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Testing in Practice
Keeping Your Tests Concise and Declarative
Droidcon Berlin - July '24
!
Ash Davies - SumUp
Android / Kotlin GDE Berlin
Cat Person
ashdavies.dev
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Why Test?
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Photos by Daniel Romero, Taylor Vick on Unsplash 3
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"Hey, I hope you had a
relaxing weekend..."
— QA Enginner (8:47 Monday morning)
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Testing
— Monkey
— End-to-End
— Acceptance
— Instrumentation
— Integration
— Regression
— Unit
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Testing
Photo by DiEGO MüLLER on Unsplash 8
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Writing Unit Tests
Pros Cons
They'll improve the quality
of my code
I don't want to
It'll take like 10 mins max
Literally everyone says that
I should
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Writing Unit Tests
Conclusion
I will not write unit tests
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⏱
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"Can we skip the unit tests,
just for this feature?"
— PM
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Developer: "I'll get back to it later..."
Narrator: "They never got back to it."
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"Tests Saved My Ass"
— Ben Kadel
youtu.be/8QvXErxv9qw 14
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"Our OKR is 100% Code
Coverage"
— Former CTO
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Tests test design as well as
logic
— Michael Feathers
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Architecture
Anti-Patterns and Code Smell
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Architecture
— Coupling
— Inheritance
— Mutability
— Polymorphism
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Problem
Solving
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Solutions Looking for a Problem
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Problem Solving
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// Let's Code
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class ThermosiphonTest {
@Mock
var heater: Heater
@Mock
var logger: Logger
@InjectMocks
lateinit var sut: Thermosiphon
@SetUp
fun setup() {
MockAnnotations.init(this)
}
@Test
fun `should heat water`() {
thermosiphon.pump()
assertTrue(heater.isHot())
}
}
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Test Doubles
Mocks
(╯°□°)╯︵ ┻━┻
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Test Doubles
Mocks
— Behaviour Verification
!
— API Insensitivity
"
— Scale Poorly
#
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class ThermosiphonTest {
@Mock
var heater: Heater
@Mock
var logger: Logger
@InjectMocks
lateinit var sut: Thermosiphon
@SetUp
fun setup() {
MockAnnotations.init(this)
whenever(heater.heat(any())).thenReturn(/* ... */
whenever(logger.log(any())).thenAnswer {
/* ... */
}
@Test
fun `should heat water`() {
thermosiphon.pump()
val argumentCaptor = argumentCaptor()
verify(heater).heat(argumentCapture.capture())
assertTrue(argumentCaptor.values[0])
}
}
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Factory Functions
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Factory Functions
interface Pump {
fun pump()
}
class Thermosiphon(
private val heater: Heater,
) : Pump
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Factory Functions
interface Pump {
fun pump()
}
fun Pump(heater: Heater): Pump {
return Thermosiphon(heater)
}
private class Thermosiphon(
private val heater: Heater,
)
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Factory Functions
CoroutineScope()
CompletableDeferred()
Channel()
MutableStateFlow()
List()
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Factory Functions
public actual fun lazy(mode: LazyThreadSafetyMode, initializer: () -> T): Lazy =
when (mode) {
LazyThreadSafetyMode.SYNCHRONIZED -> SynchronizedLazyImpl(initializer)
LazyThreadSafetyMode.PUBLICATION -> SafePublicationLazyImpl(initializer)
LazyThreadSafetyMode.NONE -> UnsafeLazyImpl(initializer)
}
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Functional Interfaces
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Functional Interfaces
fun interface Pump {
fun pump()
}
fun Pump(heater: Heater): Pump {
return Thermosiphon(heater)
}
private class Thermosiphon(
private val heater: Heater,
)
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Functional Interfaces
fun interface Pump {
fun pump()
}
fun Pump(heater: Heater) = Pump {
/* ... */
}
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Functional Interfaces
fun interface Pump {
fun pump(): Boolean
}
- val pump = mock {
- whenever(heat()).thenReturn(true)
- }
+ val pump = Pump { true }
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class ThermosiphonTest {
@Test
fun `should heat water`() {
var isHot = false
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(heater)
