This is the "Basically Redux" talk that Cody gave at Android Listeners in May 2018. This talk goes over what Redux is along with some of the other relevant information and how it can be used on Android and clean up RxJava.

1. “Basically Redux”
Cody Engel
@POTUS404
Senior Engineer

2. @POTUS404
Redux Basics

3. @POTUS404Redux Basics
Actions
Actions are payloads of information that send
data from your application to your store. They
are the only source of information for the
store. Actions do not describe how the actions
will change the state.
https://redux.js.org/basics/actions

4. @POTUS404Redux Basics
Actions
{
type: ADD_TODO,
text: 'Build my first Redux app'
}

5. @POTUS404Redux Basics
Reducers
Reducers specify how the application's state
changes in response to actions sent to the
store. Remember that actions only describe
what happened, but don't describe how the
application's state changes.
https://redux.js.org/basics/reducers

6. @POTUS404Redux Basics
Reducers
function todoApp(state = initialState, action) {
switch (action.type) {
case SET_VISIBILITY_FILTER:
return Object.assign({}, state, {
visibilityFilter: action.filter
})
default:
return state
}
}

7. @POTUS404Redux Basics
The Store
The Store brings the actions and reducers
together. It holds the application’s state and
allows state to be updated and observed on.
You’ll only ever have one store in a Redux
application.
https://redux.js.org/basics/store

8. @POTUS404Redux Basics
Data Flow
Redux follows a strict unidirectional data flow.
This means data in your systems follows a
predictable path which is easier to
understand.
https://redux.js.org/basics/data-flow

9. @POTUS404Redux Basics
Data Flow
Actions Reducers Store
State Tree Updated

10. @POTUS404
Algebraic Data
Types

11. @POTUS404Algebraic Data Types
Product Type
A product type is a structure made up of
multiple other types.

12. @POTUS404Algebraic Data Types
Product Type
data class Gumball(
val size: GumballSize,
val color: GumballColor
)

13. @POTUS404Algebraic Data Types
Sum Type
A structure used to hold a value that could
take on several different, but fixed types.

14. @POTUS404Algebraic Data Types
Sum Type
sealed class GumballSize {
object Small : GumballSize()
object Medium : GumballSize()
object Large : GumballSize()
}

15. @POTUS404
Pure Functions

16. @POTUS404Pure Functions
Pure Functions
A pure function is a function where the return
value is only determined by its input values,
without observable side effects.

17. @POTUS404Pure Functions
fun add(left: Int, right: Int): Int {
return left + right
}
Pure Functions

18. @POTUS404Pure Functions
fun giveMeAGumball(
gumballSize: GumballSize,
gumballColor: GumballColor
): Gumball {
return Gumball(gumballSize, gumballColor)
}
Pure Functions

19. @POTUS404
Finite-state
Machines

20. @POTUS404Finite-state Machine
A finite state machine is one that has a
limited or finite number of possible states.
Finite-state Machines

21. @POTUS404Finite-state Machines

22. @POTUS404
• How many gumballs are left?
• Is it currently dispensing?
Gumball Machine
States
Finite-state Machines

23. @POTUS404
REDUCE!

24. @POTUS404REDUCE!
fun main(args: Array<String>) {
val gumballMachine = GumballMachine()
while(true) {
if (gumballMachine.hasGumballs) {
println("Purchased a ${gumballMachine.buyGumball()}")
} else {
println("Sorry, no more gumballs.")
print("Would you like to refill the gumball machine? y/n: ")
if (readLine() == "y") {
gumballMachine.refill()
println()
} else {
return
}
}
}
}
Gumball Machine Usage

25. @POTUS404REDUCE!
fun main(args: Array<String>) {
val gumballMachine = GumballMachine()
while(true) {
if (gumballMachine.hasGumballs) {
println("Purchased a ${gumballMachine.buyGumball()}")
} else {
println("Sorry, no more gumballs.")
print("Would you like to refill the gumball machine? y/n: ")
if (readLine() == "y") {
gumballMachine.refill()
println()
} else {
return
}
}
}
}
Gumball Machine Usage

26. @POTUS404REDUCE!
fun main(args: Array<String>) {
val gumballMachine = GumballMachine()
while(true) {
if (gumballMachine.hasGumballs) {
println("Purchased a ${gumballMachine.buyGumball()}")
} else {
println("Sorry, no more gumballs.")
print("Would you like to refill the gumball machine? y/n: ")
if (readLine() == "y") {
gumballMachine.refill()
println()
} else {
return
}
}
}
}
Gumball Machine Usage

