Migrating to Swift 6 Tutorial

Swift 6 appeared at WWDC 2024, and all of us rushed emigrate all our apps to it … effectively, not likely. We had been fairly pleased with what we acquired 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 every little thing, and it felt like a good suggestion to attend some time.

One yr later, the migration path appears lots smoother, with tons extra guideposts. Hold studying to learn how a lot simpler it’s grow to be.

From Single-Thread to Concurrency

The purpose 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 every little thing on the primary thread: Begin with synchronous execution on the primary thread — if each operation is quick sufficient, your app’s UI received’t dangle.
2. async/await: If it’s essential to carry out a sluggish operation, create and await an async perform to do the work. This perform nonetheless runs on the primary 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 primary thread is free to do another work. When the URLSession technique finishes, your perform is able to resume execution on the primary thread, normally to offer some new knowledge to show to the person.
3. Concurrency: As you add extra asynchronous operations to the primary thread, your app’s UI would possibly grow to be much less responsive. Profile your app with Devices to seek 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 process teams to run sub-tasks in parallel to benefit from the a number of CPUs on the system.

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 varieties like struct as a lot as doable.

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

The important characteristic of an isolation area is the protection it supplies. 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’ll be able to 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 outdoors their isolation area. A GlobalActor should have a static property known as shared that exposes an actor occasion that you just 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’ll be able to name it from wherever: if you happen to name it from the primary actor, it would keep on the primary actor. Should you name it from a background thread, it would 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 features 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 initiatives can have MainActor because the default isolation area, so each operation runs on the primary thread except you do one thing to maneuver work onto a background thread. The primary thread is serial, so mutable MainActor objects may 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 venture’s Construct Settings. Allow these settings one by one, 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 venture’s Construct Settings, these are in Swift Compiler — Upcoming Options:

Upcoming Options strategies in Xcode Construct Settings

Upcoming Options strategies in Xcode Construct Settings

Observe: I don’t see an actual match for GlobalConcurrency, but it surely 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 initiatives can have these default settings for the opposite two Swift Compiler — Concurrency settings:

• Approachable Concurrency: Sure: Permits a collection 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 permits a number of Upcoming Options, together with two of the swift.org’s migration technique strategies:

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 an alternative.

Getting Began

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

TheMet is a venture 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.
  /// This can be 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 completely 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 primary thread. You’ll quickly repair this!