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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
    }
})