Ios 由于'；内部'；保护级别

我创建了一个pod库

库包含一个协议。当我从cocoapods安装库并尝试符合我得到的协议时，由于“内部”保护级别错误，委托无法访问

议定书：

@objc公共协议TDDropdownListDelegate{
函数列表（发送方：UIButton）
}

协议功能：

open func listTapped(sender: UIButton) {
    activate(sender: sender)
    delegate?.listTapped(sender: sender)
}

我得到的代码是错误的

import UIKit
import TDDropdownList

class ViewController: UIViewController,TDDropdownListDelegate {
let a = TDDropdownList(frame: CGRect(x: 100 , y: 100 , width: 200 , height: 75))
override func viewDidLoad() {
    a.initialize(data: ["Emre","Duman"])

    a.delegate = self  // LINE I GOT THE ERROR

    self.view.addSubview(a)
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
}
func listTapped(sender: UIButton) {
    print(sender.currentTitle!)
}
override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}


}