ios - WKWebView 需要在 load() 之后延迟设置其 scrollView 的 contentOffset 和 zoomScale

Question

我写了一些代码来保存 WKWebView 的 ScrollView 的 contentOffset 和 zoomScale，因此它们可以在 webView 加载后恢复。我发现设置这些 scrollView 属性只能使用延迟(例如使用 DispatchQueue.main.asyncAfter())。为什么这是必要的？有没有更好的方法来实现这个？

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")!))
    }
}

根据 AJ B 建议更新(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}

Answer 1

我正在做类似的事情，试图在加载后获取内容高度，然后将 webview 的容器调整为该高度。我为此苦苦挣扎了一段时间，我发现最好的方法是观察 WKWebView 的 ScrollView 内容高度，当内容高度以相同的大小重复出现时，您就知道它已完全加载。这有点老套，但对我来说一直有效。我想知道是否有人也知道更好的解决方案。

//This was an example html string that would demonstrate the issue

var html = "<html><body><p>We're no strangers to love  You know the rules and so do I  A full commitment's what I'm thinking of  You wouldn't get this from any other guy    I just want to tell you how I'm feeling  Gotta make you understand    Never gonna give you up, never gonna let you down  Never gonna run around and desert you  Never gonna make you cry, never gonna say goodbye  Never gonna tell a lie and hurt you    We've known each other for so long  Your heart's been aching but you're too shy to say it  Inside we both know what's been going on  We know the game and we're gonna play it    And if you ask me how I'm feeling  Don't tell me you're too blind to see    Never gonna give you up, never gonna let you down  Never gonna run around and desert you  Never gonna make you cry, never gonna say goodbye  Never gonna tell a lie and hurt you    Never gonna give you up, never gonna let you down  Never gonna run around and desert you  Never gonna make you cry, never gonna say goodbye  Never gonna tell a lie and hurt you    We've known each other for so long  Your heart's been aching but you're too shy to say it  Inside we both know what's been going on  We know the game and we're gonna play it    I just want to tell you how I'm feeling  Gotta make you understand    Never gonna give you up, never gonna let you down  Never gonna run around and desert you  Never gonna make you cry, never gonna say goodbye  Never gonna tell a lie and hurt you</p><p><img src=\"https://i.ytimg.com/vi/dQw4w9WgXcQ/maxresdefault.jpg\" width=\"\" height=\"\" style=\"display:block;height:auto;max-width:100%;width:100%;\"></p></body></html>"

我注入(inject)了一些 javascript 来发送准备状态和 dom 加载的事件，并打印当时的大小以及 didFinish 导航功能。这是打印的内容:

// Adjusting the container height in didFinish navigation function 

started navigation
committed navigation
content height = 0.0
Javascript: Ready state change interactive | content height = 0.0
Javascript: DOM content loaded | content height = 0.0
Javascript: Ready state change complete | content height = 0.0
ended navigation content height = 0.0 (didFinish navigation)
content size observed = 1638.0
height constraint = Optional(0.0)
content size observed = 691.666666666667
height constraint = Optional(0.0)
content size observed = 1171.0
height constraint = Optional(0.0)
content size observed = 2772.0
height constraint = Optional(0.0)
content size observed = 2772.0
height constraint = Optional(0.0)
content size observed = 2772.0
height constraint = Optional(0.0)
content size observed = 2772.0
height constraint = Optional(0.0)
content size observed = 2772.0
height constraint = Optional(0.0)
content size observed = 2772.0
height constraint = Optional(0.0)
content size observed = 2772.0
height constraint = Optional(0.0)

// Adjusting the container height after content size repeats itself KVO

started navigation
committed navigation
content height = 0.0
Javascript: Ready state change interactive | content height = 0.0
Javascript: DOM content loaded | content height = 0.0
Javascript: Ready state change complete | content height = 0.0
ended navigation content height = 0.0 (didFinish navigation)
content size observed = 1638.0
height constraint = Optional(1.0)
content size observed = 691.666666666667
height constraint = Optional(1.0)
content size observed = 691.666666666667
height constraint = Optional(1.0)
content size observed = 691.666666666667
height constraint = Optional(691.666666666667)
content size observed = 691.666666666667
height constraint = Optional(691.666666666667)
content size observed = 691.666666666667
height constraint = Optional(691.666666666667)
content size observed = 691.666666666667
height constraint = Optional(691.666666666667)
content size observed = 691.666666666667
height constraint = Optional(691.666666666667)
content size observed = 691.666666666667
height constraint = Optional(691.666666666667)
content size observed = 691.666666666667
height constraint = Optional(691.666666666667)