看起来你试图调用函数performGetRequest，但是你在开始的时候用func定义了它。
如果这不是你的意图，也许你正在尝试定义函数，但是你必须小心完成处理程序。你可以看到正确的方法来完成它。
最后，这里有一个函数的question，它具有相同的名称和类似的问题，我不知道你的代码是否基于这个，但是看看这个问题和它的答案可能会更清楚。

var desiredChannelsArray = ["eminemvevo", "Google", "disney"]

    var channelIndex = 0

    var channelsDataArray: Array<Dictionary<NSObject, AnyObject>>.Type?

    override func viewDidLoad() {
        super.viewDidLoad()

    }  class helper {
        func performGetRequest(targetURL: NSURL!, completion: (data: NSData?, HTTPStatusCode: Int, error: NSError?) -> Void) {

            let request = NSMutableURLRequest(URL: targetURL)
            request.HTTPMethod = "GET"

            let sessionConfiguration = NSURLSessionConfiguration.defaultSessionConfiguration()

            let session = NSURLSession(configuration: sessionConfiguration)

            let task = session.dataTaskWithRequest(request, completionHandler: { (data, response, error) -> Void in
                dispatch_async(dispatch_get_main_queue(), { () -> Void in
                    completion(data: data, HTTPStatusCode: (response as! NSHTTPURLResponse).statusCode, error: error)})})

            task.resume()
        }
    }

    func getChannelDetails(useChannelIDParam: Bool) {
        var urlString: String!
        if !useChannelIDParam {
            urlString = "https://www.googleapis.com/youtube/v3/channels?part=contentDetails,snippet&forUsername=\(desiredChannelsArray[channelIndex])&key=\(apiKey)"
        } else {
        }

        let targetURL = NSURL(string: urlString)

        func performGetRequest(targetURL: NSURL!, completion: (data: NSData?, HTTPStatusCode: Int, error: NSError?) -> Void) {

            if HTTPStatusCode == 200 && error == nil{
                // Convert the JSON data to a dictionary.

                let resultsDict = NSJSONSerialization.JSONObjectWithData(data!, options: nil, error: nil) as! Dictionary<NSObject, AnyObject>}

            let items: AnyObject! = resultsDict["items"] as AnyObject!
            let firstItemDict = (items as! Array<AnyObject>)[0] as! Dictionary<NSObject, AnyObject>

            let snippetDict = firstItemDict["snippet"] as! Dictionary<NSObject, AnyObject>

            var desiredValuesDict: Dictionary<NSObject, AnyObject> = Dictionary<NSObject, AnyObject>()
            desiredValuesDict["title"] = snippetDict["title"]
            desiredValuesDict["description"] = snippetDict["description"]
            desiredValuesDict["thumbnail"] = ((snippetDict["thumbnails"] as! Dictionary<NSObject, AnyObject>)["default"] as! Dictionary<NSObject, AnyObject>)["url"]

            desiredValuesDict["playlistID"] = ((firstItemDict["contentDetails"] as! Dictionary<NSObject, AnyObject>)["relatedPlaylists"] as! Dictionary<NSObject, AnyObject>)["uploads"]

            self.channelsDataArray.append(desiredValuesDict)

            self.tblVideos.reloadData()

            ++self.channelIndex
            if self.channelIndex < self.desiredChannelsArray.count {
                self.getChannelDetails(useChannelIDParam)
            }
            else {
                self.viewWait.hidden = true
            }
        } else {

            print("HTTP Status Code = \(HTTPStatusCode)")
            print("Error while loading channel details: \(error)") }
    }
}
}