Introduction to Functional-Reactive Programming v2

v2.0

Transcript

1. Grzegorz Dyrda
   Geeky Devs Studio
   Introduction to
   Functional-Reactive
   Programming
   

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2. Functional-Reactive
   Programming
   

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3. Functional-Reactive
   Programming
   Why?
   

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4. Why?
   • Java 9 + Reactive Streams (JEP 266)
   • Spring WebFlux, Eclipse Vert.x
   • Angular, Cycle.js, React (Native)
   • Android Architecture Components,
   Room, Retrofit… Airbnb MvRx
   

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5. Functional-Reactive
   

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6. Functional + Reactive
   

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7. Functional
   Part 1:
   

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8. You cannot understand
   Functional-Reactive programming,
   if you don’t understand
   Functional programming.
   

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9. What is Functional Programming?
   

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10. Functional Programming
    Programming style in which we avoid
    sharing state and data mutation,
    in favor of composing functions and
    passing data from one to another.
    

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11. Functional Programming
    …is about moving from

    the imperative

    to declarative style.
    

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12. The world’s most popular
    functional language is…
    Excel
    

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13. =IF(A6="", IF(C5<>"", C5, IFERROR(MATCH("?*", A6:$A$20, 0)-1,
    ROWS(A6:$A$20))&" in "&VLOOKUP(A5,$H$6:$I$8, 2, 0)), "")
    Excel
    =IF(MONTH($C$3)<>MONTH($C$3-(WEEKDAY($C$3;1)-($A$1-1))-
    IF((WEEKDAY($C$3;1)-($A$1-1))<=0;7;0)+(ROW(F7)-J3-
    ROW($D$6))*7+(COLUMN(F7)-COLUMN($D$6)+1));"";$C$3-
    (WEEKDAY($C$3;1)-($A$1-1))-IF((WEEKDAY($C$3;1)-
    ($A$1-1))<=0;7;0)+(ROW(F7)-J3-ROW($D$6))*7+(COLUMN(F7)-
    COLUMN($D$6)+1))
    

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14. Language Support
    

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15. • Functions as first-class citizens
    • Higher-order functions
    • Lambda expressions
    • Immutable data types
    • Functions are closures
    Language Support
    JAVA 8+
    APPROVED
    

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16. Example
    

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17. Client says:
    “Make a sum of all our prices

    that are higher than $20,

    discounted by 10%”
    Example
    

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18. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    

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19. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    var sum = 0.0
    for (var i = 0; i <= prices.size; i++) {

    if (price[i] > 20) {

    sum += price[i] * 0.9

    }}

    }}
    val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    

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20. var sum = 0.0
    for (var i = 0; i < prices.size; i++) {

    if (price[i] > 20) {

    sum += price[i] * 0.9

    }}

    }}
    Example
    val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    

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21. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    var sum = 0.0
    for (price in prices) {

    if (price > 20) {

    sum += price * 0.9

    }}

    }}
    

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22. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    var sum = 0.0
    for (price in prices) {

    if (price > 20) {

    sum += price * 0.9

    }}

    }}
    

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23. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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24. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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25. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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26. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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27. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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28. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    Important:
    1. No import = stdlib
    2. Cross-platform
    

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29. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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30. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    // Returns list containing only elements matching [predicate]
    fun filter(predicate: (T) -> Boolean): List
    

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31. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    // Returns list containing only elements matching [predicate]
    fun filter(predicate: (T) -> Boolean): List
    

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32. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    // Returns list containing elements transformed by [transform]
    fun map(transform: (T) -> R): List
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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33. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    // Returns list containing elements transformed by [transform]
    fun map(transform: (T) -> R): List
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    

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34. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    Single, consistent, higher-level API.
    More readable & closer to the Business Intent
    

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35. val prices = listOf(20, 12, 15, 21, 25, 28, 30)
    Example
    val sum = prices

    .filter { price -> price > 20 }

    .map { price -> price * 0.9 }

    .sum()
    var sum = 0.0
    for (price in prices) {

    if (price > 20) {

    sum += price * 0.9

    }}

    }}
    

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36. Another
    Example
    

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37. You’re a Blog admin:
    “Calculate average blogpost length
    of all your blog’s Users
    that speak Polish”
    Example
    

