加载后,WKWebView需要延迟设置其scrollView的contentOffset和zoomScale()

contentOffset 和 zoomScale ,因此可以在Web视图加载后恢复。我发现设置那些scrollView属性只能使用延迟(例如 DispatchQueue.main.asyncAfter() . 为什么这是必要的?有没有更好的方法来实现这一点?

import UIKit
import WebKit

class ViewController: UIViewController, WKNavigationDelegate {
    var contentOffset = CGPoint.zero
    var zoomScale: CGFloat = 1.0
    lazy var webView: WKWebView = {
        let wv = WKWebView(frame: CGRect.zero)
        wv.translatesAutoresizingMaskIntoConstraints = false
        wv.allowsBackForwardNavigationGestures = true
        wv.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        wv.navigationDelegate = self
        return wv
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(webView)
        webView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        webView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        webView.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
        webView.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        reload()
    }

    @IBAction func refreshTapped(_ sender: UIBarButtonItem) {
        reload()
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        // Without some delay, the restoration doesn't happen!
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.25, execute: {
            self.webView.scrollView.setZoomScale(self.zoomScale, animated: false)
            self.webView.scrollView.setContentOffset(self.contentOffset, animated: false)
        })
    }

    private func reload() {
        contentOffset = webView.scrollView.contentOffset
        zoomScale = webView.scrollView.zoomScale;
        webView.load(URLRequest(url: URL(string: "https://www.google.com")!))
    }
}

(抱歉重复)

import UIKit
import WebKit

class ViewController: UIViewController {
    private static let keyPath = "webView.scrollView.contentSize"
    private static var kvoContext = 0

    private var contentOffset: CGPoint?
    private var zoomScale: CGFloat?
    private var lastContentSize: CGSize?
    private var repeatedSizeCount = 0

    lazy var webView: WKWebView = {
        let wv = WKWebView(frame: CGRect.zero)
        wv.translatesAutoresizingMaskIntoConstraints = false
        wv.allowsBackForwardNavigationGestures = true
        wv.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        return wv
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(webView)
        webView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        webView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        webView.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
        webView.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        addObserver(self, forKeyPath: ViewController.keyPath, options: .new, context: &ViewController.kvoContext)
        reload()
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        removeObserver(self, forKeyPath: ViewController.keyPath)
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        guard let newSize = change?[.newKey] as? CGSize, context == &ViewController.kvoContext else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
            return
        }

        print("Observed: \(NSStringFromCGSize(newSize))")

        // Nothing to restore if these are nil
        guard let offset = contentOffset, let zoom = zoomScale else { return }

        guard let lastSize = lastContentSize, lastSize == newSize else {
            print("Waiting for size to settle")
            lastContentSize = newSize
            return
        }

        repeatedSizeCount += 1

        guard repeatedSizeCount >= 4 else {
            print("Size repeated \(repeatedSizeCount) time(s)")
            return
        }

        print("Settled - set saved zoom and offset")
        contentOffset = nil
        zoomScale = nil
        lastContentSize = nil
        repeatedSizeCount = 0
        webView.scrollView.setZoomScale(zoom, animated: false)
        webView.scrollView.setContentOffset(offset, animated: false)
    }

    @IBAction func refreshTapped(_ sender: UIBarButtonItem) {
        contentOffset = webView.scrollView.contentOffset
        zoomScale = webView.scrollView.zoomScale;
        reload()
    }

    private func reload() {
        print("Reload: \(NSStringFromCGSize(webView.scrollView.contentSize))")
        webView.load(URLRequest(url: URL(string: "https://www.google.com")!))
    }
}

打印以下内容:

Reload: {781, 1453}
Observed: {781, 1453}
Waiting for size to settle
Observed: {320, 595}
Waiting for size to settle
Observed: {320, 595}
Size repeated 1 time(s)
Observed: {320, 595}
Size repeated 2 time(s)
Observed: {320, 595}
Size repeated 3 time(s)
Observed: {320, 595}
Settled - set saved zoom and offset
Observed: {781, 1453}
Observed: {781, 1453}
Observed: {781, 1453}