Setting Scroll View's zoomScale and hitting back button crashes program - swift

Question

I was trying to make a program where you click an image and it segues to show it on full screen, scaled so that there is no whitespace around (CS 193P assignment 4 task 7 for anyone who knows it). View controllers are embedded in navigation controller. It turns out that the program crashes with EXC_BAD_ACCESS (code = EXC_I386_GPFLT) when I hit the back button in the navigation bar.
I found out that I can prevent this crash by scrolling to the top of the image and then going back. Another way to completely prevent it is to comment out the line that sets scrollView's zoomScale. Here's the code I used for loading and handling the image:
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()
}
}
I tried to make a simple application which segues into ImageViewController from a View Controller that has only a button and is embedded in navigation controller, and the crash is still the same. Here's the code I used in the first view controller:
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
}
}
}
What causes this and am I doing something wrong here? I'm using Xcode 6.3 beta 3.

Answer 1

It seems that the problem is actually with the scrollView's delegate, as I was able to fix this with adding
override func viewWillDisappear(animated: Bool) {
super.viewWillDisappear(animated)
scrollView.delegate = nil
}