27. @POTUS404REDUCE!
fun main(args: Array<String>) {
val gumballMachine = GumballMachine()
while(true) {
if (gumballMachine.hasGumballs) {
println("Purchased a ${gumballMachine.buyGumball()}")
} else {
println("Sorry, no more gumballs.")
print("Would you like to refill the gumball machine? y/n: ")
if (readLine() == "y") {
gumballMachine.refill()
println()
} else {
return
}
}
}
}
Gumball Machine Usage

28. @POTUS404REDUCE!
fun main(args: Array<String>) {
val gumballMachine = GumballMachine()
while(true) {
if (gumballMachine.hasGumballs) {
println("Purchased a ${gumballMachine.buyGumball()}")
} else {
println("Sorry, no more gumballs.")
print("Would you like to refill the gumball machine? y/n: ")
if (readLine() == "y") {
gumballMachine.refill()
println()
} else {
return
}
}
}
}
Gumball Machine Usage

29. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}
val hasGumballs
get() = state.remainingGumballs > 0
private val gumballReducer = GumballReducer()
private var state = GumballState()
fun buyGumball(): Gumball {
state = gumballReducer.reduce(state, GumballAction.BuyGumball)
Thread.sleep(timeToDispense)
state = gumballReducer.reduce(state, GumballAction.Dispensed)
return Gumball
}
fun refill() {
state = gumballReducer.reduce(state, GumballAction.RefillGumballs)
}
}

30. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}
val hasGumballs
get() = state.remainingGumballs > 0
private val gumballReducer = GumballReducer()
private var state = GumballState()
fun buyGumball(): Gumball {
state = gumballReducer.reduce(state, GumballAction.BuyGumball)
Thread.sleep(timeToDispense)
state = gumballReducer.reduce(state, GumballAction.Dispensed)
return Gumball
}
fun refill() {
state = gumballReducer.reduce(state, GumballAction.RefillGumballs)
}
}

31. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}

32. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}
val hasGumballs
get() = state.remainingGumballs > 0
private val gumballReducer = GumballReducer()
private var state = GumballState()
fun buyGumball(): Gumball {
state = gumballReducer.reduce(state, GumballAction.BuyGumball)
Thread.sleep(timeToDispense)
state = gumballReducer.reduce(state, GumballAction.Dispensed)
return Gumball
}
fun refill() {
state = gumballReducer.reduce(state, GumballAction.RefillGumballs)
}
}

33. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}
val hasGumballs
get() = state.remainingGumballs > 0
private val gumballReducer = GumballReducer()
private var state = GumballState()
fun buyGumball(): Gumball {
state = gumballReducer.reduce(state, GumballAction.BuyGumball)
Thread.sleep(timeToDispense)
state = gumballReducer.reduce(state, GumballAction.Dispensed)
return Gumball
}
fun refill() {
state = gumballReducer.reduce(state, GumballAction.RefillGumballs)
}
}

34. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}
val hasGumballs
get() = state.remainingGumballs > 0
private val gumballReducer = GumballReducer()
private var state = GumballState()
fun buyGumball(): Gumball {
state = gumballReducer.reduce(state, GumballAction.BuyGumball)
Thread.sleep(timeToDispense)
state = gumballReducer.reduce(state, GumballAction.Dispensed)
return Gumball
}
fun refill() {
state = gumballReducer.reduce(state, GumballAction.RefillGumballs)
}
}

35. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}
val hasGumballs
get() = state.remainingGumballs > 0
private val gumballReducer = GumballReducer()
private var state = GumballState()
fun buyGumball(): Gumball {
state = gumballReducer.reduce(state, GumballAction.BuyGumball)
Thread.sleep(timeToDispense)
state = gumballReducer.reduce(state, GumballAction.Dispensed)
return Gumball
}
fun refill() {
state = gumballReducer.reduce(state, GumballAction.RefillGumballs)
}
}

36. @POTUS404REDUCE!
The Gumball Machine
class GumballMachine(private val timeToDispense: Long = 50) {
object Gumball {
override fun toString() = "$color Gumball"
private val color
get() = when(Random().nextInt(8) + 1) {
1 -> "Blue"
2 -> "Red"
3 -> "Green"
4 -> "Yellow"
5 -> "Pink"
6 -> "Purple"
7 -> "Orange"
else -> "White"
}
}
val hasGumballs
get() = state.remainingGumballs > 0
private val gumballReducer = GumballReducer()
private var state = GumballState()
fun buyGumball(): Gumball {
state = gumballReducer.reduce(state, GumballAction.BuyGumball)
Thread.sleep(timeToDispense)
state = gumballReducer.reduce(state, GumballAction.Dispensed)
return Gumball
}
fun refill() {
state = gumballReducer.reduce(state, GumballAction.RefillGumballs)
}
}

