Updating closures to Swift 3 - @escaping

Question

I've updated my code to Xcode 8.0 beta 6 but I got stuck with what seems to be about the new non escaping closure default. In the following code Xcode suggests to add @escaping in front of completion: in the first line of the below code, but that still won't compile and goes in circles. *

(EDIT: In fact, @escaping should be added in after completion:, as Xcode suggests. The alert may still show but cleaning and compiling will remove it.)* How should this code be re-written / fixed to work in the updated Swift 3? I've had a look in the new manual but I couldn't find proper code samples.

func doSomething(withParameter parameter: Int, completion: () -> ()) {
    // Does something

    callSomeOtherFunc(withCompletion: completion)
  }

// Calling the method and execute closure 
doSomething(withParameter: 2) {
  // do things in closure
}

Any help much appreciated!

Answer 1

Swift 3: closure parameter attributes are now applied to the parameter type, and not the parameter itself

Prior to Swift 3, the closure attributes @autoclosure and @noescape used to be attributes to the closure parameter, but are now attributes to the parameter type; see the following accepted Swift evolution proposal:

• SE-0049: Move @noescape and @autoclosure to be type attributes

Your specific question pertain to parameter type attribute @escaping (for which the same new rule applies), as described in the accepted Swift evolution proposal to let closure parameters be non-escaping by default:

• SE-0103: Make non-escaping closures the default

These proposals are now both implemented in the beta stage of Xcode 8 (see release notes for Xcode 8 beta 6; dev. account login needed for access)

New in Xcode 8 beta 6 - Swift Compiler: Swift Language

Closure parameters are non-escaping by default, rather than explicitly being annotated with @noescape. Use @escaping to indicate that a closure parameter may escape. @autoclosure(escaping) is now written as @autoclosure @escaping. The annotations @noescape and @autoclosure(escaping) are deprecated. (SE-0103)

...

New in Xcode 8 beta – Swift and Apple LLVM Compilers: Swift Language

The @noescape and @autoclosure attributes must now be written before the parameter type instead of before the parameter name. [SE-0049]

Hence, you use the non-default @escaping attribute as follows; applied to the type of the closure parameter, rather than the parameter itself

func doSomething(withParameter parameter: Int, completion: @escaping () -> ()) {
    // ...
}

---

(Including my answer to a question in an upvoted comment below, as comments are not persistent data on SO)

@Cristi Băluță: "What does escaping do? Never seen this keywords before swift3 auto-conversion ... "

See e.g. the link to the SE-0103 evolution proposal above (as well as the quoted text from the beta 6 release notes): previously, closure parameters were escaping by default (hence no need for the existence of an explicit annotation for escaping), but are now instead non-escaping, by default. Hence the addition of @escaping to explicitly annotate that a closure parameter may escape (contrary to its default behaviour). This also explains why @noescape is now deprecated (no need to annotate the default behaviour).

For explaining what it means that a closure parameter is escaping, I quote the Language Reference - attributes:

"Apply this attribute to a parameter’s type in a method or function declaration to indicate that the parameter’s value can be stored for later execution. This means that the value is allowed to outlive the lifetime of the call."