设置Scroll View的zoomScale和点击后退按钮会导致程序崩溃

我试着制作一个程序,你点击一个图像,它会在全屏上显示,缩放以使周围没有空白(CS 193P任务4任务7,任何知道它的人都可以)。视图控制器嵌入在导航控制器中。结果是程序崩溃了 EXC_BAD_ACCESS (code = EXC_I386_GPFLT) 当我按下导航栏上的后退按钮时。

我发现我可以通过滚动到图像顶部然后返回来防止这种崩溃。另一种完全防止这种情况的方法是注释掉设置scrollView的zoomScale的行。下面是我用于加载和处理图像的代码:

import UIKit

class ImageViewController: UIViewController, UIScrollViewDelegate {

    @IBOutlet weak var scrollView: UIScrollView! {
        didSet {
            scrollView.contentSize = imageView.frame.size
            scrollView.delegate = self
            scrollView.minimumZoomScale = 0.03
            scrollView.maximumZoomScale = 5.0
        }
    }

    func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView? {
        return imageView
    }

    var imageURL: NSURL? {
        didSet {
            image = nil
            if view.window != nil {
                fetchImage()
            }
        }
    }

    private func fetchImage() {
        if let url = imageURL {
            let qos = Int(QOS_CLASS_USER_INITIATED.value)
            dispatch_async(dispatch_get_global_queue(qos, 0)) {
                let imageData = NSData(contentsOfURL: url)
                dispatch_async(dispatch_get_main_queue()) {
                    if url == self.imageURL {
                        if imageData != nil {
                            self.image = UIImage(data: imageData!)
                        } else {
                            self.image = nil
                        }
                    }
                }
            }
        }
    }

    private var imageView = UIImageView()

    private var image: UIImage? {
        get { return imageView.image }
        set {
            imageView.image = newValue
            imageView.sizeToFit()
            scrollView?.contentSize = imageView.frame.size
            scrollViewDidScrollOrZoom = false
            autoScale()
        }
    }

    private var scrollViewDidScrollOrZoom = false

    private func autoScale() {
        if scrollViewDidScrollOrZoom {
            return
        }
        if let sv = scrollView {
            if image != nil {
                sv.zoomScale = max(sv.bounds.size.width / image!.size.width, sv.bounds.size.height / image!.size.height)
                sv.contentOffset = CGPoint(x: (imageView.frame.size.width - sv.frame.size.width) / 2, y: (imageView.frame.size.height - sv.frame.size.height) / 2)
                scrollViewDidScrollOrZoom = false
            }
        }
    }


    func scrollViewDidEndZooming(scrollView: UIScrollView, withView view: UIView!, atScale scale: CGFloat) {
        scrollViewDidScrollOrZoom = true
    }

    func scrollViewDidScroll(scrollView: UIScrollView) {
        scrollViewDidScrollOrZoom = true
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        scrollView.addSubview(imageView)
    }

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)
        if image == nil {
            fetchImage()
        }
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        autoScale()
    }
}

我试图制作一个简单的应用程序,它从一个只有一个按钮并嵌入在导航控制器中的视图控制器切换到ImageViewController,但崩溃仍然是一样的。下面是我在第一个视图控制器中使用的代码:

class FirstViewController: UIViewController {

    let url = NSURL(string: "https://developer.apple.com/swift/images/swift-og.png")

    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        if segue.identifier == "Show Full Image" {
            let ivc = segue.destinationViewController as! ImageViewController
            ivc.imageURL = url
        }
    }
}

是什么原因导致的?我在这里做错了什么?我正在使用Xcode 6.3 beta 3。