Introduction to Swift Keypaths

Keypaths were added in Swift 4. They're a fantastic feature but sometimes it feels difficult to find good situations for using them. However, when understood and used the right way, keypaths allow you to implement additional abstractions into your codebase that would be much harder with generics or protocols. In order to be able to do that, though, it is useful to intuitively understand when to apply the Keypath pattern.

This talk will first explain the different types of keypaths that exist and then go through real world examples to showcase how they can be leveraged in your own projects.

Transcript

1. 
   Introduction to Swift KeyPaths
   

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2. @terhechte
   お早う!
   2
   # Benedikt Terhechte
   # Twitter: @terhechte
   # Swift Guides: appventure.me
   # Swift Podcast: contravariance.rocks
   

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3. @terhechte
   What are KeyPaths
   # Swift 4
   # Type-Safe shortcuts to Read / Write properties
   # Composable
   3
   

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4. Not related to Objective-C's KeyPaths
   [object valueForKeyPath: @“user.address.city.zip"]
   object.valueForKey(#keyPath(Object.firstName))
   

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5. "
   What do we want to achieve?
   

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6. struct ProfileSettings {
   var displayName: String
   var shareUpdates: Bool
   var score: Float
   }
   struct PrivacySettings {
   var passcode: Bool
   var addByID: Bool
   var blackList: [String]
   }
   

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7. protocol SettingsEntry {
   ???
   }
   

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8. Abstract Over Types
   struct ProfileSettings {
   var displayName: String
   var shareUpdates: Bool
   var score: Float
   }
   struct PrivacySettings {
   var passcode: Bool
   var addByID: Bool
   var blackList: [String]
   }
   String, Bool, Float
   Bool, Bool, [String]
   

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9. @terhechte
   KeyPaths
   # Protocols make it difficult to abstract over very different
   types
   # KeyPaths allow to do this
   9
   

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10. @terhechte
    Goal
    # Develop generic app settings
    # Settings of any shape / type
    # To achieve that, we will learn about KeyPaths
    10
    

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11. @terhechte
    Agenda
    # Intro
    # KeyPath Theory
    # Practical Example
    # Tips And Tricks
    # KeyPath Libraries
    11
    

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12. Intro
    #
    

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13. Example:
    player[keyPath: \User.username] = “Link”
    struct User {
    var username: String
    }
    var player = User(username: “Mario”]
    

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14. Example:
    player[keyPath: \User.username] =
    “Link”
    

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15. let nameKeyPath = \User.username
    player[keyPath: nameKeyPath] = “Luigi”
    

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16. Abstract the access to the property "username" into
    a variable that can be moved and stored
    let nameKeyPath = \User.username
    

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17. \User.username
    KeyPath
    Root: User Value: String
    KeyPath
    

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18. Nesting:
    struct Address {
    let street: String
    }
    struct User {
    let address: Address
    }
    let x = \User.address.street.count
    KeyPath
    

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19. 
    Theory
    

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20. Types of Keypaths
    ➡ ⏪ ↔ ⏺
    

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21. @terhechte
    KeyPath
    # Read-Only KeyPath with Root and Value
    # let properties
    21
    

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22. KeyPath
    struct User {
    let username: String
    }
    let kp: KeyPath = \User.username
    let player = User(username: “Mario”)
    print(player[keyPath: kp])
    // Error. Read Only KeyPath
    player[keyPath: kp] = “Luigi"
    

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23. @terhechte
    WritableKeyPath
    # Read / Write KeyPath
    # var properties
    23
    

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24. WritableKeyPath
    struct User { var username: String }
    var player = User(username: “Mario”)
    player[keyPath: \User.username] = “Luigi”
    

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25. @terhechte
    ReferenceWritableKeyPath
    # Read / Write KeyPath for Class types
    # Useful for mutating properties of let roots
    25
    

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26. @terhechte
    PartialKeyPath
    # KeyPaths of different Values with the same Root
    # Also Read-Only
    26
    

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27. PartialKeyPath
    // String
    let a: PartialKeyPath = \User.name
    // Float
    let c: PartialKeyPath = \User.quote
    // Int
    let b: PartialKeyPath = \User.age
    

