iOS10-在deinit上发生NSKeyValueObservation崩溃

我NSKeyValueObservation用来观察子类中的属性WKWebView.

它在iOS 11上运行良好,但deinit在iOS 10上崩溃.

控制台上的打印错误日志

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'An instance 0x15209e600 of class Rakuemon.WebView was deallocated while key value observers were still registered with it. Current observation info:  (
 Context: 0x0, Property: 0x170643ba0>
 Context: 0x0, Property: 0x170643330>
 Context: 0x0, Property: 0x1706437b0>
)'

代码

class WebView: WKWebView {

    // MARK: - Properties

    weak var delegate: WebViewDelegate?

    // MARK: - Private properties

    private var contentSizeObserver: NSKeyValueObservation?
    private var loadingObserver: NSKeyValueObservation?
    private var estimatedProgressObserver: NSKeyValueObservation?
    private var titleObserver: NSKeyValueObservation?

    // MARK: - Life cycle

    override init(frame: CGRect, configuration: WKWebViewConfiguration) {
        super.init(frame: frame, configuration: configuration)
        setupObserver()
    }

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

// MARK: - Private functions

private extension WebView {
    func setupObserver() {
        cOntentSizeObserver= scrollView.observe(\.contentSize, options: [.old, .new], changeHandler: { [unowned self] _, change in
            guard let oldSize = change.oldValue, let newSize = change.newValue, oldSize != newSize else { return }
            self.delegate?.webView?(self, didChangeSizeFrom: oldSize, to: newSize)
        })

        loadingObserver = observe(\.isLoading, changeHandler: { [unowned self] _, _ in
            self.delegate?.webViewIsLoading?(self)
        })

        estimatedProgressObserver = observe(\.estimatedProgress, options: [.new], changeHandler: { [unowned self] _, change in
            guard let newValue = change.newValue else { return }
            self.delegate?.webView?(self, didChangeEstimatedProgress: newValue)
        })

        titleObserver = observe(\.title, options: [.new], changeHandler: { [unowned self] _, change in
            guard let title = change.newValue else { return }
            self.delegate?.webView?(self, didChangeTitle: title ?? "")
        })
    }
}

题

我还发现contentSizeObserver,观察到的scrollView.contentSize,不是属性self,没有导致崩溃.

那么,self通过iOS 10 观察其属性的正确方法是什么NSKeyValueObservation？或取消注册？

我最终弄清楚如何删除removeObserver(_:forKeyPath)iOS 10上的观察者.

// Environment: Xcode 11.1, Swift 5.1, iOS 10.3
deinit {
    if #available(iOS 11.0, *) {} else if let observer = observer {
        removeObserver(observer, forKeyPath: "foo")
    }
}