ios – WKWebView显示灰色背景,pdf内容在viewcontroller开关上不可见

我从WKWebView中的主包或文档目录加载pdf文件.它最初加载完美,但它显示灰色背景,如果我在tabbar控制器中切换选项卡后回到同一屏幕,则pdf内容将不可见.这是我用来加载的代码

class ViewController: UIViewController {

    @IBOutlet var progressview: UIProgressview!
    @IBOutlet var webView: WKWebView!

    var pdfURLS: URL?

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view,typically from a nib.

        pdfURLS = Bundle.main.url(forresource: "97_pdf",withExtension: "pdf",subdirectory: nil,localization: nil)
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        print("pdfURLS: \(String(describing: pdfURLS))")
    }

    @IBACtion func loadAction(_ sender: Any) {
        if let pdfURL = pdfURLS  {
            self.webView.loadFileURL(pdfURL,allowingReadAccessTo: pdfURl.deletingLastPathComponent())
        }
    }
}

有人对这个问题有所了解吗？

附有图像参考.如果我使用不推荐使用的UIWebView,则不存在此类问题,但我不想使用已弃用的库.

let prevIoUsZoomScale = webView.scrollView.zoomScale
let prevIoUsContentOffset = webView.scrollView.contentOffset

if let currentItem = webView.BACkForWARDList.currentItem {
    webView.go(to: currentItem)
}

DispatchQueue.main.asyncAfter(deadline: .Now() + .milliseconds(100)) {
    webView.scrollView.setZoomScale(prevIoUsZoomScale,animated: falsE)
    webView.scrollView.setContentOffset(prevIoUsContentOffset,animated: falsE)
}