swift - urlSession.dataTaskWithRequest(request)的含义

Question

当我在网络开发章节中阅读有关swift的书时，我遇到了一些我看不懂的代码。代码如下:

let sessionTask = urlSession.dataTaskWithRequest(request) {
            (data, response, error) in

            handler(response, data)
            } 

这个函数在swift中的原型(prototype)是:

public func dataTaskWithRequest(request: NSURLRequest, completionHandler: (NSData?, NSURLResponse?, NSError?) -> Void) -> NSURLSessionDataTask

可以看到，原型(prototype)有2个参数，一个是request，一个是completionHandler。但是在上面的代码中，它还有一个参数。而且我也无法理解花括号中的代码，3 个变量数据、响应、错误从何而来？我找不到这 3 个变量的任何定义。谁能帮助我理解代码，在此先感谢。

Answer 1

它被称为尾随闭包，如果该函数是最后一个参数，它是将一个函数传递给另一个函数的一种更简洁的方法。同样的代码可以写成:

let sessionTask = NSURLSession.sharedSession()
let request = NSURLRequest()
sessionTask.dataTaskWithRequest(request, completionHandler: {(data: NSData?, response: NSURLResponse?, error: NSError?) -> Void in

})

If you need to pass a closure expression to a function as the function’s final argument and the closure expression is long, it can be useful to write it as a trailing closure instead. A trailing closure is a closure expression that is written outside of (and after) the parentheses of the function call it supports

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html#//apple_ref/doc/uid/TP40014097-CH11-ID102

func aFunction(callback: (done: Bool) -> Void) {
  let finished = true
  callback(done: finished)
}

aFunction { (done) -> Void in
  print("we are done \(done)")
}