David Rodrigues. Senior iOS Engineer at Babylon Health, UK.

1. FormsKitDavid Rodrigues,@dmcrodrigues
Babylon Health
1

2. Forms? !
2

3. 3

4. Butbefore starting some
context...
4

5. Managing state can bean
extremelyhardtask
» Mutability
» Observation
» Thread-safety
5

6. To buildaformweare
especiallyinterested in
mutabilityand observation
6

7. Thread-safetyisalso
fundamentalbut outofscope
forthistalk
7

8. final class Property<Value> {
private var handlers: [(Value) -> Void] = []
var value: Value {
didSet { handlers.forEach { $0(value) } }
}
init(value: Value) {
self.value = value
}
func observe(_ handler: @escaping (Value) -> Void) {
self.handlers.append(handler)
handler(value)
}
func map<T>(_ transform: @escaping (Value) -> T) -> Property<T> {
let property = Property<T>(value: transform(value))
observe { [weak property] value in property?.value = transform(value) }
return property
}
}
8

9. ⚠⚠⚠⚠⚠⚠⚠⚠⚠⚠⚠
Please notethis isafairly
simple implementation !
⚠⚠⚠⚠⚠⚠⚠⚠⚠⚠⚠
9

10. Propertygive usanicewayto observe
changes
1> let a = Property(value: 1)
2> a.observe { value in print("Value: (value)") }
"Value: 1"
3> a.value = a.value + 2
"Value: 3"
10

11. Andto derive newstates
1> let a = Property(value: 1.0)
2> let b = a.map { value in "(value) as String" }
4> b.value
"1.0 as String"
5> a.value = a.value * 10
6> a.value
10.0
7> b.value
"10.0 as String"
11

12. Backto forms...
12

13. 13

14. // Sign-Up Table View DataSource
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return 12
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
switch indexPath.row {
case 0:
// Facebook button
case 1:
// Or text
case 2:
// First Name
case 3:
// Last Name
...
}
}
14

15. Adding or removing elements requires:
» update the total number of elements
» shift all indices affected
Moving elements requires:
» shift all indices affected
15

16. Stillmanageable?
16

17. 17

18. func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
switch indexPath.row {
case 0:
// Account selected
case 1:
switch consultantType {
case .gp:
switch dateAndTimeAvailable {
case true:
...
case false:
...
}
case .specialist:
...
}
case ???:
...
...
}
}
18

19. Buthowcanwe getthose
fancyanimationswhen
something changes?
19

20. 20

21. Index pathsare
aweak system,
hard to manage
and maintain21

22. Especially with
dynamic changes
22

23. Whatthen?
23

24. Let's forgeteverythingand
startfrom scratch...
24

25. What'saform?
25

26. Aform isacollection
of components following
asequential order
26

27. 27

28. A form isa collection
ofcomponents following
a sequentialorder
28

29. Colection ofcomponents
withasequentialorder?
!
29

30. ArrayAn ordered, random-access collection
1> let array = [0, 1, 2]
2> array
[
0,
1,
2
]
30

31. We can representaform
withacollection of
components
31

32. » Text
» Text Input
» Password
» Selection
» Buttons
» ...
32

33. enum Component {
case text(String)
case textInput(placeholder: String, ...)
case password(placeholder: String, ...)
case selection(...)
case button(title: String, ...)
case facebookButton(...)
case toggle(title: String, ...)
...
}
33

34. let components: [Component] = [
.facebookButton(...), // Row 0
.text("OR"), // Row 1
.textInput(placeholder: "First name", ...), // Row 2
.textInput(placeholder: "Last name", ...), // Row 3
.textInput(placeholder: "Email", ...), // Row 4
.password(placeholder: "Password", ...), // Row 5
...
]
34

35. Byusinganorderedcollectionwecan
derive therespective
state (indexpaths)witha
clear and
declarative way 35

36. Adding, removing or moving
elements is super easy✨
let components: [Component] = [
.textInput(placeholder: "First name", ...), // Row 0
.textInput(placeholder: "Last name", ...), // Row 1
.textInput(placeholder: "Email", ...), // Row 2
.password(placeholder: "Password", ...), // Row 3
...
]
36

37. Buthow dowe go
fromacollection of
componentstoa
form? !
37

38. protocol Form {
var components: Property<[FormComponent]> { get }
}
protocol FormComponent {
func matches(with component: FormComponent) -> Bool
func render() -> UIView & FormComponentView
}
protocol FormComponentView {
...
}
enum Component: FormComponent {
...
}
38