thermosiphon.pump()
assertTrue(isHot)
}
}
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class ThermosiphonTest {
@Test
fun `should heat water`() {
var isHot = false
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(heater)
thermosiphon.pump()
assertTrue(isHot)
}
@Test
fun `should cool down after heating`() {
/** ... */
}
}
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class ThermosiphonTest {
@Test
fun `should heat water`() {
var isHot = false
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(heater)
thermosiphon.pump()
assertTrue(isHot)
}
@Test
fun `should cool down after heating`() {
/** ... */
}
@Test
fun `should not catch fire when flying`() {
/** ... */
}
}
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class ThermosiphonTest {
@Test
fun `should heat water`() {
var isHot = false
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(heater)
thermosiphon.pump()
assertTrue(isHot)
}
@Test
fun `should cool down after heating`() {
/** ... */
}
@Test
fun `should not catch fire when flying`() {
/** ... */
}
@Test
fun `should not become sentient`() {
/** ... */
}
}
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class ThermosiphonTest {
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(heater)
var isSentient = false
var hasExploded = false
var isHot = false
@Test
fun `should heat water`() {
/* ... */
}
@Test
fun `should cool down after heating`() {
/** ... */
}
@Test
fun `should not catch fire when flying`() {
/** ... */
}
@Test
fun `should not become sentient`() {
/** ... */
}
}
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class ThermosiphonTest {
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(
overflow = SmallOverflowTank(),
heater = heater,
)
var isSentient = false
var hasExploded = false
var isHot = false
@Test
fun `should heat water`() {
/* ... */
}
@Test
fun `should cool down after heating`() {
/** ... */
}
@Test
fun `should not catch fire when flying`() {
/** ... */
}
@Test
fun `should not become sentient`() {
/** ... */
}
}
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class ThermosiphonTest {
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(
overflow = SmallOverflowTank(),
aiEngine = GeminiEngine(),
heater = heater,
)
var isSentient = false
var hasExploded = false
var isHot = false
@Test
fun `should heat water`() {
/* ... */
}
@Test
fun `should cool down after heating`() {
/** ... */
}
@Test
fun `should not catch fire when flying`() {
/** ... */
}
@Test
fun `should not become sentient`() {
/** ... */
}
}
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class ThermosiphonTest {
val heater = Heater { isHot = true }
val thermosiphon = Thermosiphon(
overflow = SmallOverflowTank(),
aiEngine = GeminiEngine(),
heater = heater,
launchCodes = emptyList(),
)
var isSentient = false
var hasExploded = false
var isHot = false
@Test
fun `should heat water`() {
/* ... */
}
@Test
fun `should cool down after heating`() {
/** ... */
}
@Test
fun `should not catch fire when flying`() {
/** ... */
}
@Test
fun `should not become sentient`() {
/** ... */
}
}
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class ThermosiphonTest {
@Test
fun `should heat water`() {
var isHot = false
val thermosiphon = thermosiphon(
onHeat = { isHot = true },
)
thermosiphon.pump()
assertTrue(isHot)
}
}
fun thermosiphon(
onHeat: () -> Unit
) = Thermosiphon(
overflow = SmallOverflowTank(),
aiEngine = GeminiEngine(),
heater = Heater(onHeat),
launchCodes = emptyList(),
)
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DRY
Don't Repeat Yourself
— Remove duplication
— High code reusability
— Isolating change
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DAMP
Descriptive AND Meaningful Phrases
— Some duplication permitted
— Declarative syntax
— Meaningful naming
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DRY vs DAMP
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Documentation
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Documentation
/**
* Make sure not to change this thing back to the previous implementation,
* because it breaks on that one specific device in production,
* when opening the user profile in France using a German locale.
**/
fun doMyThing() {
/* ... */
}
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Documentation
/**
* Make sure not to change this thing back to the previous implementation,
* because it breaks on that one specific device in production,
* when opening the user profile in France using a German locale.
*
* Update: We changed this to be a remote resolution, this shouldn't happen?
* We haven't seen it happen in production anymore, but just leave this
* comment here, incase it occurs again...
**/
fun doMyThingRefactoredV63() {
/* ... */
}
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Documentation
/**
* Make sure not to change this thing back to the previous implementation,
* because it breaks on that one specific device in production,
* when opening the user profile in France using a German locale.
*
* Update: We changed this to be a remote resolution, this shouldn't happen?
