Swift 可选的转义闭包参数 [英] Swift optional escaping closure parameter

问题描述

给定:

typealias Action = () -> ()

var action: Action = { }

func doStuff(stuff: String, completion: @escaping Action) {
    print(stuff)
    action = completion
    completion()
}

func doStuffAgain() {
    print("again")
    action()
}

doStuff(stuff: "do stuff") { 
    print("swift 3!")
}

doStuffAgain()

有什么办法可以使 completion 参数(和 action)的类型为 Action? 并保持 @escaping ?

Is there any way to make the completion parameter (and action) of type Action? and also keep @escaping ?

更改类型会出现以下错误:

Changing the type gives the following error:

@escaping 属性只适用于函数类型

@escaping attribute only applies to function types

删除 @escaping 属性，代码编译并运行，但似乎不正确，因为 completion 闭包正在转义函数的作用域.

Removing the @escaping attribute, the code compiles and runs, but doesn't seem to be correct since the completion closure is escaping the scope of the function.

推荐答案

有一个 SR-2552 报告 @escaping 无法识别函数类型别名.这就是错误 @escaping 属性仅适用于函数类型的原因.您可以通过扩展函数签名中的函数类型来解决:

There is a SR-2552 reporting that @escaping is not recognizing function type alias. that's why the error @escaping attribute only applies to function types. you can workaround by expanding the function type in the function signature:

typealias Action = () -> ()

var action: Action? = { }

func doStuff(stuff: String, completion: (@escaping ()->())?) {
    print(stuff)
    action = completion
    completion?()
}

func doStuffAgain() {
    print("again")
    action?()
}

doStuff(stuff: "do stuff") {
    print("swift 3!")
}

doStuffAgain()

编辑 1::

我实际上使用的是 xcode 8 beta 版本，其中错误 SR-2552 是还没有解决.修复该错误，引入了一个仍然打开的新错误(您面临的错误).请参阅 SR-2444.

I was actually under a xcode 8 beta version where the bug SR-2552 was not resolved yet. fixing that bug, introduced a new one(the one you're facing) that is still open. see SR-2444.

解决方法 @Michael Ilseman 指出的临时解决方案是从可选函数类型中删除 @escaping 属性，使函数保持转义.

The workaround @Michael Ilseman pointed as a temporary solution is remove the @escaping attribute from optional function type, that keep the function as escaping.

func doStuff(stuff: String, completion: Action?) {...}

编辑 2::

SR-2444 已关闭，明确声明参数位置中的闭包 不是 转义，需要用 @escaping 标记它们以使它们转义，但可选参数 是 隐式转义，因为 ((Int)->())? 是 Optional<(Int)->()> 的同义词，可选闭包是转义的.

The SR-2444 has been closed stating explicitly that closures in parameters positions are not escaping and need them to be marked with @escaping to make them escaping, but the optional parameters are implicitly escaping, since ((Int)->())? is a synonyms of Optional<(Int)->()>, optional closures are escaping.

这篇关于Swift 可选的转义闭包参数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！