39. ⚠ Disclaimer ⚠
The following examplesare
based on MVVM
39

40. 40

41. Butthis can be
appliedtoanything,
including MVC ifyou
are wondering !
41

42. class SignUpViewModel: Form {
let components: Property<[Component]>
init(...) {
self.components = Property(value: [
.facebookButton(...),
.text("OR"),
.textInput(placeholder: "First name", ...),
.textInput(placeholder: "Last name", ...),
.textInput(placeholder: "Email", ...),
.password(placeholder: "Password", ...),
...
])
}
}
42

43. final class FormTableViewDataSource: NSObject, UITableViewDataSource {
private var currentComponents: [FormComponent] = []
init(tableView: UITableView, components: Property<[FormComponent]>) {
components.observe { components in
self.currentComponents = components
tableView.reloadData()
}
}
}
open class FormViewController<F: Form>: UIViewController {
private let tableView = UITableView()
private let dataSource: FormTableViewDataSource
init(form: F) {
dataSource = FormTableViewDataSource(
tableView: tableView,
components: form.components
)
...
}
}
43

44. final class SignUpViewController: FormViewController {
init(viewModel: SignUpViewModel) {
super.init(form: viewModel)
}
}
44

45. Ok butwhatifwe needa
dynamic collection of
components? !
45

46. Quick Example:
Sign-Inwithtwotypes ofauthentication
» Email + Password
» Email + Membership ID
46

47. 47

48. enum SignInType {
case standard
case membership
}
class SignInViewModel: Form {
let components: Property<[Component]>
init(...) {
self.components = ??? !
}
}
48

49. switch signInType {
case .standard:
components = Property(value: [
.textInput(placeholder: "Email", ...),
.password(placeholder: "Password", ...),
.toggle(title: "Sign-In with Membership ID", ...)
.button(title: "Submit", ...)
])
case .membership:
components = Property(value: [
.textInput(placeholder: "Email", ...),
.toggle(title: "Sign-In with Membership ID", ...)
.textInput(placeholder: "Membership ID", ...),
.button(title: "Submit", ...)
])
}
49

50. Firstwe needatriggerto change from one
statetothe other
.toggle(title: "Sign-In with Membership ID", ...)
50

51. enum Component: FormComponent {
...
case toggle(title: String, isOn: Property<Bool>)
...
}
Nowwe can define component's initialvalue
and observeanychanges
51

52. enum SignInType {
case standard
case membership
}
class SignInViewModel: Form {
private isMembershipSelected = Property(value: false)
let components: Property<[Component]>
init(...) {
self.components = ??? !
}
}
52

53. enum SignInType { case standard, membership }
class SignInViewModel: Form {
private isMembershipSelected = Property(value: false)
private signInType: Property<SignInType>
let components: Property<[Component]>
init(...) {
self.signInType = isMembershipSelected.map { isSelected in
return isSelected ? .membership : .standard
}
self.components = ??? !
}
}
53

54. enum SignInType { case standard, membership }
class SignInViewModel: Form {
private isMembershipSelected = Property(value: false)
private signInType: Property<SignInType>
let components: Property<[Component]>
init(...) {
self.signInType = ...
self.components = signInType.map { signInType in
switch signInType {
case .standard:
...
case .membership:
...
}
}
}
}
54

55. self.components = signInType.map { signInType in
switch signInType {
case .standard: return [
...
.toggle(
title: "Sign-In with Membership ID",
isOn: isMembershipSelected
)
...
]
case .membership: return [
...
.toggle(
title: "Sign-In with Membership ID",
isOn: isMembershipSelected
)
...
]
}
}
55

56. 56

57. switch signInType {
case .standard:
components = Property(value: [
.textInput(placeholder: "Email", ...),
...
.toggle(title: "Sign-In with Membership ID", ...)
.button(title: "Submit", ...)
])
case .membership:
components = Property(value: [
.textInput(placeholder: "Email", ...),
.toggle(title: "Sign-In with Membership ID", ...)
...
.button(title: "Submit", ...)
])
}
57

58. Weare repeatingthe same
components for each
state... can'twe compose
this inabetterway?
58

59. Wearerepeatingthesamecomponents
foreachstate...can'twe compose
thisinabetterway?
59

60. FormBuilder
60

61. struct FormBuilder<Component: FormComponent> {
let components: [Component]
public static var empty: FormBuilder {
return FormBuilder()
}
init(components: [Component] = []) {
self.components = components
}
}
61