37. @POTUS404REDUCE!
sealed class GumballAction : Action {
object BuyGumball : GumballAction()
object RefillGumballs : GumballAction()
object Dispensed : GumballAction()
}
The Gumball Actions

38. @POTUS404REDUCE!
data class GumballState(
val remainingGumballs: Int = 100,
val isDispensing: Boolean = false
) : State
The Gumball State

39. @POTUS404REDUCE!
class GumballReducer : Reducer<GumballState, GumballAction> {
override fun reduce(state: GumballState, action: GumballAction): GumballState {
return when(action) {
is GumballAction.BuyGumball -> {
if (state.isDispensing) {
throw IllegalStateException(“…”)
} else if (state.remainingGumballs <= 0) {
throw IllegalStateException(“…”)
}
state.copy(
remainingGumballs = state.remainingGumballs - 1,
isDispensing = true
)
}
is GumballAction.RefillGumballs -> state.copy(
remainingGumballs = 100,
isDispensing = false
)
is GumballAction.Dispensed -> state.copy(isDispensing = false)
}
}
}
The Gumball Reducer

40. @POTUS404REDUCE!
class GumballReducer : Reducer<GumballState, GumballAction> {
override fun reduce(state: GumballState, action: GumballAction): GumballState{
return when(action) {
is GumballAction.BuyGumball -> {
if (state.isDispensing) {
throw IllegalStateException(“…”)
} else if (state.remainingGumballs <= 0) {
throw IllegalStateException(“…”)
}
state.copy(
remainingGumballs = state.remainingGumballs - 1,
isDispensing = true
)
}
is GumballAction.RefillGumballs -> state.copy(
remainingGumballs = 100,
isDispensing = false
)
is GumballAction.Dispensed -> state.copy(isDispensing = false)
}
}
}
The Gumball Reducer

41. @POTUS404REDUCE!
class GumballReducer : Reducer<GumballState, GumballAction> {
override fun reduce(state: GumballState, action: GumballAction): GumballState {
return when(action) {
is GumballAction.BuyGumball -> {
if (state.isDispensing) {
throw IllegalStateException(“…”)
} else if (state.remainingGumballs <= 0) {
throw IllegalStateException(“…”)
}
state.copy(
remainingGumballs = state.remainingGumballs - 1,
isDispensing = true
)
}
is GumballAction.RefillGumballs -> state.copy(
remainingGumballs = 100,
isDispensing = false
)
is GumballAction.Dispensed -> state.copy(isDispensing = false)
}
}
}
The Gumball Reducer

42. @POTUS404REDUCE!
class GumballReducer : Reducer<GumballState, GumballAction> {
override fun reduce(state: GumballState, action: GumballAction): GumballState {
return when(action) {
is GumballAction.BuyGumball -> {
if (state.isDispensing) {
throw IllegalStateException(“…”)
} else if (state.remainingGumballs <= 0) {
throw IllegalStateException(“…”)
}
state.copy(
remainingGumballs = state.remainingGumballs - 1,
isDispensing = true
)
}
is GumballAction.RefillGumballs -> state.copy(
remainingGumballs = 100,
isDispensing = false
)
is GumballAction.Dispensed -> state.copy(isDispensing = false)
}
}
}
The Gumball Reducer

43. @POTUS404REDUCE!
class GumballReducer : Reducer<GumballState, GumballAction> {
override fun reduce(state: GumballState, action: GumballAction): GumballState {
return when(action) {
is GumballAction.BuyGumball -> {
if (state.isDispensing) {
throw IllegalStateException(“…”)
} else if (state.remainingGumballs <= 0) {
throw IllegalStateException(“…”)
}
state.copy(
remainingGumballs = state.remainingGumballs - 1,
isDispensing = true
)
}
is GumballAction.RefillGumballs -> state.copy(
remainingGumballs = 100,
isDispensing = false
)
is GumballAction.Dispensed -> state.copy(isDispensing = false)
}
}
}
The Gumball Reducer

44. @POTUS404REDUCE!
class GumballReducer : Reducer<GumballState, GumballAction> {
override fun reduce(state: GumballState, action: GumballAction): GumballState {
return when(action) {
is GumballAction.BuyGumball -> {
if (state.isDispensing) {
throw IllegalStateException(“…”)
} else if (state.remainingGumballs <= 0) {
throw IllegalStateException(“…”)
}
state.copy(
remainingGumballs = state.remainingGumballs - 1,
isDispensing = true
)
}
is GumballAction.RefillGumballs -> state.copy(
remainingGumballs = 100,
isDispensing = false
)
is GumballAction.Dispensed -> state.copy(isDispensing = false)
}
}
}
The Gumball Reducer