* We haven't seen it happen in production anymore, but just leave this
* comment here, incase it occurs again...
*
* Update: It's happening again, but for Italian uses in Australia,
* I really hope our Italian QA enginner doesn't go on vacation to Melbourne again...
**/
fun doMyThingRefactoredToBeMoreSafeIHopeV91() {
if (user.locale == Locale.ITALY) {
/* Hacky hack McHackFace */
}
}
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Documentation
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Documentation: Tests
fun `should store in user specific locale when device is in another country()
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Documentation: Tests
fun `should store in user specific locale when device is in another country() {
val intendedTarget = ...
val expectedLocale = ...
val actualLocale = doMyThing()
assertEquals(expectedLocale, actualLocal)
}
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commit d4c2d156e78cd579662ac7a658b00ca5aa17fd5d (HEAD -> main, origin/main, origin/HEAD)
Author: Ash Davies <[email protected]>
Date: Sun Jun 23 19:19:44 2024 +0200
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class ThermosiphonTest {
@Test
fun `should heat water`() {
var isHot = false
val thermosiphon = thermosiphon(
onHeat = { isHot = true },
)
thermosiphon.pump()
assertTrue(isHot)
}
}
fun thermosiphon(
onHeat: () -> Unit
) = Thermosiphon(
overflow = SmallOverflowTank(),
aiEngine = GeminiEngine(),
heater = Heater(onHeat),
launchCodes = emptyList(),
)
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What about
Context?!
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Test Doubles
Mocks
fun Context(checkSelfPermission: (String) -> Int): Context = mock {
whenever(it.checkSelfPermission(any())).thenAnswer { invocation ->
checkSelfPermission(invocation.arguments[0] as String)
}
}
val context = Context { PackageManager.PERMISSION_GRANTED }
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Refactoring
Interface Segregation
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Refactoring
Interface Segregation
class MenuProvider(
private val navStateStore: NavStateStore = NavStateStore(),
) {
fun get() = combine(navStateStore.isEnabled, /* ... */) {
/* ... */
}
}
class NavStateStore {
val isEnabled: Flow = /* ... */
fun setIsEnabled(value: Boolean) { /* ... */ }
fun getLastSet(): Long { /* ... */ }
}
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Refactoring
Interface Segregation
class MenuProvider(
private val navStateStore: NavStateStore = NavStateStore(),
) {
fun get() = combine(navStateStore.isEnabled, /* ... */) {
/* ... */
}
}
class NavStateStore {
val isEnabled: Flow = /* ... */
fun setIsEnabled(value: Boolean) { /* ... */ }
fun getLastSet(): Long { /* ... */ }
}
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Refactoring
Interface Segregation
interface NavStateStore {
val isEnabled: Flow
}
class InMemoryNavStateStore : NavStateStore {
override val isEnabled: Flow = /* ... */
fun setIsEnabled(value: Boolean) { /* ... */ }
fun getLastSet(): Long { /* ... */ }
}
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Refactoring
Interface Segregation
interface NavStateStore {
val isEnabled: Flow
}
class PreferencesNavStateStore : NavStateStore {
override val isEnabled: Flow = /* ... */
fun setIsEnabled(value: Boolean) { /* ... */ }
fun getLastSet(): Long { /* ... */ }
}
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Refactoring
Interface Segregation
✨
interface NavStateStore {
val isEnabled: Flow
}
class SentientNavStateStore : NavStateStore {
override val isEnabled: Flow = /* ... */
fun setIsEnabled(value: Boolean) { /* ... */ }
fun getLastSet(): Long { /* ... */ }
}
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Refactoring
Interface Segregation
interface NavStateStore {
val isEnabled: Flow
}
class NavStateStoreImpl : NavStateStore {
override val isEnabled: Flow = /* ... */
fun setIsEnabled(value: Boolean) { /* ... */ }
fun getLastSet(): Long { /* ... */ }
}
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Refactoring
Objects
class MenuProvider(
private val menuDefaults: MenuStateDefaults = MenuStateDefaults,
) {
/* ... */
}
object MenuStateDefaults {
val iconWidth = 24
}
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Refactoring
Objects
class MenuProvider(
private val menuProperties: MenuStateProperties = MenuStateProperties.Default,
) {
/* ... */
}
interface MenuStateProperties {
val iconWidth: Int
companion object Default : MenuStateProperties {
override val iconWidth = 24
}
}
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Refactoring
Objects
!
