SKStoreProductViewController没有显示在swift中（应用内购买）

Question

我正在尝试将应用内购买添加到我的iOS应用中。我按照教程编写了以下代码：

import UIKit
import StoreKit

let secondViewController: SKStoreProductViewController = SKStoreProductViewController();

class GameViewController: UIViewController, UITextFieldDelegate, GKGameCenterControllerDelegate, SKStoreProductViewControllerDelegate{   

func buycoin(){
  println("buycoin")
    secondViewController.delegate = self
    var someitunesid:String = "coin200a"
    var productparameters = [SKStoreProductParameterITunesItemIdentifier: someitunesid]
    secondViewController.loadProductWithParameters(productparameters, {
        (success:Bool!, error: NSError!) -> Void in
        if success == true{
        self.presentViewController(secondViewController, animated: true, completion: nil)
            println("succes")
    }
        else{
            NSLog("%@", error)
            println("nosucces")
        }

    })
}

func productViewControllerDidFinish(viewController: SKStoreProductViewController!) {
    secondViewController.dismissViewControllerAnimated(true, completion: nil)
}

}

此代码工作正常。当函数buycoin()激活时，我只在控制台中打印buycoin，没有别的。

我做错了什么？