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38. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    

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39. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val users = db.getAllUsers()
    

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40. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = ???
    val users = db.getAllUsers()
    

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41. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users
    val users = db.getAllUsers()
    

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42. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { ??? }}
    val users = db.getAllUsers()
    

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43. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }}
    val users = db.getAllUsers()
    

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44. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .map { ??? }}
    val users = db.getAllUsers()
    

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45. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .map { user -> user.posts }}
    val users = db.getAllUsers()
    

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46. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .map { user -> user.posts }
    // Return type is: List>
    // But we need: List
    val users = db.getAllUsers()
    

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47. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .map { user -> user.posts }
    val users = db.getAllUsers()
    

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48. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }
    val users = db.getAllUsers()
    

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49. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }
    // Returns a single list with all elements yielded from
    // [transform] invoked on on each element of original collection.
    fun flatMap(transform: (T) -> Iterable): List
    val users = db.getAllUsers()
    

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50. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }
    // Returns a single list with all elements yielded from
    // [transform] invoked on on each element of original collection.
    fun flatMap(transform: (T) -> Iterable): List
    val users = db.getAllUsers()
    

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51. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }
    // Returns a single list with all elements yielded from
    // [transform] invoked on on each element of original collection.
    fun flatMap(transform: (T) -> Iterable): List
    val users = db.getAllUsers()
    

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52. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }
    val users = db.getAllUsers()
    

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53. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }

    .map { ??? }}
    val users = db.getAllUsers()
    

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54. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }}
    val users = db.getAllUsers()
    

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55. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .average()
    val users = db.getAllUsers()
    

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56. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .average()
    val users = db.getAllUsers()
    // Easy to maintain and modify
    

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57. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .filter { user -> user.language == "PL" }

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .average()
    val users = db.getAllUsers()
    

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58. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .take(100)

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .average()
    val users = db.getAllUsers()
    

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59. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .take(100)

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .average()
    val users = db.getAllUsers()
    

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60. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .take(100)

    .flatMap { user -> user.posts }
    .map { post -> post.body.length }
    .count()
    val users = db.getAllUsers()
    

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61. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .take(100)

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .reduce { acc, length -> acc + length }
    val users = db.getAllUsers()
    

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62. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .take(100)

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .reduce { acc, length -> acc + length }}
    val users = db.getAllUsers()
    // Returns a single value computed by accumulating values
    // returned from applying [operation] to each element.
    fun reduce(operation: (acc: S, T) -> S): S
    

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63. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .take(100)

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .reduce { acc, length -> acc + length }}
    val users = db.getAllUsers()
    // Returns a single value computed by accumulating values
    // returned from applying [operation] to each element.
    fun reduce(operation: (acc: S, T) -> S): S
    

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64. data class Post(val title: String, val body: String)

    data class User(val language: String, val posts: List)
    Example
    val avgPolishPostLength = users

    .take(100)

    .flatMap { user -> user.posts }

    .map { post -> post.body.length }

    .fold(0) { acc, _ -> acc + 1 }}
    val users = db.getAllUsers()
    // Accumulates value starting with [initial] and applying
    // [operation] to current accumulator value and each element.
    fun fold(initial: R, operation: (acc: R, T) -> R): R
    

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65. Benefits
    

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66. • No explicit mutation, reassignment,
    iteration etc. = less error prone
    • Low-level details controlled by the library
    = may be optimized under the hood
    • More expressive, more readable
    • Easy to enhance & modify the logic
    • Closer to the business intent
    Benefits
    

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67. Functional
    -vs-
    Object-Oriented
    

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68. Functional
    -vs-
    Object-Oriented
    

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69. Functional
    +
    Object-Oriented
    

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70. // Returns list containing only elements matching [predicate]
    fun filter(predicate: (T) -> Boolean): List
    // Returns list containing elements transformed by [transform]
    fun map(transform: (T) -> R): List
    // Flattens nested lists into single list
    fun flatMap(transform: (T) -> Iterable): List
    // Accumulates value by applying [operation]
    fun reduce(operation: (acc: S, T) -> S): S
    // Accumulates value, starting with [initial]
    fun fold(initial: R, operation: (acc: R, T) -> R): R
    Common Specialized Functions
    

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73. Functional…
    

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74. Functional + Reactive
    

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75. Reactive
    Part 2:
    

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76. What does is mean
    to be “Reactive”?
    

