假设您有一个从网络下载文件的下载功能，并且希望在下载任务完成时收到通知.
typealias CompletionHandler = (success:Bool) -> Void
func downloadFileFromURL(url: NSURL,completionHandler: CompletionHandler) {
// download code.
let flag = true // true if download succeed,false otherwise
completionHandler(success: flag)
}
// How to use it.
downloadFileFromURL(NSURL(string: "url_str")!, { (success) -> Void in
// When download completes,control flow goes here.
if success {
// download success
} else {
// download fail
}
})
希望有帮助.