ImageViewController2.swift

class ImageViewController: UIViewController
{
    //1. model
//    var imageURL: URL?
    
    //5.
    var imageURL: URL? {
        didSet {
            image = nil
            //fetchImage()
            //9.
            if view.window != nil {
                fetchImage()
            }
        }
    }
    //11.
    @IBOutlet weak var scrollView: UIScrollView! {
        didSet {
            //13.
            scrollView.delegate = self
            //12.
            scrollView.minimumZoomScale = 0.03
            scrollView.maximumZoomScale = 1.0
            
            scrollView.contentSize = imageView.frame.size
            scrollView.addSubview(imageView)
        }
    }
    
    @IBOutlet weak var spinner: UIActivityIndicatorView!
    //6.
    private func fetchImage() {
        if let url = imageURL {
            spinner.startAnimating()
            //19
            DispatchQueue.global(qos:.userInitiated).async { [weak self] in
                //try try! try?
                let urlContents = try? Data(contentsOf: url)
                if let imageData = urlContents, url == self?.imageURL {
                    //20
                    DispatchQueue.main.async {
                        self?.image = UIImage(data: imageData)
                    }
                }
            }
        }
    }

    //16註解掉？？
    //4.
    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(imageView)
        //7.
        //imageURL = DemoURL.stanford
        
    }
    
    //8.
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        //10.
        if image == nil {
            fetchImage()
        }
    }
    
    //2.
    fileprivate var imageView = UIImageView()
    
    
    //3.
    private var image: UIImage? {
        get {
            return imageView.image
        }
        set{
            imageView.image = newValue
            imageView.sizeToFit()
            scrollView?.contentSize = imageView.frame.size
            //21 spinner?
            spinner?.stopAnimating()
        }
    }
}

extension ImageViewController: UIScrollViewDelegate {
    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }
}