiceTest {
@Test
fun `should emit temperature updates in Fahrenheit`() = runTest {
// given
val testDataSource = object : TemperatureDataSource {
override fun observeTemperatureUpdates(): Flow<TemperatureData> = flow {
delay(1)
emit(TemperatureData("TestCity", 10.0))
emit(TemperatureData("TestCity2", 20.0))
delay(1)
emit(TemperatureData("TestCity", 30.0))
emit(TemperatureData("TestCity3", 40.0))
emit(TemperatureData("TestCity2", 50.0))
delay(1)
}
}
val service = TemperatureService(testDataSource, backgroundScope)
// when
val emitted = mutableListOf<Fahrenheit>()
service.observeTemperature("TestCity")
.onEach { emitted.add(it) }
.launchIn(backgroundScope)
delay(10)
// then
assertEquals(listOf(Fahrenheit(50.0), Fahrenheit(86.0)), emitted)
}
@Test
fun `should store last known temperature update in Fahrenheit`() = runTest {
// given
val testDataSource = object : TemperatureDataSource {
override fun observeTemperatureUpdates(): Flow<TemperatureData> = flow {
delay(100)
emit(TemperatureData("TestCity", 10.0))
delay(100)
emit(TemperatureData("TestCity2", 20.0))
delay(100)
emit(TemperatureData("TestCity", 30.0))
delay(100)
emit(TemperatureData("TestCity3", 40.0))
delay(100)
emit(TemperatureData("TestCity2", 50.0))
}
}
val service = TemperatureService(testDataSource, backgroundScope)
// when
val emitted = mutableListOf<Fahrenheit>()
service.observeTemperature("TestCity")
.onEach { emitted.add(it) }
.launchIn(backgroundScope)
delay(150)
assertEquals(Fahrenheit(50.0), service.getLastKnown("TestCity"))
assertEquals(Fahrenheit(50.0), service.getAllLastKnown()["TestCity"])
delay(200)
assertEquals(Fahrenheit(86.0), service.getLastKnown("TestCity"))
assertEquals(Fahrenheit(86.0), service.getAllLastKnown()["TestCity"])
}
@Test
fun `should emit last known temperature update on start`() = runTest {
// given
val testDataSource = object : TemperatureDataSource {
override fun observeTemperatureUpdates(): Flow<TemperatureData> = flow {
delay(100)
emit(TemperatureData("TestCity", 10.0))
delay(100)
emit(TemperatureData("TestCity2", 20.0))
}
}
val service = TemperatureService(testDataSource, backgroundScope)
service.observeTemperature("TestCity").first()
assertEquals(100, currentTime)
// when
val result = service.observeTemperature("TestCity").first()
// then
assertEquals(Fahrenheit(50.0), result)
assertEquals(100, currentTime)
// when
val result2 = service.observeTemperature("TestCity2").first()
// then
assertEquals(Fahrenheit(68.0), result2)
}
@Test
fun `should always have only one temperature updates observer`() = runTest {
// given
val temperatureUpdatesSource = MutableSharedFlow<TemperatureData>()
var observersCounter = 0
val testDataSource = object : TemperatureDataSource {
override fun observeTemperatureUpdates(): Flow<TemperatureData> = temperatureUpdatesSource
.onStart { observersCounter++ }
.onCompletion { observersCounter++ }
}
val service = TemperatureService(testDataSource, backgroundScope)
// then
delay(10)
assertEquals(1, observersCounter)
// when
val emitted = mutableListOf<Fahrenheit>()
val jobs = (1..10).map {
service.observeTemperature("TestCity$it")
.onEach { emitted.add(it) }
.launchIn(backgroundScope)
}
delay(10)
// then
assertEquals(1, observersCounter)
// when
jobs.forEach { it.cancel() }
delay(10)
// then
assertEquals(1, observersCounter)
}
@Test
fun `should store temperature updates even if a city is not observed`() = runTest {
// given
val temperatureUpdatesSource = MutableSharedFlow<TemperatureData>()
var observersCounter = 0
val testDataSource = object : TemperatureDataSource {
override fun observeTemperatureUpdates(): Flow<TemperatureData> = temperatureUpdatesSource
.onStart { observersCounter++ }
.onCompletion { observersCounter++ }
}
val service = TemperatureService(testDataSource, backgroundScope)
delay(1)
// when
temperatureUpdatesSource.emit(TemperatureData("A", 10.0))
temperatureUpdatesSource.emit(TemperatureData("B", 20.0))
temperatureUpdatesSource.emit(TemperatureData("C", 30.0))
temperatureUpdatesSource.emit(TemperatureData("D", 40.0))
delay(1)
// then
assertEquals(Fahrenheit(temperature = 50.0), service.getLastKnown("A"))
assertEquals(Fahrenheit(temperature = 68.0), service.getLastKnown("B"))
assertEquals(Fahrenheit(temperature = 86.0), service.getLastKnown("C"))
assertEquals(Fahrenheit(temperature = 104.0), service.getLastKnown("D"))
assertEquals(
mapOf(
"A" to Fahrenheit(temperature = 50.0),
"B" to Fahrenheit(temperature = 68.0),
"C" to Fahrenheit(temperature = 86.0),
"D" to Fahrenheit(temperature = 104.0)
), service.getAllLastKnown()
)
}
}