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28. func acceptKeyPath (_ keyPath: PartialKeyPath) {
    …
    }
    acceptKeyPath(\User.age)
    acceptKeyPath(\User.username)
    

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29. @terhechte
    AnyKeyPath
    # No Root, no Value
    # Type-Erased KeyPath.
    # Very useful to keep different types together
    29
    

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30. AnyKeyPath
    let keyPaths: [AnyKeyPath]
    = [\User.username, \String.count]
    KeyPath KeyPath
    AnyKeyPath
    

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31. @terhechte
    You can cast types back
    # AnyKeyPath as? WritableKeyPath
    # PartialKeyPath as? KeyPath
    31
    

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32. KeyPath Composition
    

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33. struct User {
    let address: Address
    }
    struct Address {
    let street: String
    }
    

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34. KeyPath Composition
    // User -> address
    let addressKeyPath = \User.address
    // Address -> street
    let streetKeyPath = \Address.street
    // User -> address -> street
    let userStreetKeyPath = addressKeyPath
    .appending(path: streetKeyPath)
    

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35. KeyPath Composition
    let userStreetKeyPath = addressKeyPath
    .appending(path: streetKeyPath)
    

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36. User ➜ address
    Address ➜ street
    User ➜ street
    +
    =
    

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37. ☑ Theory
    

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38. 
    Practical Example
    

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39. Generic way of handling App
    Settings
    

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41. struct Settings {
    }
    

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42. struct Settings {
    var profileSettings: ProfileSettings
    var privacySettings: PrivacySettings
    }
    

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43. struct Settings {
    var profileSettings: ProfileSettings
    var privacySettings: PrivacySettings
    }
    struct ProfileSettings {
    var displayName: String
    var shareUpdates: Bool
    }
    

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44. struct Settings {
    var profileSettings: ProfileSettings
    var privacySettings: PrivacySettings
    }
    struct ProfileSettings {
    var displayName: String
    var shareUpdates: Bool
    }
    struct PrivacySettings {
    var passcode: Bool
    var addByID: Bool
    }
    

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45. Title
    Value / Type
    Subtitle
    

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46. Settings Entry Struct
    struct SettingsEntry {
    let keyPath: AnyKeyPath
    let title: String
    let subtitle: String
    let help: String
    ...
    }
    

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47. Settings Entry Struct
    struct SettingsEntry {
    let keyPath: AnyKeyPath let title: String
    let subtitle: String
    let help: String
    ...
    }
    

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48. Settings Entry Struct
    struct SettingsEntry {
    let keyPath: AnyKeyPath
    let title: String let subtitle: String
    let help: String
    ...
    }
    

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49. Simpler Demo
    struct SettingsEntry {
    let keyPath: AnyKeyPath
    let title: String
    }
    

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50. Allow types to return settings
    protocol SettingsProvider {
    var settings: [SettingsEntry] { get }
    }
    

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51. struct Settings {
    var profileSettings: ProfileSettings
    var privacySettings: PrivacySettings
    }
    

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52. extension Settings: SettingsProvider {
    var settings: [SettingsEntry] {
    return […]
    }
    }
    

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53. extension Settings: SettingsProvider {
    var settings: [SettingsEntry] {
    return [
    SettingsEntry(
    keyPath: \Settings.profileSettings,
    title: "Profile"),
    SettingsEntry(
    keyPath: \Settings.privacySettings,
    title: "Privacy")
    ]
    }
    }
    

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54. extension ProfileSettings: SettingsProvider {
    var settings: [SettingsEntry] {
    return […]
    }
    }
    

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55. extension ProfileSettings: SettingsProvider {
    var settings: [SettingsEntry] {
    return [
    SettingsEntry(
    keyPath: \ProfileSettings.displayName,
    title: "Display Name"),
    SettingsEntry(
    keyPath: \ProfileSettings.shareUpdates,
    title: "Share Profile Media Updates")
    ]
    }
    }
    

