NEW: Learn to build the incredible iOS 15 Weather app today! >>

How to make parts of an actor nonisolated

Paul Hudson    @twostraws   

Updated for Xcode 13.2

All methods and mutable properties inside an actor are isolated to that actor by default, which means they cannot be accessed directly from code that’s external to the actor. Access to constant properties is automatically allowed because they are inherently safe from race conditions, but if you want you can make some methods excepted by using the nonisolated keyword.

Actor methods that are non-isolated can access other non-isolated state, such as constant properties or other methods that are marked non-isolated. However, they cannot directly access isolated state like an isolated actor method would; they need to use await instead.

To demonstrate non-isolated methods, we could write a User actor that has three properties: two constant strings for their username and password, and a variable Boolean to track whether they are online. Because password is constant, we could write a non-isolated method that returns the hash of that password using CryptoKit, like this:

import CryptoKit
import Foundation

actor User {
    let username: String
    let password: String
    var isOnline = false

    init(username: String, password: String) {
        self.username = username
        self.password = password
    }

    nonisolated func passwordHash() -> String {
        let passwordData = Data(password.utf8)
        let hash = SHA256.hash(data: passwordData)
        return hash.compactMap { String(format: "%02x", $0) }.joined()
    }
}

let user = User(username: "twostraws", password: "s3kr1t")
print(user.passwordHash())

Download this as an Xcode project

I’d like to pick out a handful of things in that code:

  1. Marking passwordHash() as nonisolated means that we can call it externally without using await.
  2. We can also use nonisolated with computed properties, which in the previous example would have made nonisolated var passwordHash: String. Stored properties may not be non-isolated.
  3. Non-isolated properties and methods can access only other non-isolated properties and methods, which in our case is a constant property. Swift will not let you ignore this rule.

Non-isolated methods are particularly useful when dealing with protocol conformances such as Hashable and Codable, where we must implement methods to be run from outside the actor.

For example, if we wanted to make our User actor conform to Codable, we’d need to implement encode(to:) ourselves as a non-isolated method like this:

actor User: Codable {
    enum CodingKeys: CodingKey {
        case username, password
    }

    let username: String
    let password: String
    var isOnline = false

    init(username: String, password: String) {
        self.username = username
        self.password = password
    }

    nonisolated func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(username, forKey: .username)
        try container.encode(password, forKey: .password)
    }
}

let user = User(username: "twostraws", password: "s3kr1t")

if let encoded = try? JSONEncoder().encode(user) {
    let json = String(decoding: encoded, as: UTF8.self)
    print(json)
}

Download this as an Xcode project

Hacking with Swift is sponsored by Essential Developer

SPONSORED Learn the most up-to-date techniques and strategies for testing new and legacy Swift code in this free practical course for iOS devs who want to become complete Senior iOS Developers.

Learn more

Sponsor Hacking with Swift and reach the world's largest Swift community!

Similar solutions…

BUY OUR BOOKS
Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 3.0/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.