ios - 试图翻转 UIImage View 但是当翻转回来时我收到一个 nil

Question

我试图将 UIImageView 翻转到另一侧以显示其背后的内容。第一次翻转时一切正常，显示其背后的图像。问题是当我尝试将其翻转回来以隐藏图像时。我得到一个错误，它试图找到一个 nil。感谢您的帮助。

import UIKit

class ViewController: UIViewController {
@IBOutlet weak var mainLeftView: UIView!
@IBOutlet weak var back: UIImageView!
@IBOutlet weak var front: UIImageView!





override func viewDidLoad() {
    super.viewDidLoad()


    let singleTap = UITapGestureRecognizer(target: self, action: Selector("tappedItem"))
    singleTap.numberOfTapsRequired = 1







    mainLeftView.addGestureRecognizer(singleTap)
     mainLeftView.userInteractionEnabled = true;
    mainLeftView.addSubview(front)

}



var showingFront = true;

func tappedItem() {


   /*     TRIED METHOD NUMBER ONE


    var views: (front: UIImageView, back: UIImageView)

    if ((self.front.superview) != nil){
        views = (front: self.front, back: self.back)
    } else{
        views = (front: self.back, back: self.front)
    }
    UIView.transitionFromView(views.front, toView: views.back, duration: 1.0, options:UIViewAnimationOptions.TransitionFlipFromRight , completion: nil) 

*/

  // METHOD NUMBER TWO


      if (showingFront){
      UIView.transitionFromView(front, toView: back, duration: 0.5, options: UIViewAnimationOptions.TransitionFlipFromRight, completion: nil)
        showingFront = false;
    }

    else{
        UIView.transitionFromView(back, toView: front, duration: 0.5, options: UIViewAnimationOptions.TransitionFlipFromLeft, completion: nil)
        showingFront = true;

    }

}

Answer 1

使用强而不是弱。

@IBOutlet strong var mainLeftView: UIView!
@IBOutlet strong var back: UIImageView!
@IBOutlet strong var front: UIImageView!