KeyPaths in swift

2.1 ReferenceWritableKeyPath
2.2 WritableKeyPath
2.3 KeyPath
2.4 PartialKeyPath
2.5 AnyKeyPath

1 - What are Keypaths in swift

KeyPath is an object that stores a reference to a property of a type, that keyPath object allow us later on to get/set their underlying value ( defer/stash getter and setter evaluation when needed ) using subscript operator. you can then pass these keypaths objects around to create an API with shorter and cleaner syntax.

let's see with an example, suppose we have a class Person:

class User {
    var fullName: String
    var email: String?
    var age: Int

    init(fullName: String, email: String? = nil, age: Int) {
        self.fullName = fullName
        self.email = email
        self.age = age
    }
}

given the User we can then create a keyPath object for any instance property ( fullName, email, age ) of the User type and use that keyPath later on to get/set the property value on a specific user instance:

let user = User(fullName: "Mike", age: 16)
let fullNameKeyPath = \User.fullName
// get fullName value using fullNameKeyPath
print( user[keyPath: fullNameKeyPath] ) // Mike
// set fullName value using fullNameKeyPath
user[keyPath: fullNameKeyPath] = "John"
print( user[keyPath: fullNameKeyPath] ) // John

and as you can see, the keyPath expression have the form \Root.path and the KeyPath declaration is

class KeyPath<Root, Value> : PartialKeyPath<Root>

keyPath takes two generic types: Root Type ( User is the example ) and Value Type which is the Type of the property the keyPath refers to ( String in case fullName, Int for age, String? for email ).

Note: at compile time, a key-path expression ( \Root.path ) is replaced by an instance of the keyPath class of the appropriate keyPath type ( we have 5 types of keypaths )

you can also make the keyPath expression nested or compound like,

class User {
   // ...
}
let user = User(fullName: "Mike", age: 16)
let fullNameEmptyKeyPath = \User.fullName.isEmpty
print(user[keyPath: fullNameEmptyKeyPath]) // false

let user = User(fullName: "Mike", age: 16)
let fullNamePath = \User.fullName
let isEmptyPath = \String.isEmpty

let fullNameIsEmptyKeyPath = fullNamePath.appending(path: isEmptyPath) // merge the two paths
print( user[keyPath: fullNameIsEmptyKeyPath] ) // read keyPath value -> false

and here the keyPath Root type is User and the Value is Bool ( isEmpty is Bool ) so the fullNameIsEmptyKeyPath is of type KeyPath<User, Bool>

2 - Types of KeyPaths

we have 5 types of keypaths represented by a hierarchy of 5 generic classes: ReferenceWritableKeyPath inherits from WritableKeyPath and WritableKeyPath inherits from KeyPath and so on.

Note: that hierarchy means ( for example ) if you have a function that takes KeyPath<Root,Type> as a parameter then you can pass also a keyPath of type WritableKeyPath and ReferenceWritableKeyPath as a parameter to hat function because both of theme inherits from KeyPath<Root,Type> but you cannot pass in a parameter of type PartialKeyPath or AnyKeyPath

let's explore the different keypaths types one by one and see when they can be used.

2.1 ReferenceWritableKeyPath

ReferenceWritableKeyPath works with Root of reference types ( class ) and mutable properties ( var ), this allows to read and write ( change ) the property value.

This is the one we have seen already in the previous example, as the User is class/reference type and the fullName property is var

class User {
   var fullName: String
    var email: String?
    var age: Int
  // ...
}
let fullNameKeyPath = \User.fullName
print(type(of: fullNameKeyPath)) // ReferenceWritableKeyPath<User, String>

Note: it doesn't matter wether the instance itself is var or let, what matters is the keyPath property should be mutable ( var ) to be able to change/write its value.

