如何将闭包提取为类型别名？

Question

I'm a bit lost with typealias, the following chunk of code works fine, which I would like to refactor into using typealias.

NEVPNManager.shared().loadFromPreferences(completionHandler: vpnLoadHandler)

func vpnLoadHandler(_: (Error?)) -> Void {

}

I would like to create a typealias instead:

typealias vpnCompleteClosure = (_: (Error?)) -> Void

func vpnLoadHandler(complete: @escaping vpnCompleteClosure) {
}

NEVPNManager.shared().loadFromPreferences(completionHandler: vpnLoadHandler)

Unfortunately this doesn't compile anymore:

Cannot convert value of type '(@escaping vpnCompleteClosure) -> ()' (aka '((Optional) -> ()) -> ()') to expected argument type '(Error?) -> Void'

What am I missing?

UPDATE:

open func loadFromPreferences(completionHandler: @escaping (Error?) -> Swift.Void)

Answer 1

loadFromPreferences(completionHandler:) is expecting a parameter, completion that is of type (Error?) -> Void Your typealias is passing a ((Optional) -> Void) -> Void Your error is telling you you have mismatching signatures.

That being said, you need to figure out what the signature of the top level method is. What are you calling your completionHandler on? What are you expecting to get back from it.

I have to admit Im confused as to your purpose of passing vpnLoadHandler(complete:) into loadFromPreferences(completionHandler:)

Your method will end up looking like (((Error?)-> Void) -> Void) -> Void

write your typealias as:

 typealias VPNCompletionHandler = (Error?) -> Void

 func vpnLoadHandler(completion: @escaping VPNCompletionHandler)