45. @POTUS404
RxJava

46. @POTUS404RxJava
Observable.fromPublisher<CompanyAction> { subscriber ->
0.until(320).map {
if (it % 11 == 0) CompanyAction.Add(Requisition.requisition) else CompanyAction.Hire(Requisition.requisition,
Employee.employee)
}.forEach { subscriber.onNext(it) }.also { subscriber.onComplete() }
}
.reduceWith({ Company("ActiveCampaign") }, { prevState, action ->
when(action) {
is CompanyAction.Hire -> {
val employees = prevState.employees.toMutableList()
employees.add(action.employee)
prevState.copy(employees = employees)
}
is CompanyAction.Add -> {
val requisitions = prevState.requisitions.toMutableList()
requisitions.add(action.requisition)
prevState.copy(requisitions = requisitions)
}
}
})
.subscribe { state ->
println("These numbers are most likely inaccurate.")
println("ActiveCampaign has ${state.requisitions.size} job openings with around ${state.employees.size} employees.")
println("There are ${state.employees.filter { it.name == "Cody" }.size} employees named Cody.")
println("Here is the raw state output:")
println(state)
}

47. @POTUS404RxJava
Observable.fromPublisher<CompanyAction> { subscriber ->
0.until(320).map {
if (it % 11 == 0) CompanyAction.Add(Requisition.requisition)
else CompanyAction.Hire(Requisition.requisition, Employee.employee)
}
.forEach { subscriber.onNext(it) }
.also { subscriber.onComplete() }
}
.reduceWith({ Company("ActiveCampaign") }, { prevState, action ->
when(action) {
is CompanyAction.Hire -> {
val employees = prevState.employees.toMutableList()
employees.add(action.employee)
prevState.copy(employees = employees)
}
is CompanyAction.Add -> {
val requisitions = prevState.requisitions.toMutableList()
requisitions.add(action.requisition)
prevState.copy(requisitions = requisitions)
}
}
})
.subscribe { state ->
println("These numbers are most likely inaccurate.")
println("ActiveCampaign has ${state.requisitions.size} job openings with around ${state.employees.size} employees.")
println("There are ${state.employees.filter { it.name == "Cody" }.size} employees named Cody.")
println("Here is the raw state output:")
println(state)
}

48. @POTUS404RxJava
Observable.fromPublisher<CompanyAction> { subscriber ->
0.until(320).map {
if (it % 11 == 0) CompanyAction.Add(Requisition.requisition) else CompanyAction.Hire(Requisition.requisition, Employee.employee)
}.forEach { subscriber.onNext(it) }.also { subscriber.onComplete() }
}
.reduceWith({ Company("ActiveCampaign") }, { prevState, action ->
when(action) {
is CompanyAction.Hire -> {
val employees = prevState.employees.toMutableList()
employees.add(action.employee)
prevState.copy(employees = employees)
}
is CompanyAction.Add -> {
val requisitions = prevState.requisitions.toMutableList()
requisitions.add(action.requisition)
prevState.copy(requisitions = requisitions)
}
}
})
.subscribe { state ->
println("These numbers are most likely inaccurate.")
println("ActiveCampaign has ${state.requisitions.size} job openings with around ${state.employees.size} employees.")
println("There are ${state.employees.filter { it.name == "Cody" }.size} employees named Cody.")
println("Here is the raw state output:")
println(state)
}

49. @POTUS404RxJava
Observable.fromPublisher<CompanyAction> { subscriber ->
0.until(320).map {
if (it % 11 == 0) CompanyAction.Add(Requisition.requisition) else CompanyAction.Hire(Requisition.requisition, Employee.employee)
}.forEach { subscriber.onNext(it) }.also { subscriber.onComplete() }
}
.reduceWith({ Company("ActiveCampaign") }, { prevState, action ->
when(action) {
is CompanyAction.Hire -> {
val employees = prevState.employees.toMutableList()
employees.add(action.employee)
prevState.copy(employees = employees)
}
is CompanyAction.Add -> {
val requisitions = prevState.requisitions.toMutableList()
requisitions.add(action.requisition)
prevState.copy(requisitions = requisitions)
}
}
})
.subscribe { state ->
println("These numbers are most likely inaccurate.")
println("ActiveCampaign has ${state.requisitions.size} job openings
with around ${state.employees.size} employees.")
println("There are ${state.employees.filter { it.name == "Cody" }.size}
employees named Cody.")
println("Here is the raw state output:")
println(state)
}

50. These numbers are most likely inaccurate.
ActiveCampaign has 30 job openings with around 290 employees.
There are 19 employees named Cody.
Here is the raw state output:
Company(name=ActiveCampaign,
requisitions=[Requisition(name=Full Stack Engineer, department=Engineering),
…)
@POTUS404RxJava

51. Cody Engel
@POTUS404
Senior Engineer
(We’re Hiring)
Thanks!