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77. var a = 1
    var b = 2
    var c = a + b
    print("c = $c")
    What does is mean
    to be “Reactive”?
    // ???
    

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78. var a = 1
    var b = 2
    var c = a + b
    print("c = $c")
    What does is mean
    to be “Reactive”?
    // c = 3
    

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79. var a = 1
    var b = 2
    var c = a + b
    a = 2
    print("c = $c")
    What does is mean
    to be “Reactive”?
    // ???
    

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80. var a = 1
    var b = 2
    var c = a + b
    a = 2
    print("c = $c")
    What does is mean
    to be “Reactive”?
    // c = 3
    

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81. The world’s most popular
    reactive system is…
    Excel
    

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82. Excel
    

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83. What is Reactive Programming?
    

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84. Reactive Programming
    Programming with

    Asynchronous Event Streams
    

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85. Reactive Programming
    Programming with

    Asynchronous Event Streams
    

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86. Observer Pattern
    on Steroids
    Reactive Programming
    

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87. Observer Pattern
    on Functional Steroids
    Reactive Programming
    

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88. 1 5 12 3 7 4
    This can be anything: Number,
    Object, Blog Post etc.
    

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89. 1
    filter(x > 4)
    5 12 3 7 4
    

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90. 1
    filter(x > 4)
    5 12 7
    5 12 3 7 4
    

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91. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    5 12 3 7 4
    

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92. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    

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93. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    So far everything was…

    Synchronous
    

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94. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    Introducing…

    Time
    

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95. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    

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96. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    

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97. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    

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98. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    

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99. 1
    filter(x > 4)
    5 12 7
    map(x + 2)
    7 14 9
    5 12 3 7 4
    

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100. 1
     filter(x > 4)
     5 12 7
     map(x + 2)
     7 14 9
     5 12 3 7 4
     

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101. 5 12 7
     map(x + 2)
     (Asynchronous Event)
     Stream
     Events
     Operator
     

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102. 5 12 7
     map(x + 2)
     Stream
     Events
     Infinite
     Operator
     

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103. 5 12 7
     map(x + 2)
     Stream
     Events
     Error
     Operator
     

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104. 5 12 7
     map(x + 2)
     Stream
     Events
     Complete
     Operator
     

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105. Implementation
     

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106. Akka
     Reactor
     Reactive Extensions
     Vert.x
     xstream
     Implementation
     

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107. Akka
     Reactor
     Reactive Extensions
     Vert.x
     xstream
     Implementation
     

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108. Rx
     Implementation
     

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109. • RxJava
     • RxJS
     • Rx.NET
     • RxScala
     • RxClojure
     • RxCpp
     Implementation
     • RxPython
     • RxGo
     • RxKotlin
     • RxSwift
     • RxPHP
     • RxDart
     

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110. Stream
     

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111. Stream
     Observable
     

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112. Stream
     Observable
     List
     -vs-
     

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113. Creating a Stream
     

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114. Observable.just(1)
     Observable.just(true)
     Observable.just(user)
     Observable.just("one", "two", "three")
     Creating a Stream
     Observable.fromIterable(users)
     Observable.fromArray(usersArray)
     Observable.fromCallable(this::sendRequest)
     Observable.fromFuture(future)
     Observable.defer(...)
     Observable.create(...)
     

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115. Observing a Stream
     

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116. Observable.just(1))
     Observing a Stream
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = 1
     

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117. Observing a Stream
     // Subscribes the given Observer to this Observable instance.
     fun subscribe(observer: Observer): Unit
     interface Observer {

     fun onNext(t: T)

     fun onError(e: Throwable)

     fun onComplete()
     ...

