ios - 为什么 UIScrollView 的内容不居中？

Question

我在 UIContentView 中嵌入了一个 UIImageView 以使其可缩放和平移 ( reference 。问题是图像没有正确居中。它看起来像整个图像内容移出了左上角:

虽然打印的内容偏移量接近于零:内容偏移量为:(0.0, -20.0)

这是我的实现:

import UIKit

class ZoomableImageView: UIScrollView {

    private struct Constants {
        static let minimumZoomScale: CGFloat = 0.5;
        static let  maximumZoomScale: CGFloat = 6.0;
    }


    // public so that delegate can access
    public let imageView = UIImageView()

    // gw: must be called to complete a setting
    public func setImage(image: UIImage) {
        imageView.image = image
        imageView.bounds = CGRect(x: 0, y: 0, width: image.size.width, height: image.size.height)

        self.contentSize = image.size


        let scaleFitZoomScale: CGFloat = min(
        self.frame.width / image.size.width ,
        self.frame.height / image.size.height
        )


        // reset scale and offset on each resetting of image
        self.zoomScale = scaleFitZoomScale
    }


    // MARK - constructor
    init() {
        // gw: there is no super.init(), you have to use this constructor as hack
        super.init(frame: CGRect(x: 0, y: 0, width: 0, height: 0)) // gw: relies on autolayout constraint later

        minimumZoomScale = Constants.minimumZoomScale
        maximumZoomScale = Constants.maximumZoomScale

        addSubview(imageView)


    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }



}

View Controller :

class ViewController: UIViewController, UIScrollViewDelegate {


    let zoomableImageView  = ZoomableImageView()


    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(zoomableImageView)
        zoomableImageView.delegate = self

    }

    override func viewDidLayoutSubviews() {

        zoomableImageView.frame = view.frame
        let image = UIImage(imageLiteralResourceName: "kelly")
        zoomableImageView.setImage(image: image)

    }

    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        // print("scale factor is: \(scrollView.zoomScale)")
        return zoomableImageView.imageView
    }

    func scrollViewDidZoom(_ scrollView: UIScrollView) {
         print("scale factor is: \(scrollView.zoomScale)")
    }

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        print("content offset is: \(scrollView.contentOffset)")
    }



}

但是，如果我不将 UIImageView 和 UIScrollView 包装在自定义类下，图像的居中效果会很好:

class ViewController: UIViewController, UIScrollViewDelegate {


    let imageView: UIImageView = {
        let image = UIImage(imageLiteralResourceName: "kelly")
        let imageView = UIImageView(image: image)
        imageView.bounds = CGRect(x: 0, y: 0, width: image.size.width, height: image.size.height)
        return imageView
    } ()

    let scrollView: UIScrollView = {
        let scrollView = UIScrollView()
        return scrollView
    } ()
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        view.addSubview(scrollView)
        scrollView.addSubview(imageView)


        scrollView.delegate = self
        self.scrollView.minimumZoomScale = 0.5;
        self.scrollView.maximumZoomScale = 6.0;

    }

    override func viewDidLayoutSubviews() {
        scrollView.frame = view.frame

        if let size = imageView.image?.size {
            scrollView.contentSize = size
        }



    }

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


}

我是否在第一次实现中遗漏了什么？

Answer 1

在你的ZoomableImageView类，您正在设置 bounds 当它应该是 frame

所以改变:

imageView.bounds = CGRect(x: 0, y: 0, width: image.size.width, height: image.size.height) 

到

imageView.frame =  CGRect(x: 0, y: 0, width: image.size.width, height: image.size.height)