Exercise: TemperatureService
You are working on a TemperatureService class that should provide temperature updates for a given city. The service should use a TemperatureDataSource to get temperature updates. The service should:
- Provide a
FlowofFahrenheittemperatures for a given city (updates filtered by city, and converted from Celsius). - Store the last known temperature for each city (at this point, you can store temparature for only those cities that are currently observed).
- Provide the last known temperature for a given city.
- Provide all last known temperatures.
class TemperatureService(
private val temperatureDataSource: TemperatureDataSource,
backgroundScope: CoroutineScope,
) {
private val lastKnownTemperature =
ConcurrentHashMap<String, Fahrenheit>()
fun observeTemperature(city: String): Flow<Fahrenheit> =
TODO()
fun getLastKnown(city: String): Fahrenheit? =
lastKnownTemperature[city]
fun getAllLastKnown(): Map<String, Fahrenheit> =
lastKnownTemperature.toMap()
private fun celsiusToFahrenheit(celsius: Double) =
Fahrenheit(celsius * 9 / 5 + 32)
}
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in coroutines/flow/TemperatureService.kt. You can find there starting code and unit tests.
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> =
TODO()
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)
}
}