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56. extension PrivacySettings: SettingsProvider {
    var settings: [SettingsEntry] {
    return [
    SettingsEntry(
    keyPath: \PrivacySettings.addByID,
    title: "Allow add me by ID"),
    SettingsEntry(
    keyPath: \PrivacySettings.passcode,
    title: "Passcode Lock")
    ]
    }
    }
    

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57. func editSettings(provider: inout SettingsProvider) {
    }
    var appSettings = Settings()
    editSettings(provider: &appSettings)
    

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58. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    }
    }
    

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59. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    let value = provider[keyPath: setting.keyPath]
    ...
    }
    }
    

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60. Nested Providers
    Settings { profileSettings {
    displayName: String,
    shareUpdates: Bool
    },
    ...
    }
    

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61. Nested Providers
    Settings {
    profileSettings { displayName: String,
    shareUpdates: Bool
    },
    ...
    }
    

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62. Nested Providers
    Settings {
    profileSettings {
    displayName: String, shareUpdates: Bool
    },
    ...
    }
    

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63. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    let value = provider[keyPath: setting.keyPath]
    if let nested = value as? SettingsProvider {
    ...
    } else {
    ...
    }
    }
    }
    

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64. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    let value = provider[keyPath: setting.keyPath]
    if let nested = value as? SettingsProvider {
    for item in nested.settings {
    

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65. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    let value = provider[keyPath: setting.keyPath]
    if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    

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66. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    let value = provider[keyPath: setting.keyPath]
    if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    \Settings.PrivacySettings
    

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67. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    let value = provider[keyPath: setting.keyPath]
    if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    \PrivacySettings.passcode
    

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68. func editSettings(provider: inout SettingsProvider) {
    for setting in provider.settings {
    let value = provider[keyPath: keyPath]
    if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    updateSetting(keyPath: joined,
    title: item.title)
    ...
    }
    

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69. Settings ➜ privacySettings
    PrivacySettings ➜ passcode
    Settings ➜ passcode
    +
    =
    

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70. func editSettings(provider: inout SettingsProvider) {
    func updateSetting(keyPath: AnyKeyPath,
    title: String) {
    let value = provider[keyPath: setting.keyPath]
    if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    updateSetting(keyPath: joined,
    title: item.title)
    ...
    }
    

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71. ☑ Settings: SettingsProvider {
    ☑ profileSettings: SettingsProvider {
    },
    ☑ privacySettings: SettingsProvider {
    },
    }
    

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72. ☑ Settings: SettingsProvider {
    ☑ profileSettings: SettingsProvider {
    displayName: String,
    shareUpdates: Bool
    },
    ☑ privacySettings: SettingsProvider {
    },
    }
    

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73. if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    updateSetting(keyPath: joined,
    title: item.title)
    }
    }
    } else {
    }
    }
    

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74. if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    updateSetting(keyPath: joined,
    title: item.title)
    }
    }
    } else {
    if let writable =
    keyPath as? WritableKeyPath {
    provider[keyPath: writable] = true
    }
    }
    

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75. if let nested = value as? SettingsProvider {
    for item in nested.settings {
    if let joined =
    keyPath.appending(path: item.keyPath) {
    updateSetting(keyPath: joined,
    title: item.title)
    }
    }
    } else {
    if let writable =
    keyPath as? WritableKeyPath {
    titleLabel.text = setting.title
    provider[keyPath: writable] = true
    }
    }
    

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76. Nested Providers
    Settings {
    profileSettings {
    displayName: String,
    shareUpdates: Bool },
    ...
    }
    

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77. The Final Code:
    

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78. func editSettings(provider: inout Root) {
    func updateSetting(keyPath: AnyKeyPath, title: String) {
    let value = provider[keyPath: keyPath]
    if let nestedProvider = value as? SettingsProvider {
    for item in nestedProvider.settings {
    if let joined = keyPath.appending(path: item.keyPath) {
    updateSetting(keyPath: joined, title: item.title)
    }
    }
    } else if let writable = keyPath as? WritableKeyPath {
    provider[keyPath: writable] = true
    }
    }
    for setting in provider.settings {
    updateSetting(keyPath: setting.keyPath, title: setting.title)
    }
    }
    

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79. What did we achieve?
    