     }}
     // Subscribes to an ObservableSource and provides callbacks.
     fun subscribe(onNext: (T) -> Unit,
     onError: (Throwable) -> Unit,
     onComplete: () -> Unit): Unit
     

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118. Observable.just(1))
     Observing a Stream
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = 1
     

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119. Observable.just(true))
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = true
     Observing a Stream
     

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120. Observable.just(user))
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = User(language="PL",posts=...)
     Observing a Stream
     

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121. Observable.just("one", "two", "three")
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = one
     // Item = two
     // Item = three
     Observing a Stream
     

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122. Observable.fromIterable(users))
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = User(language="PL",posts=...)
     // Item = User(language="EN",posts=...)
     // Item = User(language="PL",posts=...)
     // Item = User(language="PL",posts=...)
     ...
     Observing a Stream
     

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123. Observable.fromIterable(users))
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = User(language="PL",posts=...)
     // Item = User(language="EN",posts=...)
     // Item = User(language="PL",posts=...)
     // Item = User(language="PL",posts=...)
     ...
     Where are my Steroids?
     Observing a Stream
     

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124. Transforming a Stream
     

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125. Observable.just("5", "8", "3", "7")
     .subscribe { item ->

     print("Item = $item")

     }}
     Transforming a Stream
     

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126. Observable.just("5", "8", "3", "7")
     .map(String::toInt)
     .subscribe { item ->

     print("Item = $item")

     }}
     Transforming a Stream
     

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127. Observable.just("5", "8", "3", "7")
     .map(String::toInt)
     .filter { item -> item > 5 }
     .subscribe { item ->

     print("Item = $item")

     }}
     // Item = 8
     // Item = 7
     Transforming a Stream
     

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129. “There’s an App for that”
     Apple, 2009
     

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130. “There’s an Operator for that”
     Reactive Extensions, 2018
     

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131. ReactiveX Operators
     Creating:
     • create
     • defer
     • empty
     • error
     • fromArray
     • fromCallable
     • fromFuture
     • interval
     • just
     • …
     Transform/Filter:
     • buffer
     • flatMap
     • groupBy
     • map
     • scan
     • window
     • debounce
     • distinct
     • filter
     • …
     Combining:
     • combineLatest
     • withLatestFrom
     • merge
     • switch
     • zip
     Threading:
     • subscribeOn
     • observeOn
     • …
     

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132. Introducing Time
     

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133. Observable.interval(1, TimeUnit.SECONDS)
     Interval
     .subscribe { item ->

     print("Item = $item")

     }}
     //00:00: Item = 0
     //00:01: Item = 1
     //00:02: Item = 2
     //00:03: Item = 3
     //00:04: Item = 4
     //...
     

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134. Observable.interval(1, TimeUnit.SECONDS)
     Interval
     .map { item -> item * item }
     .subscribe { item ->

     print("Item = $item")

     }}
     

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135. Observable.interval(1, TimeUnit.SECONDS)
     Interval
     .map { item -> item * item }.
     .take(3)
     .subscribe { item ->

     print("Item = $item")

     }}
     //00:00: Item = 0
     //00:01: Item = 1
     //00:02: Item = 4
     

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136. // RxBinding
     RxView.clicks(button) // Observable
     .map { click -> 1 }
     .scan { acc, next -> acc + next }
     UI Events
     // Similar to reduce(), but accumulates values over time,
     // each time passing the result of [operation] downstream.
     fun scan(operation: (acc: T, T) -> T): Observable
     

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137. // RxBinding
     RxView.clicks(button) // Observable
     .map { click -> 1 }
     .scan { acc, next -> acc + next }
     UI Events
     .subscribe { count ->

     print("Count = $count")

     }
     // Count = 1
     // Count = 2
     // Count = 3
     // Count = 4
     // ...
     

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138. // Retrofit
     myService.getUsers() // Observable
     .map { user -> user.posts.size }
     .reduce { acc, next -> acc + next }
     Network Requests
     .subscribe { count ->

     print("Post count = $count")

     }
     // Post count = 120
     

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139. Threading
     

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140. fun fetchUser(): User { /* fetch from network */ }
     Threading
     Observable.fromCallable(this::fetchUser)
     .subscribe { user ->

     print("User = $user")

     }}
     // Error: Network on the Main Thread!
     