class MenuProvider(
private val menuProperties: MenuStateProperties = MenuStateProperties(),
) {
/* ... */
}
interface MenuStateProperties {
val iconWidth: Int
companion object {
operator fun invoke(): MenuStateProperties {
/* ... */
}
}
}
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Refactoring
Factory Function
private object MenuStateDefaults {
val iconWidth = 24
}
class MenuProvider(
private val menuProperties: MenuStateProperties = MenuStateProperties(),
) {
/* ... */
}
fun interface MenuStateProperties {
val iconWidth: Int
}
fun MenuStateProperties(
iconWidth: Int = MenuStateDefaults.iconWidth,
) = object : MenuStateProperties {
override val iconWidth = iconWidth
}
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Testing
Assertions
data class WaterState(
val temperature: Int,
)
fun interface Heater {
fun heat(water: WaterState): WaterState
}
@Test
fun `should produce water for English Breakfast Tea`() {
val heater = Heater { it.copy(temperature = 95) }
val thermosiphon = Thermosiphon(heater)
val initial = WaterState(21)
val state = thermosiphon.pump(initial)
assertTrue(state.temperature >= 95)
}
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Testing
Assertions
data class WaterState(
val temperature: Int,
+ val filtered: Boolean,
)
fun interface Heater {
fun heat(water: WaterState): WaterState
}
@Test
fun `should produce water for English Breakfast Tea`() {
val heater = Heater { it.copy(temperature = 95) }
val thermosiphon = Thermosiphon(heater)
val initial = WaterState(21, false)
val state = thermosiphon.pump(initial)
assertTrue(state.temperature >= 95)
}
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Testing
Assertions
data class WaterState(
val filtered: Boolean,
val temperature: Int,
)
fun interface Heater {
fun heat(water: WaterState): WaterState
}
@Test
fun `should produce water for English Breakfast Tea`() {
val heater = Heater { it.copy(temperature = 95) }
val thermosiphon = Thermosiphon(heater)
val initial = WaterState(21, true)
val expected = WaterState(
filtered = true,
temperature = 95,
)
assertEquals(expected, thermosiphon.pump(initial))
}
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Testing
Assertions
data class WaterState(
val filtered: Boolean,
val temperature: Int,
)
fun interface Heater {
fun heat(water: WaterState): WaterState
}
@Test
fun `should produce water for English Breakfast Tea`() {
val heater = Heater { it.copy(temperature = 95) }
val thermosiphon = Thermosiphon(heater)
val initial = WaterState(21, true)
val expected = WaterState(
filtered = true,
temperature = 95,
)
assertEquals(expected, thermosiphon.pump(initial))
}
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Conclusion
General
— Prefer functional interfaces wherever possible
— Utilise factory functions to isolate behaviour
— Segregate behaviour into smaller interfaces
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Conclusion
Testing
— Avoid using mocks unless absolutely necessary
— Restrict test behaviour to function body
— Keep individual tests idempotent
— It's OK to repeat yourself
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But wait,
there's more!
Out-Takes
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Out-Takes: Flaky the Little
Flake
android-
review.googlesource.com/c/
platform/frameworks/support/
+/2776638
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Out-Takes: TDD
private fun isEven(number: Int): Boolean {
// Added to pass unit test
if (number == 11) {
return false
}
// Added to pass unit test
if (number == 11) {
return false
}
// Fix for Ticket 12846
if (number == 11407) {
return false
}
// Fix for Ticket 14336
if (number == 9) {
return false
}
return true
}
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Out-Takes:
Unreachable
State
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Thank You!
Ash Davies - SumUp
Android / Kotlin GDE Berlin
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Additional
— blog.kotlin-academy.com/item-30-consider-factory-
functions-instead-of-constructors-e1c747fc475
— medium.com/@june.pravin/mocking-is-not-practical-
use-fakes-e30cc6eaaf4e
— testing.googleblog.com/2024/02/increase-test-
fidelity-by-avoiding-mocks.html
— handstandsam.com/2020/06/08/wrapping-mockito-
mocks-for-reusability/
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