62. We havethe container, now
we onlyneed methodsto
compose
62

63. Butwhymethodswhenwe
have operators?
63

64. precedencegroup ChainingPrecedence {
associativity: left
higherThan: TernaryPrecedence
}
// Compose with a new component
infix operator |-+ : ChainingPrecedence
// Compose with another builder
infix operator |-* : ChainingPrecedence
64

65. struct FormBuilder {
...
static func |-+(
builder: FormBuilder,
component: Component
) -> FormBuilder {
...
}
static func |-* (
builder: FormBuilder,
components: [Component]
) -> FormBuilder {
...
}
}
65

66. func signInStandardComponents(for signInType: SignInType) -> FormBuilder {
guard let signInType == .standard else { return .empty }
return FormBuilder.empty
|-+ .password(placeholder: "Password", ...)
}
func signInMembershipComponents(for signInType: SignInType) -> FormBuilder {
guard let signInType == .standard else { return .empty }
return FormBuilder.empty
|-+ .textInput(placeholder: "Membership ID", ...)
}
self.components = signInType.map { signInType in
let builder = FormBuilder<Component>.empty
|-+ .textInput(placeholder: "Email", ...)
|-* signInStandardComponents(for: signInType)
|-+ .toggle(title: "Sign-In with Membership ID", isOn: isMembershipSelected)
|-* signInMembershipComponents(for: signInType)
|-+ .button(title: "Submit", ...)
return builder.components
}
66

67. Composingabuilderwithanother
builder is powerfulbutmaybewe can
have something simpler
67

68. // Compose components pending on a boolean condition
infix operator |-? : ChainingPrecedence
struct FormBuilder<Component: FormComponent> {
...
static func |-? (
builder: FormBuilder,
validator: Validator<FormBuilder<Component>>
) -> FormBuilder {
...
}
}
struct Validator<T> {
// `iff` stands for "if and only if" from math and logic
static func iff(
_ condition: @autoclosure @escaping () -> Bool,
generator: @escaping (T) -> T
) -> Validator<T> {
...
}
}
68

69. self.components = signInType.map { signInType in
let builder = FormBuilder<Component>.empty
|-+ .textInput(placeholder: "Email", ...)
|-? .iff(signInType == .standard) {
$0 |-+ .password(placeholder: "Password", ...)
}
|-+ .toggle(title: "Sign-In with Membership ID", isOn: isMembershipSelected)
|-? .iff(signInType == .membership) {
$0 |-+ .textInput(placeholder: "Membership ID", ...)
}
|-+ .button(title: "Submit", ...)
return builder.components
}
69

70. Nowwe haveadynamic collection of
componentsand consequentlya
dynamic rendering
70

71. Nowwehaveadynamiccollectionofcomponentsand
consequentlyadynamic rendering !
71

72. We can definearendererto
renderthe collection of
components for each state
72

73. protocol Renderer {
associatedtype FormState
func render(state: FormState) -> [FormComponent]
}
struct SignInRenderer: Renderer {
func render(state: SignInType) -> [FormComponent] {
let builder = FormBuilder<Component>.empty
|-+ .textInput(placeholder: "Email", ...)
|-? .iff(signInType == .standard) {
$0 |-+ .password(placeholder: "Password", ...)
}
|-+ .toggle(title: "Sign-In with Membership ID", isOn: isMembershipSelected)
|-? .iff(signInType == .membership) {
$0 |-+ .textInput(placeholder: "Membership ID", ...)
}
|-+ .button(title: "Submit", ...)
return builder.components
}
}
73

74. enum SignInType { case standard, membership }
class SignInViewModel: Form {
private isMembershipSelected = Property(value: false)
private signInType: Property<SignInType>
let components: Property<[Component]>
init(...) {
self.signInType = isMembershipSelected.map { isSelected in
return isSelected ? .membership : .standard
}
let renderer = SignInRenderer(
isMembershipSelected: isMembershipSelected,
...
)
self.components = signInType.map(renderer.render(state:))
}
}
74

75. Andwiththis canwe have
fancyanimations? !
75

76. YES76

77. final class FormTableViewDataSource: NSObject, UITableViewDataSource {
private var currentComponents: [FormComponent] = []
init(tableView: UITableView, components: Property<[FormComponent]>) {
components.observe { components in
self.currentComponents = components
tableView.reloadData() // !
}
}
}
77

