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Introduction to Swift KeyPaths
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@terhechte
お早う!
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# Benedikt Terhechte
# Twitter: @terhechte
# Swift Guides: appventure.me
# Swift Podcast: contravariance.rocks
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@terhechte
What are KeyPaths
# Swift 4
# Type-Safe shortcuts to Read / Write properties
# Composable
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Not related to Objective-C's KeyPaths
[object valueForKeyPath: @“user.address.city.zip"]
object.valueForKey(#keyPath(Object.firstName))
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"
What do we want to achieve?
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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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protocol SettingsEntry {
???
}
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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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@terhechte
KeyPaths
# Protocols make it difficult to abstract over very different
types
# KeyPaths allow to do this
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@terhechte
Goal
# Develop generic app settings
# Settings of any shape / type
# To achieve that, we will learn about KeyPaths
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@terhechte
Agenda
# Intro
# KeyPath Theory
# Practical Example
# Tips And Tricks
# KeyPath Libraries
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Intro
#
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Example:
player[keyPath: \User.username] = “Link”
struct User {
var username: String
}
var player = User(username: “Mario”]
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Example:
player[keyPath: \User.username] =
“Link”
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let nameKeyPath = \User.username
player[keyPath: nameKeyPath] = “Luigi”
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Abstract the access to the property "username" into
a variable that can be moved and stored
let nameKeyPath = \User.username
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\User.username
KeyPath
Root: User Value: String
KeyPath
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Nesting:
struct Address {
let street: String
}
struct User {
let address: Address
}
let x = \User.address.street.count
KeyPath
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Theory
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Types of Keypaths
➡ ⏪ ↔ ⏺
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@terhechte
KeyPath
# Read-Only KeyPath with Root and Value
# let properties
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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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@terhechte
WritableKeyPath
# Read / Write KeyPath
# var properties
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WritableKeyPath
struct User { var username: String }
var player = User(username: “Mario”)
player[keyPath: \User.username] = “Luigi”
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@terhechte
ReferenceWritableKeyPath
# Read / Write KeyPath for Class types
# Useful for mutating properties of let roots
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@terhechte
PartialKeyPath
# KeyPaths of different Values with the same Root
# Also Read-Only
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PartialKeyPath
// String
let a: PartialKeyPath = \User.name
// Float
let c: PartialKeyPath = \User.quote
// Int
let b: PartialKeyPath = \User.age
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func acceptKeyPath (_ keyPath: PartialKeyPath) {
…
}
acceptKeyPath(\User.age)
acceptKeyPath(\User.username)
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@terhechte
AnyKeyPath
# No Root, no Value
# Type-Erased KeyPath.
# Very useful to keep different types together
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AnyKeyPath
let keyPaths: [AnyKeyPath]
= [\User.username, \String.count]
KeyPath KeyPath
AnyKeyPath
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@terhechte
You can cast types back
# AnyKeyPath as? WritableKeyPath
# PartialKeyPath as? KeyPath
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KeyPath Composition
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struct User {
let address: Address
}
struct Address {
let street: String
}
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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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KeyPath Composition
let userStreetKeyPath = addressKeyPath
.appending(path: streetKeyPath)
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User ➜ address
Address ➜ street
User ➜ street
+
=
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☑ Theory
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Practical Example
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Generic way of handling App
Settings
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No content
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struct Settings {
}
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struct Settings {
var profileSettings: ProfileSettings
var privacySettings: PrivacySettings
}
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struct Settings {
var profileSettings: ProfileSettings
var privacySettings: PrivacySettings
}
struct ProfileSettings {
var displayName: String
var shareUpdates: Bool
}
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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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Title
Value / Type
Subtitle
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Settings Entry Struct
struct SettingsEntry {
let keyPath: AnyKeyPath
let title: String
let subtitle: String
let help: String
...
}
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Settings Entry Struct
struct SettingsEntry {
let keyPath: AnyKeyPath <—\PrivacySettings.passcode
let title: String
let subtitle: String
let help: String
...
}
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Settings Entry Struct
struct SettingsEntry {
let keyPath: AnyKeyPath
let title: String <— “Lock with Passcode”
let subtitle: String
let help: String
...
