错误 - UIImageView UIScrollView 播放背景视频

Question

我无法编译以下代码，大家有什么建议吗？

import UIKit

class ViewController: UIViewController, UIScrollViewDelegate {

    @IBOutlet weak var avatar: UIImageView!
    @IBOutlet weak var scrollView: UIScrollView!
    @IBOutlet weak var contentImageView: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()
        avatar.layer.cornerRadius = 5.0
        avatar.layer.borderWidth = 4.0
        avatar.layer.borderColor = UIColor.whiteColor().CGColor
        avatar.clipsToBounds = true
        scrollView.delegate = self
        contentImageView.clipsToBounds = true
    }

    override func viewDidAppear(animated: Bool) {
        let vc = self.storyboard?.instantiateViewControllerWithIdentifier("LoginScreen") as! UIViewController
        self.presentViewController(vc, animated: true, completion: nil)
    }

    func scrollViewDidScroll(scrollView: UIScrollView) {
        let yOffset = self.scrollView.contentOffset.y * 0.2
        let availableOffset = min(yOffset, 60)
        let contentRectYOffset = availableOffset / contentImageView.frame.size.height
        contentImageView.layer.contentsRect = CGRectMake(0.0, contentRectYOffset, 1, 1);
    }
}

错误是

'来自'UIViewController 的向下转换？'到 'UIViewController' 只展开 选项；你的意思是用'！'

这发生在一行

let vc = self.storyboard?.instantiateViewControllerWithIdentifier("LoginScreen") as! UIViewController

Answer 1

你强制向下转换一个已经是相同类型的对象。您只需要删除as! UIViewController 并假设存在标识符为LoginScreen 的ViewController，您可以在需要时强制解开vc 变量。

override func viewDidAppear(animated: Bool) {
    let vc = self.storyboard?.instantiateViewControllerWithIdentifier("LoginScreen")
    self.presentViewController(vc!, animated: true, completion: nil)
}

Answer 2

感谢 R P，您的解决方案很有效。

override func viewDidAppear(animated: Bool) {
let vc = self.storyboard?.instantiateViewControllerWithIdentifier("LoginScreen")
self.presentViewController(vc!, animated: true, completion: nil)

}