78. WELL
NOT
YET 78

79. Everytime our state
changes,anewcollection
ofcomponents is
generatedto reflectthat
particular state
79

80. 1> (signInViewModel.signInType, signInViewModel.components)
(SignInType.standard, [
.textInput(placeholder: "Email", ...),
.password(placeholder: "Password", ...),
.toggle(title: "Sign-In with Membership ID", ...),
.button(title: "Submit", ...)
])
2> (signInViewModel.signInType, signInViewModel.components)
(SignInType.membership, [
.textInput(placeholder: "Email", ...),
.toggle(title: "Sign-In with Membership ID", ...),
.textInput(placeholder: "Membership ID", ...),
.button(title: "Submit", ...)
])
80

81. 1> (signInViewModel.signInType, signInViewModel.components)
// (SignInType.standard, [
// .textInput(placeholder: "Email", ...),
.password(placeholder: "Password", ...),
// .toggle(title: "Sign-In with Membership ID", ...),
// .button(title: "Submit", ...)
//])
2> (signInViewModel.signInType, signInViewModel.components)
// (SignInType.membership, [
// .textInput(placeholder: "Email", ...),
// .toggle(title: "Sign-In with Membership ID", ...),
.textInput(placeholder: "Membership ID", ...),
// .button(title: "Submit", ...)
//])
81

82. SignInType.standard ➡ SignInType.membership
» REMOVES (-)
.password(placeholder: "Password", ...)
» INSERTS (+)
.textInput(placeholder: "Membership ID", ...)
82

83. SignInType.membership ➡ SignInType.standard
» REMOVES (-)
.textInput(placeholder: "Membership ID", ...)
» INSERTS (+)
.password(placeholder: "Password", ...)
83

84. Rememberthis?
protocol FormComponent {
func matches(with component: FormComponent) -> Bool
}
84

85. Givenacertain component
we can match itagainst
another componentto
validate iftheyare
equivalentor not
85

86. 1> let componentA = Component.password(placeholder: "Password", ...)
2> let componentB = Component.password(placeholder: "Password", ...)
3> let componentC = Component.textInput(placeholder: "Membership ID", ...)
4> componentA.matches(with: componentA)
true
5> componentA.matches(with: componentB)
true
6> componentA.matches(with: componentC)
false
86

87. Then byemployingadiffing
algorithmwe can identify
anychanges betweentwo
collections ofcomponents
87

88. import Dwifft
static func diff<Value>(
_ lhs: [Value],
_ rhs: [Value]
) -> [DiffStep<Value>] where Value: Equatable
88

89. import Dwifft
static func diff<Value>(
_ lhs: [Value],
_ rhs: [Value]
) -> [DiffStep<Value>] where Value: Equatable
enum DiffStep<Value> {
case insert(Int, Value)
case delete(Int, Value)
}
89

90. import Dwifft
static func diff<Value>(
_ lhs: [Value],
_ rhs: [Value]
) -> [DiffStep<Value>] where Value: Equatable
enum DiffStep<Value> {
case insert(Int, Value)
case delete(Int, Value)
}
Value: Equatable !
90

91. struct FormNode: Equatable {
let component: FormComponent
static func ==(left: FormNode, right: FormNode) -> Bool {
return left.component.matches(with: right.component)
}
}
91

92. struct FormBuilder<Component: FormComponent> {
...
func build() -> [FormNode] {
return components.map(FormNode.init)
}
}
protocol Form {
var nodes: Property<[FormNode]> { get }
}
92

93. final class FormTableViewDataSource: NSObject, UITableViewDataSource {
private var currentNodes: [FormNode] = []
init(tableView: UITableView, nodes: Property<[FormNode]>) {
nodes.observe { nodes in
self.currentNodes = nodes
tableView.reloadData()
}
}
}
93

94. nodes.observe { nodes in
let previousNodes = self.currentNodes
self.currentNodes = nodes
tableView.beginUpdates()
for step in Dwifft.diff(previousNodes, currentNodes) {
switch step {
case let .insert(index, _):
tableView.insertRows(
at: [IndexPath(row: index, section: 0)], with: .fade)
case let .delete(index, _):
tableView.deleteRows(
at: [IndexPath(row: index, section: 0)], with: .fade)
}
}
tableView.endUpdates()
}
94

95. Timeto recap
95

96. 96

97. The conceptis highlyinspired on React
from Facebook
97

98. ⏰ Timeto close ⏰
98

99. Managing state is hard but
we cantryto minimise how
hard itbecomes
99

100. State derivation is
essentialto reduce
mutabilityandachievea
self-descriptive system
100

101. FormsKit's ultimate goalis
to helpcreatingand
managinganykind ofform
Openforextension
101

102. Open source
January'18102

103. Thank
You ! 103