Sending vs Sendable in Swift – Donny Wals

With Swift 6, we now have a completely new model of the language that has every kind of knowledge race protections built-in. Most of those protections have been round with Swift 5 in a technique or one other and in Swift 6 they’ve refined, up to date, improved, and expanded these options, making them obligatory. So in Swift 5 you may get away with sure issues the place in Swift 6 these are actually compiler errors.

Swift 6 additionally introduces a bunch of latest options, considered one of these is the sending key phrase. Sending intently pertains to Sendable, however they’re fairly completely different by way of why they’re used, what they will do, and which issues they have an inclination to resolve.

On this publish, I wish to discover the similarities and variations between Sendable and sending. By the top of this publish, you’ll perceive why the Swift crew determined to vary the closures that you just cross to duties, continuations, and job teams to be sending as a substitute of @Sendable.

When you’re not absolutely updated on Sendable, I extremely suggest that you just try my publish on Sendable and @Sendable closures. On this publish, it is most related so that you can perceive the @Sendable closures half as a result of we will be a comparability between a @Sendable closure and a sending argument.

Understanding the issue that’s solved by sending

In Swift 5, we did not have the sending key phrase. That meant that if we needed to cross a closure or a worth from one place to a different safely, we’d do this with the sendable annotation. So, for instance, Activity would have been outlined just a little bit like this in Swift 5.

public init(
  precedence: TaskPriority? = nil,
  operation: @Sendable @escaping () async -> Success
)

This initializer is copied from the Swift repository with some annotations stripped for simplicity.

Discover that the operation argument takes a @Sendable closure.

Taking a @Sendable closure for one thing like a Activity implies that that closure ought to be protected to name from another duties or isolation context. In observe, because of this no matter we do and seize within that closure should be protected, or in different phrases, it should be Sendable.

So, a @Sendable closure can basically solely seize Sendable issues.

Because of this the code under is just not protected in accordance with the Swift 5.10 compiler with strict concurrency warnings enabled.

Be aware that operating the instance under in Xcode 16 with the Swift 6 compiler in Swift 5 mode won’t throw any errors. That is as a result of Activity has modified its operation to be sending as a substitute of @Sendable at a language degree no matter language mode.

So, even in Swift 5 language mode, Activity takes a sending operation.

// The instance under requires the Swift 5 COMPILER to fail
// Utilizing the Swift 5 language mode is just not sufficient
func exampleFunc() {
  let isNotSendable = MyClass()

  Activity {
      // Seize of 'isNotSendable' with non-sendable kind 'MyClass' in a `@Sendable` closure
    isNotSendable.depend += 1
  }
}

If you wish to discover this compiler error in a venture that makes use of the Swift 6 compiler, you possibly can outline your personal perform that takes a @Sendable closure as a substitute of a Activity:

public func sendableClosure(
  _ closure: @Sendable () -> Void
  ) {
  closure()
}

When you name that as a substitute of Activity, you’ll see the compiler error talked about earlier.

The compiler error is right. We’re taking one thing that is not sendable and passing it right into a job which in Swift 5 nonetheless took a @Sendable closure.

The compiler does not like that as a result of the compiler says, “If it is a sendable closure, then it should be protected to name this from a number of isolation contexts, and if we’re capturing a non-sendable class, that isn’t going to work.”

This drawback is one thing that you’d run into often, particularly with @Sendable closures.

Our particular utilization right here is completely protected although. We’re creating an occasion of MyClass within the perform that we’re making a job or passing that occasion of MyClass into the duty.

After which we’re by no means accessing it exterior of the duty or after we make the duty anymore as a result of by the top of exampleFunc this occasion is not retained exterior of the Activity closure.

Due to this, there is no means that we will be passing isolation boundaries right here; No different place than our Activity has entry to our occasion anymore.

That’s the place sending is available in…

Understanding sending arguments

In Swift 6, the crew added a function that permits us to inform the compiler that we intend to seize no matter non-sendable state we would obtain and do not wish to entry it elsewhere after capturing it.

This permits us to cross non-sendable objects right into a closure that must be protected to name throughout isolation contexts.

In Swift 6, the code under is completely legitimate:

func exampleFunc() async {
  let isNotSendable = MyClass()

  Activity {
    isNotSendable.depend += 1
  }
}

That’s as a result of Activity had its operation modified from being @Sendable to one thing that appears a bit as follows:

public init(
  precedence: TaskPriority? = nil,
  operation: sending @escaping () async -> Success
)

Once more, it is a simplified model of the particular initializer. The purpose is so that you can see how they changed @Sendable with sending.

As a result of the closure is now sending as a substitute of @sendable, the compiler can examine that this occasion of MyClass that we’re passing into the duty is just not accessed or used after the duty captures it. So whereas the code above is legitimate, we will truly write one thing that’s not legitimate.

For instance:

func exampleFunc() async {
  let isNotSendable = MyClass()

  // Worth of non-Sendable kind ... accessed after being transferred; 
  // later accesses may race
  Activity {
    isNotSendable.depend += 1
  }

  // Entry can occur concurrently
  print(isNotSendable.depend)
} 

This alteration to the language permits us to cross non-sendable state right into a Activity, which is one thing that you will generally wish to do. It additionally makes positive that we’re not doing issues which are probably unsafe, like accessing non-sendable state from a number of isolation contexts, which is what occurs within the instance above.

In case you are defining your personal capabilities that take closures that you just wish to be protected to name from a number of isolation contexts, you’ll wish to mark them as sending.

Defining your personal perform that takes a sending closure seems as follows:

public func sendingClosure(
  _ closure: sending () -> Void
) {
  closure()
}

The sending key phrase is added as a prefix to the closure kind, much like the place @escaping would usually go.

In Abstract

You most likely will not be defining your personal sending closures or your personal capabilities that take sending arguments continuously. The Swift crew has up to date the initializers for duties, indifferent duties, the continuation APIs, and the duty group APIs to take sending closures as a substitute of @Sendable closures. Due to this, you will discover that Swift 6 permits you to do sure issues that Swift 5 would not let you do with strict concurrency enabled.

I feel it’s actually cool to know and perceive how sending and @Sendable work.

I extremely suggest that you just experiment with the examples on this weblog publish by defining your personal sending and @Sendable closures and seeing how every might be referred to as and how one can name them from a number of duties. It is also value exploring how and when every choices stops working so that you’re conscious of their limitations.

Additional studying