Key path expressions as functions
Key path expressions as functions êŽë š
Available from Swift 5.2
SE-0249 (apple/swift-evolution) introduced a marvelous shortcut that allows us to use keypaths in a handful of specific circumstances.
The Evolution proposal describes this as being able to use â\Root.value wherever functions of (Root) -> Value are allowedâ, but what it means is that if previously you sent a Car into a method and got back its license plate, you can now use Car.licensePlate instead.
This is best understood as an example, so hereâs a User type that defines four properties:
struct User {
let name: String
let age: Int
let bestFriend: String?
var canVote: Bool {
age >= 18
}
}
We could create some instance of that struct and put them into an array, like this:
let eric = User(name: "Eric Effiong", age: 18, bestFriend: "Otis Milburn")
let maeve = User(name: "Maeve Wiley", age: 19, bestFriend: nil)
let otis = User(name: "Otis Milburn", age: 17, bestFriend: "Eric Effiong")
let users = [eric, maeve, otis]
Now for the important part: if you want to get an array of all the users names, you can do so by using a key path like this:
let userNames = users.map(\.name)
print(userNames)
Previously you would have had to write a closure to retrieve the name by hand, like this:
let oldUserNames = users.map { $0.name }
This same approach works elsewhere â anywhere where previously you would have received a value and passed back one of its properties, you can now use a key path instead. For example, this will return all users who can vote:
let voters = users.filter(\.canVote)
And this will return the best friends for all users who have one:
let bestFriends = users.compactMap(\.bestFriend)
Other Changes in Swift 5.2