Swift:无法将 ImageView 置于 ScrollView 中心

Question

我正在尝试将 X 和 Y 轴中心的 imageView 显示到 scrollView 上，当我运行应用程序时，它很好地适合设备的宽度。我还想平移和缩放imageView。我已经尝试了几乎所有已发布在 SO 上的解决方案，并且也遵循了此 tutorial但不知怎的，它并不能解决我的问题。该图像是从 PDF 生成的。

我以编程方式进行布局。到目前为止我的实现:

let scrollView: UIScrollView = {
    let sv = UIScrollView()
    sv.translatesAutoresizingMaskIntoConstraints = false
    sv.backgroundColor = .lightGray
    return sv
}()

let imageView: UIImageView = {
    let iv = UIImageView()
    iv.translatesAutoresizingMaskIntoConstraints = false
    iv.contentMode = .scaleAspectFill
    return iv
}()

override func viewDidLoad() {
    super.viewDidLoad()
    scrollView.delegate = self

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

    NSLayoutConstraint.activate([
        scrollView.topAnchor.constraint(equalTo: view.topAnchor),
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor),

        imageView.topAnchor.constraint(equalTo: scrollView.topAnchor, constant: 10),
        imageView.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor, constant: 10),
        imageView.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor, constant: -10),
        imageView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: -10),

        ])

    imageView.image = obtainThumbnail()
}

fileprivate func updateMinZoomScaleForSize(_ size: CGSize) {
    let widthScale = size.width / imageView.bounds.width
    let heightScale = size.height / imageView.bounds.height
    let minScale = min(widthScale, heightScale)

    scrollView.minimumZoomScale = minScale
    scrollView.zoomScale = minScale
}

override func viewWillLayoutSubviews() {
    super.viewWillLayoutSubviews()
    updateMinZoomScaleForSize(view.bounds.size)
}

func obtainThumbnail() -> UIImage? {
    guard let url = Bundle.main.url(forResource: "drawings", withExtension: "pdf") else {return nil}

    guard
        let data = try? Data(contentsOf: url),
        let page = PDFDocument(data: data)?.page(at: 0) else {
            return nil
    }

    let pageSize = page.bounds(for: .mediaBox)
    return page.thumbnail(of: CGSize(width: pageSize.width, height: pageSize.height), for: .mediaBox)
}

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

这是我运行应用程序时想要的结果。

这是我现在运行应用程序时的结果，过度拉伸(stretch)。

有人可以告诉我我错过了什么吗？

Answer 1

我认为 View 是在 UIViewController 中延迟加载的，因此在 UI 生命周期的后期进行计算应该可以解决问题。删除 viewWillLayoutSubviews 中的调用并改为使用:

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    updateMinZoomScaleForSize(view.bounds.size)
}

似乎有效。