2.2 WritableKeyPath

WritableKeyPath works will Root of value types ( struct and enum ) and again allow us to read/write the keyPath property value of an instance taking into account the property should be var and the instance should be var also.

struct User {
    var fullName: String
    var email: String?
    var age: Int
}
var user = User(fullName: "Mike", age: 16) // instance should be var
let ageKeyPath = \User.age
// get fullName
print(user[keyPath: ageKeyPath]) // 16 
// set fullName
user[keyPath: ageKeyPath] = 20
print(user[keyPath: ageKeyPath]) // 20

print(type(of: ageKeyPath)) // WritableKeyPath<User, Int>

2.3 KeyPath

KeyPath is the simplest type, it can be used with both reference types ( classes ) and value types ( struct and enum ) as a Root type. it is used only for read property value ( read-only ) so it is only used for accessing immutable ( let ) properties.

struct User {  // or it can be -> class User 
    var fullName: String
    var email: String?
    let age: Int // Note: age is immutable
}
var user = User(fullName: "Mike", age: 16)
let ageKeyPath = \User.age
// get age
print(user[keyPath: ageKeyPath]) // 16
print(type(of: ageKeyPath)) // KeyPath<User, Int>

and if we tried changing the property value using the ageKeyPath.

user[keyPath: ageKeyPath] = 20  // error: cannot assign through subscript: 'ageKeyPath' is a read-only key path

2.4 PartialKeyPath

in the previous three keyPath types we know the two required types : the Root type ( User in our example ) and the Value Type ( String for fullName or Int for age ). But if we don't know the Value type of our keyPath or we need to be more flexible about the value type then we can use the PartialKeyPath<Root> which takes a reference to a property of any ValueType from specific Root type. this is beneficial when we need to write a functions that accepts a KeyPaths of different Value types but the same Root Type or we need to store multiple keyPaths of different Value types in an array.

struct User {
    var fullName: String
    var email: String?
    let age: Int
}

var user = User(fullName: "Mike", email: "[email protected]", age: 16)

let agePartialKeyPath: PartialKeyPath<User> = \User.age
let emailPartialKeyPath: PartialKeyPath<User> = \User.email
let fullNamePartialKeyPath: PartialKeyPath<User> = \User.fullName

let keyPathsList: [PartialKeyPath<User>] = [agePartialKeyPath,emailPartialKeyPath,fullNamePartialKeyPath]
keyPathsList.forEach { keypath in
    print(user[keyPath: keypath]) // 16, Optional("[email protected]"), Mike
}

2.5 AnyKeyPath

AnyKeyPath is the least restrictive one. it is fully type erased keyPath that can be used when the Root Type is unknown and the Value Type ( Path) is also unknown. this is very beneficial when we want to create an API that takes a keyPath of different Root types or we want to store multiple KeyPaths of different Root Types in an array.

struct Cat {
    var name: String
}

class User {
   // ...
}

var user = User(fullName: "Mike", email: "[email protected]", age: 16)

let userAgeKeyPath = \User.age // ReferenceWritableKeyPath<User, Int>
let userEmailKeyPath = \User.email // ReferenceWritableKeyPath<User, Optional<String>>
let userFullNameKeyPath = \User.fullName // ReferenceWritableKeyPath<User, String>

var cat = Cat(name: "Nala")
let catNameKeyPath = \Cat.name // WritableKeyPath<Cat, String>

let keyPathsList: [AnyKeyPath] = [userAgeKeyPath,userEmailKeyPath,userFullNameKeyPath,catNameKeyPath]

keyPathsList.forEach { keypath in
    if let keypath = keypath as? PartialKeyPath<User> {
        print(user[keyPath: keypath]) // // 16, Optional("[email protected]"), Mike
    }else if let keypath = keypath as? PartialKeyPath<Cat> {
        print(cat[keyPath: keypath]) // Nala
    }

}

here we create keypaths of different Value types and different Root types then we store them in an array of type AnyKeyPath.

since we are using AnyKeyPath we don't know any information about both the keypath Root type or the value Type and this is why we are trying to cast the keypath to PartialKeyPath<User> ( getter return Any ). we can for sure try to cast it to a more specific keypath like KeyPath<Cat, String> or KeyPath<User, String> and if the casting succeeds we know exactly both the Root Type and the Value Type.

let's recap,

KeyPath Type	Read	Write	Note
AnyKeyPath	Yes	No	unknown Root Type/ unknown Value Type
PartialKeyPath	Yes	No	known Root Type / unknown Value Type
KeyPath	Yes	No	known Root Type / known Value Type
WritableKeyPath	Yes	Yes	known Root Type / known Value Type for value types ( struct and enum ) ( var instance )
ReferenceWritableKeyPath	Yes	Yes	known Root Type / known Value Type, for class Type ( let or var instance)

