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Sendable and @Sendable closures

Available from Swift 5.5

Paul Hudson      @twostraws

SE-0302 adds support for “sendable” data, which is data that can safely be transferred to another thread. This is accomplished through a new Sendable protocol, and an @Sendable attribute for functions.

Many things are inherently safe to send across threads:

  • All of Swift’s core value types, including Bool, Int, String, and similar.
  • Optionals, where the wrapped data is a value type.
  • Standard library collections that contain value types, such as Array<String> or Dictionary<Int, String>.
  • Tuples where the elements are all value types.
  • Metatypes, such as String.self.

These have been updated to conform to the Sendable protocol.

As for custom types, it depends what you’re making:

  • Actors automatically conform to Sendable because they handle their synchronization internally.
  • Custom structs and enums you define will also automatically conform to Sendable if they contain only values that also conform to Sendable, similar to how Codable works.
  • Custom classes can conform to Sendable as long as they either inherits from NSObject or from nothing at all, all properties are constant and themselves conform to Sendable, and they are marked as final to stop further inheritance.

Swift lets us use the @Sendable attribute on functions or closure to mark them as working concurrently, and will enforce various rules to stop us shooting ourself in the foot. For example, the operation we pass into the Task initializer is marked @Sendable, which means this kind of code is allowed because the value captured by Task is a constant:

func printScore() async { 
    let score = 1

    Task { print(score) }
    Task { print(score) }

However, that code would not be allowed if score were a variable, because it could be accessed by one of the tasks while the other was changing its value.

You can mark your own functions and closures using @Sendable, which will enforce similar rules around captured values:

import Foundation 

func runLater(_ function: @escaping @Sendable () -> Void) -> Void { .now() + 3, execute: function)
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