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141. fun fetchUser(): User { /* fetch from network */ }
     Threading
     Observable.fromCallable(this::fetchUser)
     .subscribe { user ->

     print("User = $user")

     }}
     .subscribeOn(Schedulers.io())
     

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142. fun fetchUser(): User { /* fetch from network */ }
     Threading
     Observable.fromCallable(this::fetchUser)
     .subscribe { user ->

     print("User = $user")

     }}
     .subscribeOn(Schedulers.io())
     

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143. fun fetchUser(): User { /* fetch from network */ }
     Threading
     Observable.fromCallable(this::fetchUser)
     .subscribe { user ->

     print("User = $user")

     }}
     .subscribeOn(Schedulers.io())
     

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144. fun fetchUser(): User { /* fetch from network */ }
     Threading
     Observable.fromCallable(this::fetchUser)
     .subscribeOn(Schedulers.io())
     .subscribe { user ->
     statusLabel.text = user.firstName

     }}
     // Error: Accessing UI from the background thread!
     

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145. fun fetchUser(): User { /* fetch from network */ }
     Threading
     .subscribe { user ->
     statusLabel.text = user.firstName

     }}
     Observable.fromCallable(this::fetchUser)
     .subscribeOn(Schedulers.io())
     .observeOn(AndroidSchedulers.mainThread())
     

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146. fun fetchUser(): User { /* fetch from network */ }
     Threading
     .subscribe { user ->
     statusLabel.text = user.firstName

     }}
     Observable.fromCallable(this::fetchUser)
     .subscribeOn(Schedulers.io())
     .observeOn(AndroidSchedulers.mainThread())
     

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147. fun fetchUser(): User { /* fetch from network */ }
     Threading
     .subscribe { user ->
     statusLabel.text = user.firstName

     }}
     Observable.fromCallable(this::fetchUser)
     .subscribeOn(Schedulers.io())
     .observeOn(AndroidSchedulers.mainThread())
     // OK
     

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148. Combining Streams
     

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149. // Task: perform network request after each click
     Combining Streams
     // Return type: Observable>
     // But we want: Observable
     val buttonClicks = RxView.clicks(button)
     val userInfoRequest = myService.getUserInfo()
     buttonClicks
     .map { click -> myService.getUserInfo() }
     

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150. // Returns Observable that emits all items emitted by
     // Observables returned by [mapper].
     fun flatMap(mapper: (T) -> Observable): Observable
     buttonClicks
     .flatMap { click -> myService.getUserInfo() }
     val buttonClicks = RxView.clicks(button)
     val userInfoRequest = myService.getUserInfo()
     // Task: perform network request after each click
     Combining Streams
     

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151. .map { user -> user.firstName }
     .subscribe { firstName ->

     print("firstName = $firstName")

     }
     // firstName = John
     val buttonClicks = RxView.clicks(button)
     val userInfoRequest = myService.getUserInfo()
     buttonClicks
     .flatMap { click -> myService.getUserInfo() }
     // Task: perform network request after each click
     Combining Streams
     

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152. // Task: do the same thing on btn1 & btn2 click
     val btn1Clicks = RxView.clicks(button1)
     val btn2Clicks = RxView.clicks(button2)
     btn1Clicks
     .map { click -> "Click!" }
     .subscribe { msg ->

     print("msg = $msg")

     }}
     Combining Streams
     

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153. // Task: do the same thing on btn1 & btn2 click
     val btn1Clicks = RxView.clicks(button1)
     val btn2Clicks = RxView.clicks(button2)
     Observable.merge(btn1Clicks, btn2Clicks)
     .map { click -> "Click!" }
     .subscribe { msg ->

     print("msg = $msg")

     }}
     // Merges two (or more) Observables into a single one.
     fun merge(s1: Observable, s2: Observable): Observable
     Combining Streams
     

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154. Combining Streams
     

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155. Combining Streams
     

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156. Netflix
     

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157. Netflix
     

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158. Netflix
     

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159. Netflix
     

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160. Netflix
     

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161. Netflix
     

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162. Netflix
     

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163. Netflix
     

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164. Conclusions
     

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165. • FRP is great for composing/
     orchestrating multiple Event streams
     • “There’s an Operator for that”
     • Declarative Threading, Error handling,
     Cancellation of async tasks etc.
     • Do not overuse for simple tasks
     Conclusions
     

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166. How to Start?
     

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167. How to Start?
     • ReactiveX.io
     • github.com/ReactiveX
     • rxmarbles.com
     • geekydevs.com/rx
     

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168. Thank You
     

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169. Thank You
     Any questions?
     

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