3 - Swift 5.2: Key path as functions

Because closures of the form: (Root Type ) -> Value are common in Swift, Key path as functions is one of the features added to swift 5.2. it allows keypath to automatically be promoted and used wherever a functions of the form (Root ) -> Value are expected.

let's assume we have a list of users and we need to extract their full names,

struct User {
    var fullName: String
    var email: String?
    var age: Int
}

var firstUser = User(fullName: "Mike", email: "[email protected]", age: 20)
var secondUser = User(fullName: "Adam", email: "[email protected]", age: 25)
var ThirdUser = User(fullName: "John", email: "[email protected]", age: 30)
var users = [firstUser,secondUser,ThirdUser]

let usersNames = users.map { $0.fullName } // original
print(usersNames) // ["Mike", "Adam", "John"]

here we are passing to the map a function of the form ( User ) -> String return the users full names, in swift 5.2 we can instead pass a keypath for the user fullName property and pass it to the map.

let usersNames = users.map (\.fullName) // using keypath
print(usersNames)  // ["Mike", "Adam", "John"]

let fullNameKeyPath: (User) -> String = \User.fullName
let usersNames = users.map ( fullNameKeyPath )
print(usersNames)

but this way we need to be explicit about the keypath type let fullNameKeyPath: (User) -> String, because let fullNameKeyPath = \User.fullName is inferred as a keypath object not as a function ( default behavior ). by making the type explicit we are using the fullNameKeyPath as a function not as a keypath object.

let fullNameKeyPath = \User.fullName // inferred as WritableKeyPath<User, String>
let usersNames = users.map ( fullNameKeyPath ) // Error
print(usersNames)

4 - Using keyPaths

keypaths can be used on collection types to create generic algorithms and APIs with shorter and cleaner syntax. for instance let's say we want to sort an array containing elements of any Type and sort it by a specific property. with keypaths,

extension Sequence {
    func sorted<T: Comparable>(by keyPath: KeyPath<Element, T>) -> [Element] {
        return sorted { a, b in
            return a[keyPath: keyPath] < b[keyPath: keyPath]  // read
        }
    }
}

var firstUser = User(fullName: "Mike", email: "[email protected]", age: 20)
var secondUser = User(fullName: "Adam", email: "[email protected]", age: 25)
var ThirdUser = User(fullName: "John", email: "[email protected]", age: 30)
var users = [firstUser,secondUser,ThirdUser]

print( users.sorted(by: \.fullName))
// [User(fullName: "Adam", email: Optional("[email protected]"), age: 25),
// User(fullName: "John", email: Optional("[email protected]"), age: 30),
// User(fullName: "Mike", email: Optional("[email protected]"), age: 20)]
print( ["C","D","AB","A"].sorted(by: \.self) ) // ["A", "AB", "C", "D"]
print( ["C","D","AB","A"].sorted(by: \.count) ) // ["C", "D", "A", "AB"]

here we created an extension to the Sequence type, adding a new sorted(by keyPath:) that takes a keypath to sort the array based on that keypath value, so we can sort array of users by fullName, email, age or sort any array based on any Comparable property.

another example could be if we want to update a UILabel properties in a shorter syntax, we can do something like:

protocol Builder {}

extension Builder {
     func set<T>(_ keyPath:ReferenceWritableKeyPath<Self, T>,to newValue: T) -> Self {
        self[keyPath: keyPath] = newValue  // change property value
        return self
    }
}

extension UILabel: Builder { }

let label = UILabel()
    .set(\.text, to: "Hello world")
    .set(\.textColor, to: .red)
    .set(\.font, to: .systemFont(ofSize: 24, weight: .medium))

Note: if you want to apply this to any UIKit component, just make it conform to the Builder protocol just like we did with the UILabel like extension extension UIButton: Builder { }. but if we want to use this on any NSObject or any UIKit element then we can instead make the NSObject conforms to the Builder protocol extension NSObject: Builder { }.

as you can see, keypaths enable us to create many APIs with shorter syntax like creating DSL for working with layout constraints, algorithms on collections types and many other use cases.