}
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Simpler Demo
struct SettingsEntry {
let keyPath: AnyKeyPath
let title: String
}
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Allow types to return settings
protocol SettingsProvider {
var settings: [SettingsEntry] { get }
}
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struct Settings {
var profileSettings: ProfileSettings
var privacySettings: PrivacySettings
}
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extension Settings: SettingsProvider {
var settings: [SettingsEntry] {
return […]
}
}
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extension Settings: SettingsProvider {
var settings: [SettingsEntry] {
return [
SettingsEntry(
keyPath: \Settings.profileSettings,
title: "Profile"),
SettingsEntry(
keyPath: \Settings.privacySettings,
title: "Privacy")
]
}
}
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extension ProfileSettings: SettingsProvider {
var settings: [SettingsEntry] {
return […]
}
}
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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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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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func editSettings(provider: inout SettingsProvider) {
}
var appSettings = Settings()
editSettings(provider: &appSettings)
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func editSettings(provider: inout SettingsProvider) {
for setting in provider.settings {
}
}
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func editSettings(provider: inout SettingsProvider) {
for setting in provider.settings {
let value = provider[keyPath: setting.keyPath]
...
}
}
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Nested Providers
Settings { <--- Here `[SettingsEntry]`
profileSettings {
displayName: String,
shareUpdates: Bool
},
...
}
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Nested Providers
Settings {
profileSettings { <--- Here `[SettingsEntry]`
displayName: String,
shareUpdates: Bool
},
...
}
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Nested Providers
Settings {
profileSettings {
displayName: String, <--- Here `String`
shareUpdates: Bool
},
...
}
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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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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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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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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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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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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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Settings ➜ privacySettings
PrivacySettings ➜ passcode
Settings ➜ passcode
+
=
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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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☑ Settings: SettingsProvider {
☑ profileSettings: SettingsProvider {
},
☑ privacySettings: SettingsProvider {
},
}
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☑ Settings: SettingsProvider {
☑ profileSettings: SettingsProvider {
displayName: String,
shareUpdates: Bool
},
☑ privacySettings: SettingsProvider {
},
}
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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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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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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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Nested Providers
Settings {
profileSettings {
displayName: String,
shareUpdates: Bool <--- Set to true
},
...
}
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The Final Code:
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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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What did we achieve?
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No content
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@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]
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Three Tips for Using
KeyPaths
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@terhechte
1. Choose which Types to Erase
# KeyPath = \User.age
# PartialKeyPath = \User.age
# AnyKeyPath = \User.age
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@terhechte
2. You can cast types back
# AnyKeyPath as? WritableKeyPath
# PartialKeyPath as? KeyPath
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@terhechte
3. KeyPaths conform to Hashable
# Can be Keys in dictionaries
# Useful to store more information about Keys
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let meta: [PartialKeyPath: String] = [
]
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let meta: [PartialKeyPath: String] = [
\User.username: “Your Name”,
\User.age: “Your Age”
]
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let meta: [PartialKeyPath: String] = [
\User.username: “Your Name”,
\User.age: “Your Age”
]
func renderTitle(keyPath: AnyKeyPath) {
}
renderTitle(\User.username)
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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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KeyPath Libraries
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Kuery by @k_katsumi
github.com/kishikawakatsumi/Kuery
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@terhechte
Kuery
# Type-Safe Core Data Queries
# Core Data without strings
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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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KeyPathKit by @v_pradeilles
github.com/vincent-pradeilles/KeyPathKit
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@terhechte
KeyPathKit
# Useful abstractions for easier KeyPath usage
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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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Sorting
users.sorted(by: .ascending(\.lastName),
.descending(\.address.city.name))
github.com/vincent-pradeilles/KeyPathKit
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☑
Recap
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@terhechte
What did we learn
# KeyPaths are type-safe, type-erased, hashable and
composable
# Create abstractions not possible with protocols
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@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
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@terhechte 101
Simple Swift Guides: www.appventure.me
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RIANCE VARIANCE
NTRA CONTRA
A PODCAST
www.contravariance.rocks
@terhechte @BasThomas
Swift Weekly Brief
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• German Social
Network
• Based in Hamburg
• 15 Mio Users
• Native Apps on all
platforms
We’re Hiring
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THANKS!
ありがとう