在swift 4中未弹出用于选择相机或图库的警报视图

import UIKit

class ViewController: UIViewController, UIImagePickerControllerDelegate, UINavigationControllerDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    @IBAction func showImagePicker(_ sender: UIButton) {
        let alertController = UIAlertController(title: "选择图片来源", message: nil, preferredStyle: .actionSheet)
        
        let cameraAction = UIAlertAction(title: "相机", style: .default) { (action) in
            self.openCamera()
        }
        alertController.addAction(cameraAction)
        
        let galleryAction = UIAlertAction(title: "图库", style: .default) { (action) in
            self.openGallery()
        }
        alertController.addAction(galleryAction)
        
        let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
        alertController.addAction(cancelAction)
        
        present(alertController, animated: true, completion: nil)
    }
    
    func openCamera() {
        // 打开相机的代码
    }
    
    func openGallery() {
        // 打开图库的代码
    }
}