Swift-闭包表达式语法 [英] Swift - closure expression syntax

问题描述

我正在使用Alamofire库，我注意到他们使用了这种语法

i'm working with Alamofire library, i've noticed that they use this syntax

func download(method: Alamofire.Method, URLString: URLStringConvertible, headers: [String : String]? = default, #destination: Alamofire.Request.DownloadFileDestination) -> Alamofire.Request

需要4个参数作为输入，但是如果您转到文档调用他们使用以下方法的方法

that takes 4 parameters as input but if you go to the documentation to call the method they use the following

Alamofire.download(.GET, "http://httpbin.org/stream/100") { temporaryURL, response in
let fileManager = NSFileManager.defaultManager()
if let directoryURL = fileManager.URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)[0] as? NSURL {
    let pathComponent = response.suggestedFilename
    return directoryURL.URLByAppendingPathComponent(pathComponent!)
}
return temporaryURL}

仅接受2个参数（方法：和 URLString：）我认为参数 headers 是可选的，因为提供了 default 语句。
我不明白目的地的处理方式。
您能解释一下关闭的处理方式吗？
为什么在 URLString参数之后在方法调用之后打开花括号而不在调用内部？

that takes only 2 parameters (method: and URLString:) i think that the param headers is optional because provide the default statement. I don't understand how Destination is handled. Could you please explain me how the closure is handled? Why the curl braces is open AFTER the method call and not inside the call after the URLString param?

我非常感谢您可以提供的任何帮助

I really appreciate any help you can provide

Marco

推荐答案

这是尾随的闭包技术

如果方法接受闭包作为最后一个参数

It's the trailing closure technique

If a method accepts a closure as last param

class Foo {
    func doSomething(number: Int, word: String, completion: () -> ()) {

    }
}

您可以按照经典方式来称呼它：

You can call it following the classic way:

Foo().doSomething(1, word: "hello", completion: { () -> () in 
    // your code here
})

或使用尾随闭包技术：

Foo().doSomething(1, word: "hello") { () -> () in 
    // your code here
}

结果是同样，它只是一种更优雅的（IMHO）语法。

The result is the same, it just a more elegant (IMHO) syntax.

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