Sending vs Sendable in Swift – Donny Wals

With Swift 6, we have now a wholly new model of the language that has every kind of information 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 possibly can get away with sure issues the place in Swift 6 these are actually compiler errors.

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

On this submit, I wish to discover the similarities and variations between Sendable and sending. By the top of this submit, you’ll perceive why the Swift group determined to vary the closures that you just move to duties, continuations, and process teams to be sending as an alternative of @Sendable.

In the event you’re not totally updated on Sendable, I extremely advocate that you just try my submit on Sendable and @Sendable closures. On this submit, it is most related so that you can perceive the @Sendable closures half as a result of we’ll 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 move a closure or a price from one place to a different safely, we might try this with the sendable annotation. So, for instance, Job would have been outlined a bit 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 Job implies that that closure needs to be secure to name from every other duties or isolation context. In observe, which means no matter we do and seize within that closure have to be secure, or in different phrases, it have to be Sendable.

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

Because of this the code under shouldn’t be secure in keeping 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 Job has modified its operation to be sending as an alternative of @Sendable at a language stage no matter language mode.

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

// The instance under requires the Swift 5 COMPILER to fail
// Utilizing the Swift 5 language mode shouldn't be sufficient
func exampleFunc() {
  let isNotSendable = MyClass()

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

If you wish to discover this compiler error in a mission that makes use of the Swift 6 compiler, you’ll be able to outline your individual perform that takes a @Sendable closure as an alternative of a Job:

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

In the event you name that as an alternative of Job, you’ll see the compiler error talked about earlier.

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

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

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

Our particular utilization right here is completely secure although. We’re creating an occasion of MyClass within the perform that we’re making a process 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 now not retained exterior of the Job closure.

Due to this, there is not any manner that we’ll be passing isolation boundaries right here; No different place than our Job has entry to our occasion anymore.

That’s the place sending is available in…

Understanding sending arguments

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

This enables us to move non-sendable objects right into a closure that must be secure to name throughout isolation contexts.

In Swift 6, the code under is completely legitimate:

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

  Job {
    isNotSendable.depend += 1
  }
}

That’s as a result of Job 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 an alternative of @sendable, the compiler can verify that this occasion of MyClass that we’re passing into the duty shouldn’t be accessed or used after the duty captures it. So whereas the code above is legitimate, we are able to truly write one thing that’s now not legitimate.

For instance:

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

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

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

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

If you’re defining your individual features that take closures that you just need to be secure to name from a number of isolation contexts, you’ll need to mark them as sending.

Defining your individual perform that takes a sending closure seems to be as follows:

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

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

In Abstract

You most likely will not be defining your individual sending closures or your individual features that take sending arguments steadily. The Swift group has up to date the initializers for duties, indifferent duties, the continuation APIs, and the duty group APIs to take sending closures as an alternative of @Sendable closures. Due to this, you will discover that Swift 6 permits you to do sure issues that Swift 5 would not assist you to do with strict concurrency enabled.

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

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

Additional studying