Migrating to Swift 6 Tutorial

Swift 6 appeared at WWDC 2024, and all of us rushed emigrate all our apps to it … nicely, not likely. We had been fairly proud of what we received at WWDC 2021 — Swift 5.5’s shiny new structured concurrency framework that helped us write protected code extra swiftly with async/await and actors. Swift 6 appeared to interrupt all the things, and it felt like a good suggestion to attend some time.

One 12 months later, the migration path appears to be like lots smoother, with tons extra guideposts. Maintain studying to learn how a lot simpler it’s turn into.

From Single-Thread to Concurrency

The objective of Swift 6.2 concurrency is to simplify your app improvement. It identifies three phases, the place you introduce concurrency explicitly, as and whenever you want it:

1. Run all the things on the principle thread: Begin with synchronous execution on the principle thread — if each operation is quick sufficient, your app’s UI gained’t dangle.
2. async/await: If you could carry out a sluggish operation, create and await an async perform to do the work. This perform nonetheless runs on the principle thread, which interleaves its work with work from different duties, like responding to the person scrolling or tapping. For instance, in case your app must obtain knowledge from a server, your asynchronous perform can do some setup then await a URLSession technique that runs on a background thread. At this level, your perform suspends, and the principle thread is free to do another work. When the URLSession technique finishes, your perform is able to resume execution on the principle thread, normally to supply some new knowledge to show to the person.
3. Concurrency: As you add extra asynchronous operations to the principle thread, your app’s UI would possibly turn into much less responsive. Profile your app with Devices to search out efficiency issues and see when you can repair the issue — velocity up the sluggish operation — with out concurrency. If not, introduce concurrency to maneuver that operation to a background thread and maybe use async let or activity teams to run sub-tasks in parallel to make the most of the a number of CPUs on the machine.

Isolation Domains

Swift 6.2 concurrency goals to eradicate knowledge races, which occur when a course of on one thread modifies knowledge whereas a course of on one other thread is accessing that knowledge. Knowledge races can solely come up when your app has mutable objects, which is why Swift encourages you to make use of let and worth sorts like struct as a lot as doable.

The primary instruments to forestall knowledge races are knowledge isolation and isolation domains:

The crucial function of an isolation area is the protection it gives. Mutable state can solely be accessed from one isolation area at a time. You’ll be able to go mutable state from one isolation area to a different, however you possibly can by no means entry that state concurrently from a special area. This assure is validated by the compiler.

There are three classes of isolation area:

1. Actor
2. World actor
3. Non-isolated

Actors shield their mutable objects by sustaining a serial queue for asynchronous requests coming from exterior their isolation area. A GlobalActor should have a static property referred to as shared that exposes an actor occasion that you simply make globally accessible — you don’t must inject the actor from one kind to a different, or into the SwiftUI setting.

From Embracing Swift concurrency:

Nonisolated code may be very versatile, as a result of you possibly can name it from anyplace: when you name it from the principle actor, it can keep on the principle actor. For those who name it from a background thread, it can keep on a background thread. This makes it a fantastic default for general-purpose libraries.

Knowledge isolation ensures that non-isolated entities can not entry the mutable state of different domains, so non-isolated capabilities and variables are all the time protected to entry from every other area.

Non-isolated is the default area at swift.org as a result of non-isolated code can not mutate state protected in one other area. Nonetheless, new Xcode 26 tasks may have MainActor because the default isolation area, so each operation runs on the principle thread until you do one thing to maneuver work onto a background thread. The primary thread is serial, so mutable MainActor objects could be accessed by at most one course of at a time.

Migrating to Swift 6.2

Swift.org Migration Information

The Swift Migration Information suggests a course of for migrating Swift 5 code to Swift 6. Whereas in Swift 5 language mode, incrementally allow Swift 6 checking in your mission’s Construct Settings. Allow these settings separately, in any order, and tackle any points that come up:

Upcoming Options recommended by swift.org’s migration technique

Upcoming Options recommended by swift.org’s migration technique

In your mission’s Construct Settings, these are in Swift Compiler — Upcoming Options:

Upcoming Options options in Xcode Construct Settings

Upcoming Options options in Xcode Construct Settings

Notice: I don’t see a precise match for GlobalConcurrency, nevertheless it would possibly be Remoted World Variables.

Then, allow full concurency checking to activate the remaining knowledge isolation checks. In Xcode, that is the Strict Concurrency Checking setting in Swift Compiler — Concurrency.

Xcode Construct Settings: Swift Compiler — Concurrency

Xcode Construct Settings: Swift Compiler — Concurrency

Xcode 26 Default Settings

New Xcode 26 tasks may have these default settings for the opposite two Swift Compiler — Concurrency settings:

• Approachable Concurrency: Sure: Allows a set of upcoming options that make simpler to work with concurrency.
• Default Actor Isolation: MainActor: Isolates code on the MainActor until you mark it as one thing else.

Enabling Approachable Concurrency permits a number of Upcoming Options, together with two of the swift.org’s migration technique options:

Upcoming Options that Approachable Concurrency permits

Upcoming Options that Approachable Concurrency permits

If this raises too many points, disable Approachable Concurrency and check out the swift.org migration technique as a substitute.

Getting Began

Use the Obtain Supplies button on the high or backside of this text to obtain the starter mission, then open it in Xcode 26 (beta).

TheMet is a mission from SwiftUI Apprentice. It searches The Metropolitan Museum of Artwork, New York for objects matching the person’s question time period.

TheMet app: seek for Persimmon

TheMet app: seek for Persimmon

TheMetService has two strategies:

• getObjectIDs(from:) constructs the question URL and downloads ObjectID values of artwork objects that match the question time period.
• getObject(from:) fetches the Object for a particular ObjectID.

TheMetStore instantiates TheMetService and, in fetchObjects(for:) calls getObjectIDs(from:) then loops over the array of ObjectID to populate its objects array.

ContentView instantiates TheMetStore and calls its fetchObjects(from:) technique when it seems and when the person enters a brand new question time period.

The pattern app makes use of this Thread extension from SwiftLee’s submit Swift 6.2: A primary have a look at the way it’s altering Concurrency to point out which threads fetchObjects(for:), getObjectIDs(from:) and getObject(from:) are working on.

nonisolated extension Thread {
  /// A comfort technique to print out the present thread from an async technique.
  /// It is a workaround for compiler error:
  /// Class property 'present' is unavailable from asynchronous contexts; 
  /// Thread.present can't be used from async contexts.
  /// See: https://github.com/swiftlang/swift-corelibs-foundation/points/5139
  public static var currentThread: Thread {
    return Thread.present
  }
}

On this tutorial, you’ll migrate TheMet to Swift 6.2 concurrency.

Construct and run and watch the console:

Retailer and Service strategies working on background threads

Retailer and Service strategies working on background threads

TheMetStore and TheMetService strategies run solely on background threads, besides when fetchObjects(for:) appends an object to objects, which ContentView shows. Nonetheless, in Swift 6.2’s three-phase app improvement course of, solely the URLSession technique must run off the principle thread. You’ll quickly repair this!