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81. @terhechte
    Abstract over properties
    # Types of any shape can now be combined into our settings
    # Can have generic UI elements to read / write the types
    # They just need to describe themselves via [SettingEntry]
    81
    

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82. 
    Three Tips for Using
    KeyPaths
    

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83. @terhechte
    1. Choose which Types to Erase
    # KeyPath = \User.age
    # PartialKeyPath = \User.age
    # AnyKeyPath = \User.age
    83
    

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84. @terhechte
    2. You can cast types back
    # AnyKeyPath as? WritableKeyPath
    # PartialKeyPath as? KeyPath
    84
    

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85. @terhechte
    3. KeyPaths conform to Hashable
    # Can be Keys in dictionaries
    # Useful to store more information about Keys
    85
    

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86. let meta: [PartialKeyPath: String] = [
    ]
    

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87. let meta: [PartialKeyPath: String] = [
    \User.username: “Your Name”,
    \User.age: “Your Age”
    ]
    

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88. let meta: [PartialKeyPath: String] = [
    \User.username: “Your Name”,
    \User.age: “Your Age”
    ]
    func renderTitle(keyPath: AnyKeyPath) {
    }
    renderTitle(\User.username)
    

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89. let meta: [PartialKeyPath: String] = [
    \User.username: “Your Name”,
    \User.age: “Your Age”
    ]
    func renderTitle(keyPath: AnyKeyPath) {
    if let title = meta[keyPath]
    titleField.text = title
    }
    }
    renderTitle(\User.username)
    

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90. 
    KeyPath Libraries
    

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91. Kuery by @k_katsumi
    github.com/kishikawakatsumi/Kuery
    

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92. @terhechte
    Kuery
    # Type-Safe Core Data Queries
    # Core Data without strings
    92
    

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93. Kuery Example
    // Before:
    NSPredicate(format: "name == %@", “Mario")
    NSPredicate(format: "age > %@", 20)
    // After:
    Query(Person.self).filter(\Person.name == “Mario")
    Query(Person.self).filter(\Person.age > 20)
    github.com/kishikawakatsumi/Kuery
    

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94. KeyPathKit by @v_pradeilles
    github.com/vincent-pradeilles/KeyPathKit
    

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95. @terhechte
    KeyPathKit
    # Useful abstractions for easier KeyPath usage
    95
    

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96. KeyPathKit
    contacts.filter(where: \.lastName == “Webb"
    && \.age < 40)
    contacts.average(of: \.age).rounded()
    contacts.between(\.age, range: 20...30)
    contacts.groupBy(\.lastName)
    github.com/vincent-pradeilles/KeyPathKit
    

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97. Sorting
    users.sorted(by: .ascending(\.lastName),
    .descending(\.address.city.name))
    github.com/vincent-pradeilles/KeyPathKit
    

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98. ☑
    Recap
    

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99. @terhechte
    What did we learn
    # KeyPaths are type-safe, type-erased, hashable and
    composable
    # Create abstractions not possible with protocols
    99
    

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100. @terhechte
     More Information
     # https://www.swiftbysundell.com/posts/the-power-of-key-paths-in-swift
     # https://www.klundberg.com/blog/swift-4-keypaths-and-you/
     # https://github.com/vincent-pradeilles/slides/blob/master/iosconfsg-2019-the-underestimated-
     power-of-keypaths.pdf
     # https://blog.slashkeys.com/practical-keypaths-in-swift-220da5ab5950
     # https://edit.theappbusiness.com/using-swift-keypaths-for-beautiful-user-preferences-c83c2f7ea7be
     # https://www.swiftbysundell.com/posts/the-power-of-key-paths-in-swift
     # https://github.com/makskovalko/FormValidation
     100
     

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101. @terhechte 101
     Simple Swift Guides: www.appventure.me
     

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102. RIANCE VARIANCE
     NTRA CONTRA
     A PODCAST
     www.contravariance.rocks
     @terhechte @BasThomas
     Swift Weekly Brief
     

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103. • German Social
     Network
     • Based in Hamburg
     • 15 Mio Users
     • Native Apps on all
     platforms
     We’re Hiring
     

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104. THANKS!
     ありがとう
     

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