Sending vs Sendable in Swift – Donny Wals

Understanding the issue that’s solved by sending

In Swift 5, we did not have the sending key phrase. That meant that if we wished to move a closure or a worth from one place to a different safely, we’d try this with the sendable annotation. So, for instance, Activity would have been outlined slightly 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 signifies that that closure must be protected to name from some other duties or isolation context. In observe, which means no matter we do and seize inside that closure should be protected, or in different phrases, it should be Sendable.

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

Which means the code under will not be protected in accordance with the Swift 5.10 compiler with strict concurrency warnings enabled.

Word that working the instance under in Xcode 16 with the Swift 6 compiler in Swift 5 mode is not going to throw any errors. That is as a result of Activity 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, Activity takes a sending operation.

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

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

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

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

When you name that as an alternative of Activity, 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 does not like that as a result of the compiler says, “If this can be 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 sometimes, particularly with @Sendable closures.

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

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

Due to this, there is not any means that we’ll 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 group added a function that permits us to inform the compiler that we intend to seize no matter non-sendable state we’d 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 protected to name throughout isolation contexts.

In Swift 6, the code under is completely legitimate:

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

  Activity {
    isNotSendable.rely += 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, this can be 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 test that this occasion of MyClass that we’re passing into the duty will not 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 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.rely += 1
  }

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

This transformation to the language permits us to move non-sendable state right into a Activity, which is one thing that you’re going to typically need to do. It additionally makes certain that we’re not doing issues which can be 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 personal capabilities that take closures that you just need to be protected to name from a number of isolation contexts, you’ll need to mark them as sending.

Defining your personal operate 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 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 may discover that Swift 6 permits you to do sure issues that Swift 5 would not help you do with strict concurrency enabled.

I feel 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 personal sending and @Sendable closures and seeing how every could be known 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