viewWillLayoutSubviews

override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        setZoomScaleFor(scrollViewSize: scrollView.bounds.size)
        if scrollView.zoomScale < scrollView.minimumZoomScale {
            scrollView.zoomScale = scrollView.minimumZoomScale
        }
        recenterImage()
    }

自动布局

    flexibleLeftMargin  自动调整view与父视图左边距，保证右边距不变
     flexibleWidth       自动调整view的宽度，保证左边距和右边距不变
     flexibleRightMargin 自动调整view与父视图右边距，以保证左边距不变
     flexibleTopMargin   自动调整view与父视图上边距，以保证下边距不变
     flexibleHeight      自动调整view的高度，以保证上边距和下边距不变
     flexibleBottomMargin自动调整view与父视图的下边距，以保证上边距不变

        scrollView.autoresizingMask = [.flexibleWidth,.flexibleHeight]

<li><h1>学习代码:<h1></li>

ViewController.swift

import UIKit

let YHRect = UIScreen.main.bounds
let YHHeight = YHRect.size.height
let YHWidth = YHRect.size.width

class ViewController: UIViewController {
    
    
    let scrollView = UIScrollView(frame: YHRect)
    let imageView = UIImageView(image: UIImage(named: "steve"))
    

    override func viewDidLoad() {
        super.viewDidLoad()
        setupBG()
        setupScrollView()
        setZoomScaleFor(scrollViewSize: scrollView.bounds.size)
        scrollView.zoomScale = scrollView.minimumZoomScale
        recenterImage()
    }
    override var preferredStatusBarStyle: UIStatusBarStyle {
        return .lightContent
    }
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        setZoomScaleFor(scrollViewSize: scrollView.bounds.size)
        if scrollView.zoomScale < scrollView.minimumZoomScale {
            scrollView.zoomScale = scrollView.minimumZoomScale
        }
        recenterImage()
    }
    
    
    //背景
    func setupBG() {
        view.layer.contents = UIImage(named: "steve")?.cgImage
        let visual = UIVisualEffectView(effect: UIBlurEffect(style: .dark))
        visual.frame = YHRect
        view.addSubview(visual)
    }
    //scrollview
    func setupScrollView() {
        /*自动布局
         flexibleLeftMargin  自动调整view与父视图左边距，保证右边距不变
         flexibleWidth       自动调整view的宽度，保证左边距和右边距不变
         flexibleRightMargin 自动调整view与父视图右边距，以保证左边距不变
         flexibleTopMargin   自动调整view与父视图上边距，以保证下边距不变
         flexibleHeight      自动调整view的高度，以保证上边距和下边距不变
         flexibleBottomMargin自动调整view与父视图的下边距，以保证上边距不变
         */
        scrollView.autoresizingMask = [.flexibleWidth,.flexibleHeight]
        scrollView.backgroundColor = .clear
        scrollView.contentSize = imageView.bounds.size
        scrollView.delegate = self
        
        scrollView.addSubview(imageView)
        view.addSubview(scrollView)
    }
    //设置缩放
    func setZoomScaleFor(scrollViewSize: CGSize) {
        let imageSize = imageView.bounds.size
        let widthScale = scrollViewSize.width/imageSize.width
        let heightScale = scrollViewSize.height/imageSize.height
        let minimunScale = min(widthScale,heightScale)
        
        scrollView.minimumZoomScale = minimunScale
        scrollView.maximumZoomScale = 3.5
    }
    //重新定位image
    func recenterImage() {
        let scrollViewSize = scrollView.bounds.size
        let imageViewSize = imageView.frame.size
        let horizontalSpace = imageViewSize.width < scrollViewSize.width ? (scrollViewSize.width - imageViewSize.width) / 2.0 : 0
        let verticalSpace = imageViewSize.height < scrollViewSize.height ? (scrollViewSize.height - imageViewSize.width) / 2.0 :0
        
        scrollView.contentInset = UIEdgeInsetsMake(verticalSpace, horizontalSpace, verticalSpace, horizontalSpace)
    }
}

extension ViewController: UIScrollViewDelegate {
    //要缩放时调用，返回需要缩放的view
    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }
    //缩放后调用
    func scrollViewDidZoom(_ scrollView: UIScrollView) {
        recenterImage()
    }
}