Xamarin HybridWebView 在 EvaluateJavaScript 中抛出异常

Question

我昨天花了几个小时找出为什么我无法访问我的混合 wkWebView 的（显然有效的）参数。在正确触发的 DidFinishNavigation 覆盖下的 iOS 部分，我在尝试 EvaluateJavaScriptAsync 甚至访问 Source 参数时总是遇到 NullObjectReference 异常。

然后我终于发现我看错了对象：我正在访问 Forms HybridWebView 对象（在 "chain" 中向上）而不是 iOS webview 对象（在 "chain" 中向下）渲染器）。在下面的代码中，DidFinishNavingation 中的 "webView" 参数代替了 webViewRenderer.Element 中的 _wkWebView（参见最后的代码部分）

现在我的代码可以工作了，但我仍然想知道我的平台特定集成的总体概念是否正确，或者表单 hybridwebview 中可用的表单函数（例如 EvaluateJavaScript）是否应该直接工作？我是否遗漏了一些 link 来将它们与底层 iOS 网络视图连接起来？

我的 HybidWebView 共享表单代码：

public class MyWebView : WebView
{
    public event EventHandler SwipeLeft;
    public event EventHandler SwipeRight;

    public void OnSwipeLeft() =>
        SwipeLeft?.Invoke(this, null);

    public void OnSwipeRight() =>
        SwipeRight?.Invoke(this, null);

    public static readonly BindableProperty UrlProperty = BindableProperty.Create(
        propertyName: "Url",
        returnType: typeof(string),
        declaringType: typeof(MyWebView),
        defaultValue: default(string));

    public string Url
    {
        get { return (string)GetValue(UrlProperty); }
        set
        {
            SetValue(UrlProperty, value);
            NotifyPropertyChanged(nameof(HTML));
        }
    }

    public static readonly BindableProperty HTMLProperty = BindableProperty.Create(
        propertyName: "HTML",
        returnType: typeof(string),
        declaringType: typeof(MyWebView),
        defaultValue: default(string));

    public string HTML
    {
        get { return (string)GetValue(HTMLProperty); }
        set
        {
            SetValue(HTMLProperty, value);
            NotifyPropertyChanged(nameof(HTML));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

和渲染器的连接 iOS 部分：

    public class MyWebViewRenderer : ViewRenderer<MyWebView, WKWebView>
{
    WKWebView _wkWebView;

    protected override void OnElementChanged(ElementChangedEventArgs<MyWebView> e)
    {
        System.Console.WriteLine("--- Loading wkWebView ---");
        base.OnElementChanged(e);

        if (Control == null)
        {
            Console.WriteLine("+++ WKW: Control null -> init");

            var config = new WKWebViewConfiguration();
            _wkWebView = new WKWebView(Frame, config);

            SetNativeControl(_wkWebView);
            _wkWebView.NavigationDelegate = new ExtendedWKWebViewDelegate(this);

        }

        if (e.NewElement != null)
        {
            Console.WriteLine("+++ WKW: Control exists -> init GestureRecognizers");

            UISwipeGestureRecognizer leftgestureRecognizer = new UISwipeGestureRecognizer(this, new Selector("SwipeEvent:"));

            leftgestureRecognizer.Direction = UISwipeGestureRecognizerDirection.Left;

            UISwipeGestureRecognizer rightgestureRecognizer = new UISwipeGestureRecognizer(this, new Selector("SwipeEvent:"));
            rightgestureRecognizer.Direction = UISwipeGestureRecognizerDirection.Right;

            leftgestureRecognizer.Delegate = new MyWebViewDelegate();
            rightgestureRecognizer.Delegate = new MyWebViewDelegate();

            this.AddGestureRecognizer(leftgestureRecognizer);
            this.AddGestureRecognizer(rightgestureRecognizer);
        }

        NSUrl baseURL = new NSUrl(App.dirNews, true);
        string viewFile = Path.Combine(App.dirNews, "view.html");
        NSUrl fileURL = new NSUrl(viewFile, false);

        if (Element?.HTML != null && Element?.HTML != "")
        {
            System.Console.WriteLine("--- Showing HTTP content ---");
            File.WriteAllText(viewFile, Element.HTML, System.Text.Encoding.UTF8);
            Control.LoadFileUrl(fileURL, baseURL);
        }
        else if (Element?.Url != null && Element?.Url != "")
        {
            System.Console.WriteLine("--- Loading Web page ---");
            System.Console.WriteLine("--- " + Element.Url + " ---");
            NSUrlRequest myRequest = new NSUrlRequest(new NSUrl(Element.Url), NSUrlRequestCachePolicy.ReloadIgnoringLocalAndRemoteCacheData, 120);
            Control.LoadRequest(myRequest);
        }

    }

最后是 iOS 中的 DidFinishLoading 事件处理程序 class:

    class ExtendedWKWebViewDelegate : WKNavigationDelegate
{

    MyWebViewRenderer webViewRenderer;

    public ExtendedWKWebViewDelegate(MyWebViewRenderer _webViewRenderer = null)
    {
        webViewRenderer = _webViewRenderer ?? new MyWebViewRenderer();
    }

    [Export("webView:didFinishNavigation:")]
    public override async void DidFinishNavigation(WKWebView webView, WKNavigation navigation)
    {
        try
        {
            var _webView = webViewRenderer.Element as WebView;
            if (_webView != null)
            {
                await System.Threading.Tasks.Task.Delay(100); // wait here till content is rendered
                _webView.HeightRequest = (double)webView.ScrollView.ContentSize.Height;

                //### get html from site
                var myRes = await webView.EvaluateJavaScriptAsync("document.body.innerHTML");

                //### Flag complete status
                MessagingCenter.Send<object, string>(this, "didFinishNavigation", myRes.ToString());

            }
        }

        catch (Exception ex)
        {
            //Log the Exception
            Console.WriteLine("*** CustomWebView Exception: " + ex.Message + "/" + ex.InnerException);
        }


    }

}

Answer 1

当我们调用行时

SetNativeControl(_wkWebView);

您在Forms中定义的WebView又被初始化了。所以你需要在 Custom Renderer 中实现大部分功能。特别是在iOS的WebView中，如果你想在它的生命周期中添加监听器或者调用一些JS方法，在Custom Renderer中实现是最好的方法（或者说唯一的方法） ) 。

Xamarin HybridWebView 在 EvaluateJavaScript 中抛出异常

Xamarin HybridWebView throwing exception at EvaluateJavaScript

custom-renderer

xamarin

wkwebview