Migrating to Swift 6 Tutorial

Swift 6 appeared at WWDC 2024, and all of us rushed emigrate all our apps to it … properly, probably not. We have been fairly proud of what we bought 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 the whole lot, and it felt like a good suggestion to attend some time.

One 12 months later, the migration path seems so much smoother, with heaps extra guideposts. Preserve studying to learn the way a lot simpler it’s turn out to be.

From Single-Thread to Concurrency

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

1. Run the whole lot 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 want to carry out a sluggish operation, create and await an async operate to do the work. This operate 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 information from a server, your asynchronous operate can do some setup then await a URLSession technique that runs on a background thread. At this level, your operate suspends, and the principle thread is free to do another work. When the URLSession technique finishes, your operate is able to resume execution on the principle thread, often to supply some new information to show to the person.
3. Concurrency: As you add extra asynchronous operations to the principle thread, your app’s UI may turn out to be much less responsive. Profile your app with Devices to search out efficiency issues and see if you happen to can repair the issue — pace 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 reap the benefits of the a number of CPUs on the machine.

Isolation Domains

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

The principle instruments to forestall information races are information isolation and isolation domains:

The vital function of an isolation area is the security it gives. Mutable state can solely be accessed from one isolation area at a time. You possibly can move mutable state from one isolation area to a different, however you may 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. International actor
3. Non-isolated

Actors defend 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 have to inject the actor from one kind to a different, or into the SwiftUI surroundings.

From Embracing Swift concurrency:

Nonisolated code may be very versatile, as a result of you may name it from wherever: if you happen to name it from the principle actor, it is going to keep on the principle actor. If you happen to name it from a background thread, it is going to keep on a background thread. This makes it an amazing default for general-purpose libraries.

Information 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 can have MainActor because the default isolation area, so each operation runs on the principle thread except you do one thing to maneuver work onto a background thread. The principle 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 challenge’s Construct Settings. Allow these settings one after the other, in any order, and deal with any points that come up:

Upcoming Options prompt by swift.org’s migration technique

Upcoming Options prompt by swift.org’s migration technique

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

Upcoming Options options in Xcode Construct Settings

Upcoming Options options in Xcode Construct Settings

Be aware: I don’t see an actual match for GlobalConcurrency, however it may be Remoted International Variables.

Then, allow full concurency checking to activate the remaining information 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 can 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 except you mark it as one thing else.

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

Upcoming Options that Approachable Concurrency allows

Upcoming Options that Approachable Concurrency allows

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 prime or backside of this text to obtain the starter challenge, then open it in Xcode 26 (beta).

TheMet is a challenge 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 put up Swift 6.2: A primary take a look at the way it’s altering Concurrency to point out which threads fetchObjects(for:), getObjectIDs(from:) and getObject(from:) are operating on.

<code>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
  }
}
</code>

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

Construct and run and watch the console:

Retailer and Service strategies operating on background threads

Retailer and Service strategies operating on background threads

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